Now we are really sailing off into terra incognito. "Here be dragons" and all that. But if you have starships, you almost have to have aliens (Isaac Asimov's Foundation trilogy being the most notable exception). The "science" is called Astrobiology, the famous "science in search of a subject". Unfortunately it only offers vague generalities. You can keep up on the latest news, but for now if you want aliens, you are going to have to create them yourself.

Suggested reading includes The Encyclopedia of Science Fiction's entry on "Aliens", Steve Colgan's Worlds of Possibility blog, Life Everywhere by David Darling, The Science of Aliens by Clifford Pickover and Aliens and Alien Societies by Stanley Schmidt.


[first use unknown]

Sometimes contrasted with `sentient' because even low animals can feel. `sapient' is usually an adjective, `sophont' usually a noun.


[first use unknown, but goes back at least to 1940s]

General SF term for an extraterrestrial or alien possessing human-level intelligence (see sophont).

Etymologically, and in mainstream English the word means "feeling" but is rare and now archaic.


[From Poul Anderson's `Polesotechnic League' stories, going back at least to 1963]

An evolved biological intelligence. Implies human-level cognitive and linguistic ability but not necessarily tool use. More specific and etymologically correct than sentient. Still less common than that term, but has been used by multiple writers.

From An SF Glossary by Eric S. Raymond (2006)

sapient, adjective \ˈsā-pē-ənt, ˈsa-\

Possessing or expressing great sagacity

sentient, adjective \ˈsen(t)-sh(ē-)ənt, ˈsen-tē-ənt\

Able to feel, see, hear, smell, or taste. Responsive to or conscious of sense impressions. Aware

ALIENS. Intelligent races who are not EARTH HUMANS. The term as such is never used for non-intelligent species, however unearthly, though in TECHJARGON these may be called Alien Life Forms. Nor is it used for Earth Humans who must register with the immigration service. In general, Aliens fall into two distinct groups, REALLY ALIENS and ALIENS WITH FOREHEAD RIDGES.

1) Really Aliens are truly unearthly. Frequently reported species include Energy Beings, HIVE ENTITIES, Giant Insectoids (who may also be Hive Entities), and Blobs of Protoplasm. The occasional intelligent bear or radish may also appear, or practically anything else. Except for the Energy Beings, most seem to be hydrocarbon life forms, but methane breathers who thrive at -200 C will sometimes turn up.

What they all have in common is that they are Really Alien. Exosemanticists have their work cut out understanding them, and exopsychologists in figuring out what they're all about. Relations between humans and Really Aliens are necessarily limited, since we have so little in common with them. Only rarely will anybody get to know one on a personal level. TRADE with them is sporadic, and even WARFARE seems less frequent than it used to be in the GOLDEN AGE. This is partly because it is not clear what we would fight them over, and partly because they may have an alarmingly high TECHLEVEL, making war with them a dangerously one-sided proposition. Dangerous at least for us. See COSMIC BACKGROUND HISTORY.

2) Aliens with Forehead Ridges. Much more common - especially in HOLLYWOOD SCIFI - than Really Aliens, these are species that look almost exactly like Earth Humans, except for some distinguishing visible feature such as, well, forehead ridges, or odd-shaped ears, or whatever. Sometimes they look rather less like humans, in which case (if friendly) they often resemble large teddy bears.

Not only do Aliens with Forehead Ridges mostly look like Earth Humans, they tend to act like Earth Humans as well, or at least one particular (real or speculative) Earth Human culture. A particular race of Aliens with Forehead Ridges may all have a culture like that of medieval Japan, or one based entirely on music, but you will very rarely find more than one culture per species. (The Vulcans and Romulans of Trek fame are a rare exception.)

Because of the similarity (or at least comprehensibility) of cultures, Earth Humans can have far more complex and intimate relations with Aliens with Forehead Ridges than with Really Aliens. We can not only communicate, Trade, and fight, but form joint business ventures, cheat each other at cards, and even fall in love.

Indeed, Aliens with Forehead Ridges raise a profound question in evolutionary biology. Convergent evolution might well produce a generally humanoid body plan, just as sharks and dolphins have a similar overall configuration. But Aliens with Forehead Ridges have much more than a general similarity to Earth Humans. They have the same secondary sex traits - as species-specific as you can get. Only their males have much facial hair, and their females often have bodacious figures. Often, indeed, they are INTERBREEDABLE species. This leads to some speculation that they may be of Earth Human descent. (Or else Earth Humans are descended from them, though this raises troublesome questions about chimpanzees.)

Perhaps because of these awkward issues, Aliens with Forehead Ridges have become much less common in written SF (save for media tie-ins) than they were some decades ago. In written SF, the KNOWN GALAXY seems increasingly to be inhabited only by Earth Humans. However, Aliens with Forehead Ridges continue to thrive in Hollywood Scifi. This is for an obvious reason: the audience wants aliens of some sort, and Aliens with Forehead Ridges are the only kind that can be played by members of the Screen Actors' Guild.

Alien Biology

What Is Life?

Life And Time

One of the first ways in which we learn to classify objects is into two groups: 1. living and 2. nonliving.

In casual encounters with the material universe, we rarely feel any difficulty here, since we usually deal with things that are clearly alive, such as a dog or a rattlesnake; or with things that are clearly nonalive, such as a brick or a typewriter.

Nevertheless, the task of defining "life" is both difficult and subtle; something that at once becomes evident if we stop to think.

Consider a caterpillar crawling over a rock. The caterpillar is alive, but the rock is not; as you guess at once, since the caterpillar is moving and the rock is not. Yet what if the caterpillar were crawling over the trunk of a tree? The trunk isn't moving, yet it is as alive as the caterpillar. Or what if a drop of water were trickling down the trunk of the tree? The water in motion would not be alive, but the motionless tree trunk would be.

It would be expecting much of anyone to guess that an oyster were alive if he came across one (for the first time) with a closed shell. Could a glance at a clump of trees in midwinter, when all are standing leafless, easily distinguish those which are alive and will bear leaves in the spring from those which are dead and will not? Is it easy to tell a live seed from a dead seed, or either from a grain of sand?

For that matter, is it always easy to tell whether a man is merely unconscious or quite dead? Modern medical advances are making it a matter of importance to decide the moment of actual death, and that is not always easy.

Nevertheless, what we call "life" is sufficiently important to warrant an attempt at a definition. We can begin by listing some of the things that living things can do, and nonliving things cannot do, and see if we end up with a satisfactory distinction for this particular twofold division of the Universe.

1. A living thing shows the capacity for independent motion against a force. A drop of water trickles downward, but only because gravity is pulling at it; it isn't moving "of its own accord." A caterpillar, however, can crawl upward against the pull of gravity.

Living things that seem to be motionless overall, nevertheless move in part. An oyster may lie attached to its rock all its adult life, but it can open and close its shell. Furthermore, it sucks water into its organs and strains out food, so that there are parts of itself that move constantly. Plants, too, can move, turning their leaves to the sun, for instance; and there are continuous movements in the substance making it up.

2. A living thing can sense and it can respond adaptively. That is, it can become aware, somehow, of some alteration in its environment, and will then produce an alteration in itself that will allow it to continue to live as comfortably as possible. To give a simple example, you may see a rock coming toward you and will quickly duck to avoid a collision of the rock with your head. Analogously, plants can sense the presence of light and water and can respond by extending roots toward the water and stems toward the light. Even very primitive life forms, too small to see with the unaided eye, can sense the presence of food or of danger; and can respond in such a way as to increase their chances of meeting the first and of avoiding the second. (The response may not be a successful one; you may not duck quickly enough to avoid the rock—but it is the attempt that counts.)

3. A living thing metabolizes. By this we mean that it can eventually convert material from its environment into its own substance. The material may not be fit for use to begin with, so it must be broken apart, moistened, or otherwise treated. It may have to be subjected to chemical change so that large and complex chemical units (molecules) are converted into smaller, simpler ones. The simple molecules are then absorbed into the living structure; some are broken down in a process that liberates energy; the rest are built up into the complex com­ ponents of the structure. Anything which is left over, or not usable, is then eliminated. The different phases of this process are sometimes given separate names: ingestion, digestion, absorption, assimilation, and excretion.

4. A living thing grows. As a result of the metabolic process, it can convert more and more of its environment into itself, becoming larger as a result.

5. A living thing reproduces. It can, by a variety of methods, produce new living things like itself.

Any object which possesses all these abilities would seem to be clearly alive; and any object which possesses none of them is clearly nonalive. Yet the situation is not at all clear-cut.

An adult human being no longer grows and many individuals never have children, but we still consider them alive even though they no longer grow and do not reproduce. Well, growth takes place at some time in life and the capacity for reproduction is potentially there.

A moth senses a flame and responds, but not adaptively; it flies into the flame and dies. Ah, but the response is ordinarily adaptive, for it is toward the light. The open flame is an exceptional condition.

A seed does not move, or seem to sense and respond—yet give it the proper conditions and it will suddenly begin to grow. The germ of life is there, even though dormant.

On the other hand, crystals in solution grow, and new crystals form. A thermostat in a house senses temperature and responds adaptively by preventing that temperature from rising too high or falling too low.

Then there is fire, which may be considered as eating its fuel, breaking it down to simpler substances, converting it into its own flaming structure, and eliminating the ash which it can't use. The flame moves constantly and, as we know, it can easily grow and reproduce itself, sometimes with catastrophic results.

Yet none of these things are alive.

We must therefore look at the properties of life more deeply, and the key lies in something stated earlier: that a drop of water can only trickle downward in response to gravity, while a caterpillar can move upward against gravity.

There are two types of changes: one which involves an increase in a property called entropy by physicists, and one which involves a decrease in that property. Changes that increase entropy take place spontaneously; that is, they will "just happen by themselves." Examples are the downhill movement of a rock, the explosion of a mixture of hydrogen and oxygen to form water, the uncoiling of a spring, the rusting of iron.

Changes that decrease entropy do not take place spontaneously. They will occur only through the influx of energy from some source. Thus, a rock can be pushed uphill; water can be separated into hydrogen and oxygen again by an electric current; a spring can be tightened by muscular action, and iron rust can be smelted back to iron, given sufficient heat. (The entropy decrease is more than balanced by the entropy increase in the energy source, but that is beside the point here.)

In general, we are usually safe in supposing that any change which is produced against a resisting force, or any change that alters something relatively simple to something relatively complex, or that alters something relatively disorderly to something relatively orderly, decreases entropy, and that none of these changes will take place spontaneously.

Yet the actions most characteristic of living things tend to involve a decrease in entropy. Living motion is very often against the pull of gravity and of other resisting forces. Metabolism, on the whole, tends to build complex molecules out of simple ones.

This is all done at the expense of energy drawn from the food or, ultimately, from sunlight, and the total entropy change in the system including food or the sun is an increase. Nevertheless, the local change, involving the living creature directly, is an entropy decrease.

Crystal growth, on the other hand, is a purely spontaneous effect, involving entropy increase. It is no more a sign of life than is the motion of water trickling down a tree trunk. Similarly, all the chemical and physical changes in a fire involve entropy increase.

We become safer, then, if we define life as the property displayed by those objects which can—either actually or potentially, either in whole or in part—move, sense, and respond, metabolize, grow, and reproduce in such a way as to decrease its entropy store.

Since one sign of decreasing entropy is increasing organization (that is, an increasing number of component parts interrelated in increasingly complex fashion), it is not surprising that living objects generally are more highly organized than their nonliving surroundings. The substance making up even the most primitive life form is far more variegated and complexly interrelated than the substance making up even the most complicated mineral.

What about life forms radically different from ours, based on altogether different kinds of chemistry, living in completely hostile (to us) environments? Could there conceivably be a silicon-based life, in place of our own carbon-based one, on a hot planet like Mercury? Could there be an ammonia-based life, in place of our own water-based one, on a cold planet like Jupiter?

We can only speculate. There is absolutely no way to tell at present.

We can wonder, though, whether human astronauts, ex­ploring a completely alien planet, would be sure of recog­nizing life if they found it. What if the structure were so dif­ferent, the properties so bizarre, that they would fail to realize they were facing something sufficiently complex and organized to be called living?

For that matter, we may be facing such a necessary broadening of the definition right here on Earth in the near future. For some time now, men have been building machines that can more and more closely imitate the action of living things. These include not merely objects that can imitate physical manipulations (as when electric eyes see us coming and open a door for us) but also objects that can imitate men's mental activities. We have computers that do more than merely compute; they translate Russian, play chess, and compose music.

Will there come a point when machines will be complex enough and flexible enough to reproduce the properties of life so extensively that it will become necessary to wonder if they are alive?

If so, we will have to bow to the facts. We will have to ignore cells and DNA and ask only: What can this thing do? And if it can play the role of life, we will have to call it living.

From Life And Time by Isaac Asimov (1979)

Our main point is that for many modern readers, a violation of the laws of thermodynamics by the author can spoil a story just as effectively as having Abraham Lincoln changing a set of spark plugs in a historical novel.

Therefore, if we travel to Mars in a story, the vehicle must operate either along physical laws we currently think we know, or at least on more or less convincing extrapolations of those laws. Furthermore, when we get there the Martians, not to mention their lapdogs, saddle horses, dinner steaks, and rheumatism, must not strike too jarring a set of notes against the background which author and reader are, it is to be hoped, visualizing together. It is permissible and even desirable to take the reader by surprise with some of these details, of course. However, his reaction to the surprise should be the urge to kick himself for failing to foresee the item, rather than resentment at the author’s ringing in a new theme.

It follows that the “hard” science fiction writer must have at least an informed layman’s grasp of biochemistry and ecology.

Even in this narrowed realm, there would seem to be two basic lines of procedure for the storyteller who needs nonhuman characters and other extraterrestrial life forms. The two are not mutually exclusive; they overlap heavily in many ways. Nevertheless they represent diiferent directions of attack on the problem, one of which is more useful if the basic story is already well set up in the author’s mind, while the other is of more use in creating and developing the story possibilities themselves.

In the first case, the qualities of the various life forms have to a considerable extent already been determined; they are demanded by the story events. Excellent recent examples occur in some of Keith Laumer’s “Retief” novels, such as the wheeled metallic natives of Quopp in Retiefs War and the even more peculiar Lumbagans in Retief's Ransom.

In other words, if the savages of Fomalhaut VII are going to kidnap the heroine by air, they must be able to fly with the weight of a human being. If the hero is going to escape from a welded-shut steel safe with the aid of his friend from Regulus IV, the friend must be able either to break or dissolve the steel, or perhaps get into and out of such spaces via the fourth dimension. These are part of the starting situation for the author, who must assume that the creations of his intellect do have the requisite powers. If he is really conscientious (or worries greatly about being laughed at by scientific purists) he will also have in the background an ecological system where these powers are of general use and which contains other creatures whose behavior and abilities fit into the same picture.

Flying must be easier on Fomalhaut VII than on Earth. Perhaps the air is denser, or the gravity weaker, or native muscle more efficient and powerful. Ordinary evolution will have been affected by the fact that flight by larger animals is possible, so there will be a much wider range of large flying organisms than we know on Earth. There will be carnivores, herbivores, and omnivores. There will be a wide range of attack and defense systems among these beings. In short, there will be more ecological niches available to large flyers, and it may be confidently expected that evolution will fill them.

Of course there will be limits, just as on Earth. Vertebrates have been flying for nearly two hundred million years, which for most of the forms involved means about the same number of generations; but we have no supersonic birds on this planet. Even the insects, which have been flying a good deal longer, haven’t gotten anywhere near Mach 1; the eight-hundred-mile-per-hour deer-bot fly which appeared in the literature during the 1930s was very definitely a mistaken observation. It would seem that our biochemistry can’t handle energy at the rates needed for supersonic flight. It is the evident existence of these limits which forces the author to assume a difierent set of conditions on the Fomalhaut planet.

Similarly, fourth-dimensional extrusion will have to be general on Regulus IV, and the local ecology will reflect the fact. There will be hide-and-seek techniques among predators and prey essentially incomprehensible to human beings, and therefore a tremendous challenge to the imagination and verbal skill of the writer.

If fourth-dimensional extrusion is not the answer chosen, then the ability to dissolve iron may have developed—which implies that free iron exists on the planet under circumstances that make the ability to dissolve it a useful one. Or…

There is, of course, a limit to the time any author can spend working out such details. Even I, a spare-time writer who seldom saddles himself with deadlines, spend some of that spare time writing the story itself. In any kind of story whatever, a certain amount of the background has to be filled in by the reader’s/listener’s imagination. It is neither possible nor desirable to do everything for him. In this first line of attack, the time and effort to be spent on detail work are reasonably limited.

Even the second line, which is my favored technique, has its limits in this respect. However, it does encourage the author to spend longer in the beginning at the straight slide-rule work. As it happens, I get most of the fun out of working out the physical and chemical nature of a planet or solar system, and then dreaming up life forms which might reasonably evolve under such conditions. The story (obviously, as some critics have been known to remark) comes afterward. My excuse for using this general technique, if one is needed, is twofold.

First, I find it more fun. This will carry smaller weight for the author who is writing for a living.

Second, it is not unusual for the nature of the planet and its life forms, once worked out, to suggest story events or even an entire plot line which would never otherwise have occurred to me. This fact should carry some weight even with the more fantasy-oriented writer, who cares less about “realism.”

I do have to admit that realism, or at least consistency, is a prime consideration with me; and as I implied some pages back with the Abraham Lincoln metaphor, even the most fantastic story can jar the most tolerant reader if the inconsistency is crude enough—anachronism is only one form of inconsistency. This sort of realism in life design has to be on at least two levels: biochemical and mechanical.


It is true that we do not yet know all the details of how even the simplest life forms work. It is still defensible to build for story purposes a creature that drinks hydrazine, and say that no one can prove this impossible. Beyond a certain point, however, I have to dismiss this as ducking out the easy way—sometimes justifiable for storytelling purposes, but jarring on the scientific sensibility. Some facts of life are very well known indeed, and to contradict them, a very good excuse and very convincing logic are needed.

For example, any life form converts energy from one form to another. On our own planet, the strongest and most active creatures use the oxygen in the atmosphere to convert food materials to carbon dioxide and water. The chemical reactions supply the needed energy. Obviously, the available oxygen would be quickly used up if there were not some other set of reactions to break down the water and carbon dioxide (actually it's the water, on this planet) to replace what is exhausted. It takes as much energy (actually more must be supplied, since no reaction is completely efiicient) to break up a molecule into its elements as is released by forming it from these elements, and any ecological system must have a long-term energy base. On this planet, as is common knowledge, the base is sunlight. There seems no need here to go into the very complicated details; few people get through high school these days (I’d like to believe) without at least a general idea of photosynthesis.

In passing, some people have the idea that fish violate this basic rule, and are some sort of perpetual motion machine, because they “breathe water.” Not so; fish use the elemental O2 gas supplied as usual by photosynthesis and dissolved in water, not the O in the H2O. Aquarium suppliers are perfectly justified in selling air pumps; they are not exploiting the innocent fish-fanciers.

Substitutes for free oxygen in energy-releasing reactions are perfectly possible chemically, and as far as anyone can tell should be possible biologically (indeed, some Earthly life forms do use other reactions). There is no chemical need for these substitutes even to be gases; but if the story calls for a nonhuman character to be drowned or strangled, obvious gaseous candidates are fluorine and chlorine. The former can run much more energetic reactions than even oxygen, while chlorine compares favorably with the gas we are all hooked on. (That last seems a justified assumption about the present readers. If it is wrong, please come and introduce yourself!)

Neither chlorine nor fluorine occurs free on this planet; but, as pointed out already, neither would oxygen if earthly life were not constantly replenishing it by photosynthesis. It has been pointed out that both these gases are odd-numbered elements and therefore in shorter universal supply than oxygen. This may well be true; but if some mad scientist were to develop a microorganism able to photosynthesize free chlorine from the chloride ion in Earth’s ocean, it wouldn’t have to do a very complete job to release as much of this gas as we now have of oxygen. Breaking down ten percent or so of the ocean salt would do the trick. Present-day biological engineering is probably not quite up to this job yet, but if you want to use the idea in a story be my guest. I don’t plan to use it myself; the crazy-scientist story is old hat now except in frankly political literature, and even the germ-from-space has been pretty well worked to death in the last forty years.

As mentioned, there is no chemical reason why the energy-producing reactants have to include gases at all. Oxidizing a pound of sugar with nitric acid will yield more energy than oxidizing the same pound with oxygen (if this seems improbable at first glance, remember the bond energy of the N2 molecule which is one of the products of the first reaction). True, raw concentrated nitric acid is rather hard on most if not all Terrestrial tissues; but we do handle hydrochloric acid—admittedly in rather dilute form in spite of the antacid-tablet ads—in our own digestive systems. I see little difiiculty in dreaming up a being able to store and utilize strong oxidizers in its system. The protective mucus our own stomachs use is only one of the possibilities.

Many chemical sources of energy are therefore possible in principle for our life forms; but one should be reasonably aware of the chemistry involved. Water or iron oxide would not be good fuels under any reasonable circumstances; there are admittedly some energy-yielding reactions involving these, but they call for special and unlikely reactants like sodium or fluorine—and if those reactants are around, we could get much more energy by using them on other substances. To get more fundamental, sunlight is not the only conceivable energy base for an ecological pyramid. It is, however, by far the most likely, assuming the planet in question has a sun. Remember, the energy source must not only be quantitatively large enough; it must be widely available in both space and time, so that life can originate and evolve to complex forms. Radioactivity and raw volcanic heat are both imaginable, but the first demands rather unusual conditions if much of it is to be on hand. Vulcanism, if Earth is a fair example, tends to be restricted in space at any one time and in time at any one location, a discouraging combination. Also, radioactive energy in its most direct form comes in high-energy quanta, furnishing an additional complication to the molecular architecture problem to be considered next.

It seems pretty certain that life, as well as needing energy, must be of complex structure. It has to do too many things for a simple machine. An organism must be able to absorb the chemicals needed for its energy, and carry out at the desired rate the reactions which they undergo. It must develop and repair its own structure (immortal, invulnerable, specially created beings are conceivable, but definitely outside the realm of this discussion). It must reproduce its own structure, and therefore keep on file a complete set of specifications—which must itself be reproducible.

Whatever mystical, symbolic, and figurate resemblances there may be between a candle flame and a living creature, the concrete differences between them seem to me to constitute a non-negotiable demand for extreme complexity in the latter.

On Earth, this complexity involves the phosphate-sugar-base polymers called popularly DNA and RNA for specifications, polypeptide and polysaccharide structures for most of the machinery, and—perhaps most fundamentally—the hydrogen bond to provide structural links which can be changed around as needed without the need for temperatures high enough to ruin the main framework.

I see no reason why other carbon compounds could not do the jobs of most of these, though I cannot offhand draw formulas for the alternates. The jobs in general depend on the shapes of the molecules, or perhaps more honestly the shapes of the force fields around them; these could presumably be duplicated closely enough by other substances.

I am rather doubtful that the cruder substitutions suggested by various writers, such as that of silicon for carbon, would actually work, though of course I cannot be sure that they wouldn’t. We have the fact that on Earth, with silicon many times more plentiful than carbon, life uses the latter. The explanations which can be advanced for this fact seem to me to be explanations as well of why silicon won’t work in life forms. (To be more specific: silicon atoms are large enough to four-coordinate with oxygen, and hence wind up in hard, crystalline, insoluble macromolecular structures—the usual run of silicate minerals. The smaller carbon atom, able to react with not more than three oxygens at once, was left free to form the water-reactive carbon dioxide gas.) True, some Earthly life such as scouring rushes, basket sponges, and foraminifera use silicon compounds in skeletal parts; but not, except in trace amounts, in active life machinery.

I also doubt that any other element could do the job of hydrogen, which I am inclined to regard as “the” essential life element, rather than the more popular carbon. Life machinery is complex, but it must have what might be called “moving parts” —structures which have to be altered in shape, or connected now one way and now another. A chemical bond weak enough to be changed without affecting the rest of the machine seems a necessity—a gasoline engine would be hard to design if springs didn’t exist and a cutting torch were needed to open the valves each cycle. The hydrogen bond (I don’t propose to explain what this is; if you don’t know, consult any beginning chemistry text) is the only thing I know of which meets this need on the molecular level.

This, however, is not much of a science fiction problem. Something like 999 out of every 1000 atoms in the universe are hydrogen atoms; even Earth, which seems to be one of the most thoroughly dehydrogenated objects in the observable part of space, has all it needs for an extensive collection of life forms. I suspect it will generally be easier for an author to use hydrogen in his homemade life forms than to work out a credible substitute.

To finish with the fundamental-structure level, one must admit that very complex electric and magnetic field structures other than those supplied ready-formed by atoms and molecules are conceivable. At this point, it really is necessary to fall back on the “we can’t say it’s impossible” excuse. Personally I would develop such life forms only if my story demanded of them some ability incompatible with ordinary matter, such as traveling through a telephone wire or existing without protection both in the solar photosphere and a cave on Pluto. At this point, simple scientific realism fades away, and I must bow out as an expert. It’s not that I’m above doing it; it’s just that practically anyone else could do it equally well.


The other principal basis for believability of life forms lies in the field of simple mechanics, much more common sense than biochemistry. For example, in spite of Edgar Rice Burroughs’s calots, a fast-running creature is far more likely to have a few long legs than a lot of short ones. Whether muscle tissue on Planet X is stronger or weaker than on Earth, muscular effort will be more efiiciently applied by fewer, longer strokes. Even if the evolutionary background for some reason started off with the ten legs (e.g., high gravity), I would expect an organism specializing in speed to develop two, or perhaps four, of them to greater length and either have the others degenerate or put them to other uses as the generations rolled on.

On the same general principle, if the creature lives on grass or the local ecological equivalent, it will probably not have much of a brain. If it doesn’t have to catch food or climb trees, it will lack any equivalent of a hand—in short, any anatomical part an organism has should either be useful to that creature in its current life, or be the degenerate remnant of something useful to its remote ancestors. Exceptions to this rule among Earthly life forms are hard to find, and may be only apparent; we simply don’t know the purpose of the organ in question. A former example was the “sail” on the backs of some Permian reptiles, now believed to be a temperature control device.

In addition to being useful itself, a structure must have been at least slightly useful through its early stages of development; it is hard to believe that a single mutation would produce a completely developed ear, but any ability to sense pressure variations would clearly be useful to an animal. Creatures must have existed showing development all the way from a slightly refined sense of touch to the present organ capable of detecting and recognizing a tiger’s footfall in a windy forest—or an out-of-tune flute in an orchestra.

Similarly with the eye. There are now alive on Earth creatures with light-sensitive organs ranging from the simple red spot of the single-celled Euglena, through pinhole cameras with complex retinas (some cephalopods), to the lens-and-iris-equipped diffraction-limited organ of most mammals and birds, complete with automatic focusing. There are also examples of parallel evolution which were good enough to help their owners survive all the way along the route: the compound mosaic-lens eyes of arthropods and, I have heard, at least one organism that scans the image of a single lens by moving a single retinal nerve over the field.

But eyes and ears are hardly original enough for a really imaginative science fiction story. What other long-range senses might an organism evolve? Could an intelligent species develop without any such sense? If so, what would be that creature’s conception of the universe? How, if at all, could sighted and hearing human beings communicate with it?

The first question at least can be partially answered without recourse to mysticism. Magnetic fields do exist, as do electric ones. Certainly some creatures can sense the latter directly (you can yourself, for that matter; bring your hand close to a highly charged object and feel what happens to the fine hairs on your skin). There is some evidence that certain species of birds can detect the earth’s magnetic field. Sound is already used in accordance with its limitations, as is scent. A gravity-sense other than the one we now use for orientation would probably not be discriminating enough, though I could certainly be wrong (read up on lunar mascons if you don’t see what I mean by lack of discrimination).

It is a little hard to envision what could be detected by a magnetic sense, and how its possessor would imagine the universe. Most substances on this planet have practically no effect on a magnetic field, and this is what makes me a little doubtful about the birds mentioned above. I can see the use of such a sense in navigation for a migratory species, but I have trouble thinking through its evolutionary development. Perhaps on a planet with widely distributed ferromagnetic material, the location of which is of life-and-death importance to the life forms, it would happen; maybe our Regulus IV character who can dissolve iron needs it for biochemical reasons.

The important point, from which we may have been wandering a trifle, is not whether I can envision such a situation in detail, but whether the author of the story can do so, and thereby avoid having to invent ad hoc a goose which lays golden eggs. If the life form in question has hearing but no sight, all right; but it should not be able to thread a needle with the aid of sonic perception. Sound waves short enough to have that kind of resolving power would demand a good deal of energy to produce, would have very poor range in air, and would incidentally be decidedly dangerous to human explorers. Of course, a story could be built on the unfortunate consequences of the rnen who were mowed down by what they thought must be a death ray, when the welcoming committee was merely trying to take a good look…

Sound does have the advantage of being able to diffract around obstacles, so that straight-line connection is not needed; light (that is, light visible to human beings) is of such short wavelength that diffraction efiects are minor. This means that the precise direction of origin of a sound ray cannot be well determined, while a good eye can measure light’s direction to a small fraction of a degree. On Earth, we both eat and keep this particular piece of cake, since we have evolved both sight and hearing.

Scent seems to have all the disadvantages and none of the advantages, as a long-range sense. However, under special circumstances even a modified nose may fill the need. In a story of my own some years ago (“Uncommon Sense,” Astounding Science Fiction, September 1945), I assumed an airless planet, so that molecules could ditfuse in nearly straight lines. The local sense organs were basically pinhole cameras, with the retinal mosaic formed of olfactory cells. Since the beings in question were not intelligent, the question of what sort of universe they believed in did not arise.

Granting the intelligence, it would have been—would still be, indeed—interesting to work out their cosmology. Naturally, the first few hours are spent wondering whether and how they could fill the intellectual gaps imposed by their lack of sight and hearing. Then, of course, the intelligent speculator starts wondering what essential details are missing from our concept of the universe, because of our lack of the sense of (you name it). This, for what my opinion is worth, is one of the best philosophical excuses for the practice of science fiction—if an excuse is needed. The molecule-seers presumably lack all astronomical data; what are we missing? This question, I hope I needn’t add, is not an excuse to go off on a mystical kick, though it is one which the mystics are quite reasonably fond of asking (and then answering with their own version of Truth). The human species has, as a matter of fact, done a rather impressive job of overcoming its sensory limitations, though I see no way of ever being sure when the job is done.

Philosophy aside, there are many more details of shape to be considered for nonhuman beings. Many of the pertinent factors have been pointed out by other writers, such as L. Sprague deCamp (“Design for Life,” Astounding Science Fiction, May-June, 1939). DeCamp reached the conclusion that an intelligent life form would have to wind up not grossly different in structure from a human being—carrying its sense organs high and close to the brain, having a limited number of limbs with a minimum number of these specialized for locomotion and the others for manipulation, having a rigid skeleton, and being somewhere between an Irish terrier and a grizzly bear in size. The lower size limits was set by the number of cells needed for a good brain, and the upper one by the bulk of body which could be handled by a brain without overspecialization. Sprague admitted both his estimates to be guesses, but I have seen no more convincing ones since. Whenever I have departed greatly from his strictures in my own stories, I have always felt the moral need to supply an excuse, at least to myself.

The need for an internal skeleton stems largely from the nature of muscle tissue, which can exert force only by contracting and is therefore much more effective with a good lever system to work with. I belittle neither the intelligence nor the strength of the octopus; but in spite of Victor Hugo and most other writers of undersea adventure, the creature’s boneless tentacles are not all that effective as handling organs. I don’t mean that the octopus and his kin are helpless hunks of meat; but if I had my choice of animals I was required to duel to the death, I would pick one of this tribe rather than one of their bonier rivals, the barracuda or the moray eel, even though neither of the latter have any prehensile organs but their jaws. (If any experienced scuba divers wish to dispute this matter of taste, go right ahead. I admit that so far, thank goodness, I am working from theory on this specific matter.)

This leads to a point which should be raised in any science fiction essay. I have made a number of quite definite statements in the preceding pages, and will make several more before finishing this chapter. Anyone with the slightest trace of intelligent critical power can find a way around most of these dicta by setting up appropriate situations. I wouldn’t dream of objecting; most of my own stories have developed from attempts to work out situations in which someone who has laid down the law within my hearing would be wrong. The Hunter in Needle was a deliberate attempt to get around Sprague’s minimum-size rule. Mission of Gravity complicated the size and speed issue by variable gravity.

And so on. If no one has the urge, imagination, and knowledge to kick specific holes in the things I say here, my favorite form of relaxation is in danger of going out with a whimper. If someone takes exception to the statement that muscles can only pull, by all means do something about it. We know a good deal about Earthly muscle chemistry these days; maybe a pushing cell could be worked out. I suspect it would need a very strong cell wall, but why not? Have fun with the idea. If you can make it plausible, you will have destroyed at a stroke many of the currently plausible engineering limitations to the shapes and power of animals. I could list examples for the rest of my available pages, but you should have more fun doing it yourself.

There is a natural temptation to make one’s artificial organisms as weird as possible in looks and behavior. Most authors seem to have learned that it is extremely hard to invent anything stranger than some of the life forms already on our planet, and many writers as a result have taken to using either these creatures as they are, or modifying them in size and habit, or mixing them together. The last, in particular, is not a new trick; the sphinx and hippogriff have been with us for some time.

With our present knowledge, though, we have to be careful about the changes and mixtures we make. Pegasus, for example, will have to remain mythological. Even if we could persuade a horse to grow wings (feathered or not), Earthly muscle tissue simply won’t fly a horse (assuming, of course, that the muscle is going along for the ride). Also, the horse would have to extract a great deal more energy than it does from its hay diet to power the flight muscles even if it could find room for them in an equine anatomy.

Actually, the realization that body engineering and life-style are closely connected is far from new. There is a story about Baron Cuvier, a naturalist of the late eighteenth and early nineteenth centuries. It seems that one night his students decided to play a practical joke, and one of them dressed up in a conglomeration of animal skins, including that of a deer. The disguised youth then crept into the baron’s bedroom and aroused him by growling, “Cuvier, wake up! I am going to eat you!”

The baron is supposed to have opened his eyes, looked over his visitor briefly, closed his eyes again and rolled over muttering, “Impossible! You have horns and hooves.” A large body of information, it would seem, tends to produce opinions in its possessor’s mind, if not always correct ones.

The trick of magnifying a normal creature to menacing size is all too common. The giant amoeba is a familar example; monster insects (or whole populations of them) even more so. It might pay an author with this particular urge to ask himself why we don’t actually have such creatures around. There is likely to be a good reason, and if he doesn’t know it perhaps he should do some research.

In the case of both amoeba and insect, the so-called “square-cube” law is the trouble. Things like strength of muscle and rate of chemical and heat exchange with the environment depend on surface or cross-section area, and change with the square of linear size; Swift's Brobdingnagians would therefore have a hundred times the strength and oxygen intake rate of poor Gulliver. Unfortunately the mass of tissue to be supported and fed goes up with the cube of linear dimension, so the giants would have had a thousand times Gulliver’s weight. It seems unlikely that they could have stood, much less walked (can you support ten times your present weight?). This is why a whale, though an air breather, suffocates if he runs ashore; he lacks the muscular strength to expand his chest cavity against its own weight. An ant magnified to six-foot length would be in even worse trouble, since she doesn’t have a mammal’s supercharger system in the first place, but merely a set of air pipes running through her system. Even if the mad scientist provided his giant ants with oxygen masks, I wouldn’t be afraid of them.

It is only because they are so small, and their weight has decreased even faster than their strength, that insects can perform the “miraculous” feats of carrying dozens of times their own weight or jumping hundreds of times their own length. This would have favored Swift’s Lilliputians, who would have been able to make some remarkable athletic records if judged on a strictly linear scale. That is, unless they had to spend too much time in eating to offset their excessive losses of body heat…

Really small creatures, strong as they may seem, either have structures that don’t seem to mind change in temperature too much (insects, small reptiles), or are extremely well insulated (small birds), or have to eat something like their own weight in food each day (shrew, hummingbird). There seems reason to believe that at least with Earthly biochemistry, the first and last of these weaknesses do not favor intelligence.

A rather similar factor operates against the idea of having a manlike creature get all his energy from sunlight, plant style. This was covered years ago by V. A. Eulach (“Those Impossible Autotrophic Men,” Astounding Science Fiction, October 1956), who pointed out that a man who tries to live like a tree is going to wind up looking much like one. He will have to increase his sunlight-intercepting area without greatly increasing his mass (in other words, grow leaves), cut down his energy demands to what leaves can supply from sunlight’s one-and-a-half-horse-power-per-square-yard (become sessile), and provide himself with mineral nutrients directly from the soil, since he can’t catch food any more (grow roots!).

Of course, we can get around some of this by hypothesizing a hotter, closer sun, with all the attendant complications of higher planet temperature. This is fun to work out, and some of us do it, but remember that a really basic change of this sort affects everything in the ecological pyramid sitting on that particular energy base—in other words, all the life on the planet.

It may look from all this as though a really careful and conscientious science fiction writer has to be a junior edition of the Almighty. Things are not really this bad. I mentioned one way out a few pages ago in admitting there is a limit to the detail really needed. The limit is set not wholly by time, but by the fact that too much detail results in a Ph.D. thesis—perhaps a fascinating one to some people, but still a thesis rather than a story. I must admit that some of us do have this failing, which has to be sharply controlled by editors.

Perhaps the most nearly happy-medium advice that can be given is this:

Work out your world and its creatures as long as it remains fun; then Write your story, making use of any of the details you have worked out which help the story. Write off the rest of the development work as something which built your own background picture—the stage setting, if you like—whose presence in your mind will tend to save you from the more jarring inconsistencies (I use this word, very carefully, rather than errors).

Remember, though, that among your readers there will be some who enjoy carrying your work farther than you did. They will find inconsistencies which you missed; depend on it. Part of human nature is the urge to let the world know how right you were, so you can expect to hear from these people either directly or through fanzine pages. Don’t let it Worry you.

Even if he is right and you are wrong, he has demonstrated unequivocally that you succeeded as a storyteller. You gave your audience a good time.


Building Blocks

Wikipedia has a nice article on Hypothetical types of biochemistry

In a science essay "Not As We Know It", Isaac Asimov notes that life on Terra is based on proteins dissolved in water solvent. He points out some other possibilities. Note that the "temperature" column has the information needed to set the borders of a solar system's circumstellar habitable zone for that particular biochemistry. Temperatures assume the planet has about 1 atmosphere worth of pressure.

Macromolecule in
at 1 Atm
Fluorosilicones in Fluorosilicones 400°? to
500°? C
Silanes (chains of silicon atoms) are too unstable. Silicones (chains alternating silicon and oxygen atoms) are more suitable for making "silicon life" protein analogues.

James Cambias notes that such life will consume carbon dioxide (and other carbon compounds) out of the air, combining it with silicon to create complex silicone compounds. Oxygen will be released but that will immediately combine with silicon to make silicon dioxide sand. The atmosphere will become depeleted in carbon dioxide. This might cool the planet off enough that fluorocarbon-sulfur life will take over the planet.
Fluorocarbons in Molten Sulfur113° to
445° C
Earth proteins are too unstable at liquid sulfur temperatures. They can be stabilized by substituting fluorine atoms for hydrogen atoms, resulting in complex fluorocarbons.

James Cambias notes that such life forms will probably evolve in an atmosphere poor in oxygen but rich in fluorine. However, such life will create atmospheres with oxygen as they release oxygen from carbon dioxide+sulfur dioxide as their metabolism creates complex fluorocarbon molecules. There actually might be enough oxygen in the atmosphere for humans to breath (but the temperature would kill them).
Proteins in Water0° to
100° C
Because water is hydrogenated oxygen, the proteins will have to have more oxygen than nitrogen in their make up. This is "life as we know it." Pretty much all life on Terra falls under this catagory.

James Cambias notes that such life will consume carbon dioxide out of the atmosphere and release oxygen, thus converting the planet's primordial atmosphere into a biologic oxygen containing atmosphere.
Proteins in Liquid Ammonia-77.7° C to
-33.4° C
Because ammonia is hydrogenated nitrogen, the proteins will have more nitrogen than oxygen in their make up. Earth proteins are too stable at liquid ammonia temperatures, ammonia life proteins will have to be more unstable than their Earth analogues.

James Cambias notes that such life forms will probably require a planet with a methane-ammonia atmosphere. As with protein-water life, it will consume carbon dioxide and produce oxygen. However, the oxygen will react with methane to produce carbon dioxide and water. The water will immediately freeze out of the atmosphere, the carbon dioxide will be consumed. Thus the atmosphere will gradually lose all its methane and become much lower in pressure.
Lipids in Liquid Methane-183.6° C to
-161.6° C
Polar liquids will not dissolve non-polar substances and vice versa (oil and water don't mix). Proteins are polar, so they won't dissolve in liquid methane. Complex protein-like polylipids will have to be used instead.

James Cambias notes that such life forms will probably require a planet with a methane-hydrogen atmosphere. As with protein-water life, it will consume carbon dioxide and produce oxygen. However, the oxygen will react with methane to produce carbon dioxide and water while the oxygen will react with hydrogen to produce more water. The water will immediately freeze out of the atmosphere, the carbon dioxide will be consumed. Thus the atmosphere will gradually lose all its methane and hydrogen thus becoming much lower in pressure.
Lipids in Liquid Hydrogen-253° C to
-240° C
Liquid hydrogen is also non-polar, so polylipids will be needed.

James Cambias notes that the temperature will be much higher in the immense pressures of a gas giant world.

In classic science fiction, the buzz-word was "Silicon-based Life". Life on Terra is based on Carbon, since carbon can join with not one, not two, not even three, but a whopping four other atoms. This allows the construction of complex molecules like proteins and DNA, a requirement for living creatures. The only other element that can do this is Silicon, so the SF writers seized it. They are also fond of harping on the fact that while most carbon-based animals on Terra exhale gaseous carbon dioxide, a poor silicon-based critter would breath out silicon dioxide, i.e.,sand. In "A Martian Odyssey" by Stanley Weinbaum is a silicon life creature that "exhales" bricks of silicon dioxide, which it uses to build a pyramid around itself.

Other chemical elements that are not impossible as the basis for alien life forms include ammonia, boron, nitrogen, and phosphorus. There are even more extreme possibilities.

There are several possibilities for the composition of alien blood.

An example of electronic life is the superconducting mentality in Sir Arthur C. Clarke's "Crusade".

One of the odder aliens is the Qax from Stephen Baxter's Timelike Infinity. Their "bodies" are organized clusters of millions of tiny whirlpools in still ponds. Another odd one was the Monolith Monsters. They were not invading aliens so much as an extraterrestrial chemical reaction. Instant monster: just add water.

Not As We Know It

      So we must strike beyond physiology and reach into chemistry, saying that all life is made up of a directing set of nucleic acid molecules which controls chemical reactions through the agency of proteins working in a watery medium.
      There is more, almost infinitely more, to the details of life, but I am trying to strip it to a basic minimum. For life-as-we-know-it, water is the indispensable background against which the drama is played out, and nucleic acids and proteins are the featured players.
      Hence any scientist, in evaluating the life possibilities on any particular world, instantly dismisses said world if it lacks water; or if it possesses water outside the liquid range, in the form of ice only or of steam only.
      (You might wonder, by the way, why I don't include oxygen as a basic essential. I don't because it isn't. To be sure, it is the substance most characteristically involved in the mechanics by which most life forms evolve energy, but it is not invariably involved. There are tissues in our body that can live temporarily in the absence of molecular oxygen, and there are microorganisms that can live indefinitely in the absence of oxygen. Life on earth almost certainly developed in an oxygen-free atmosphere, and even today there are microorganisms that can live only in the absence of oxygen. No known life form on earth, however, can live in the complete absence of water, or fails to contain both protein and nucleic acid.)
      In order to discuss life-not-as-we-know-it, let's change either the background or the feature players. Background first!

      Water is an amazing substance with a whole set of unusual properties which are ideal for life-as-we-know-it. So well fitted for life is it, in fact, that some people have seen in the nature of water a sure sign of Divine providence. This, however, is a false argument, since life has evolved to fit the watery medium in which it developed. Life fits water, rather than the reverse.
      Can we imagine life evolving to fit some other liquid, then, one perhaps not too different from water? The obvious candidate is ammonia.
      Ammonia is very like water in almost all ways. Whereas the water molecule is made up of an oxygen atom and two hydrogen atoms (H2O) for an atomic weight of 18, the ammonia molecule is made up of a nitrogen atom and three hydrogen atoms (NH3) for an atomic weight of 17. Liquid ammonia has almost as high a heat of evaporation, almost as high a versatility as a solvent, almost as high a tendency to liberate a hydrogen ion.
      In fact, chemists have studied reactions proceeding in liquid ammonia and have found them to be quite analogous to those proceeding in water, so that an "Ammonia chemistry" has been worked out in considerable detail.
      Ammonia as a background to life is therefore quite conceivable — but not on earth. The temperatures on earth are such that ammonia exists as a gas. Its boiling point at atmospheric pressure is -33.4° C. (-28° F.) and its freezing point is -77.7° C. (-108° F.).
      But other planets?
      In 1931, the spectroscope revealed that the atmosphere of Jupiter, and, to a lesser extent, of Saturn, was loaded with ammonia. The notion arose at once of Jupiter being covered by huge ammonia oceans.
      To be sure, Jupiter may have a temperature not higher than -100° C. (-148° F.), so that you might suppose the mass of ammonia upon it to exist as a solid, with atmospheric vapor in equilibrium. Too bad. If Jupiter were closer to the sun ...
      But wait! The boiling point I have given for ammonia is at atmospheric pressure — earth's atmosphere. At higher pressures, the boiling point would rise, and if Jupiter's atmosphere is dense enough and deep enough, ammonia oceans might be possible after all.
      An objection that might, however, be raised against the whole concept of an ammonia background for life, rests on the fact that living organisms are made up of unstable compounds that react quickly, subtly and variously. The proteins that are so characteristic of life-as-we-know-it must consequently be on the edge of instability. A slight rise in temperature and they break down.
      A drop in temperature, on the other hand, might make protein molecules too stable. At temperatures near the freezing point of water, many forms of non-warm-blooded life become sluggish indeed. In an ammonia environment with temperatures that are a hundred or so Centigrade degrees lower than the freezing point of water, would not chemical reactions become too slow to support life?
      The answer is twofold. In the first place, why is "slow" to be considered "too slow?" Why might there not be forms of life that live at slow motion compared to ourselves? Plants do.
      A second and less trivial answer is that the protein structure of developing life adapted itself to the temperature by which it was surrounded. Had it adapted itself over the space of a billion years to liquid ammonia temperatures, protein structures might have been evolved that would be far too unstable to exist for more than a few minutes at liquid water temperatures, but are just stable enough to exist conveniently at liquid ammonia temperatures. These new forms would be just stable enough and unstable enough at low temperatures to support fast-moving forms of life.
      Nor need we be concerned over the fact that we can't imagine what those structures might be. Suppose we were creatures who lived constantly at a temperature of a dull red heat (naturally with a chemistry fundamentally different from that we now have). Could we under those circumstances know anything about earth-type proteins? Could we refrigerate vessels to a mere 25° C., form proteins and study them? Would we ever dream of doing so, unless we first discovered life forms utilizing them?

      Anything else besides ammonia now?
      Well, the truly common elements of the universe are hydrogen, helium, carbon, nitrogen, oxygen and neon. We eliminate helium and neon because they are completely inert and take part in no reactions. In the presence of a vast preponderance of hydrogen throughout the universe, carbon, nitrogen and oxygen would exist as hydrogenated compounds. In the case of oxygen, that would be water (H2O), and in the case of nitrogen, that would be ammonia (NH3). Both of these have been considered. That leaves carbon, which, when hydrogenated, forms methane (CH4).There is methane in the atmosphere of Jupiter and Saturn, along with ammonia; and, in the still more distant planets of Uranus and Neptune, methane is predominant, as ammonia is frozen out. This is because methane is liquid over a temperature range still lower than that of ammonia. It boils at -161.6° C. (-259° F.) and freezes at -182.6° C. (-297° F.) at atmospheric pressure.
      Could we then consider methane as a possible background to life with the feature players being still more unstable forms of protein? Unfortunately, it's not that simple.
      Ammonia and water are both polar compounds; that is, the electric charges in their molecules are unsymmetrically distributed. The electric charges in the methane molecule are symmetrically distributed, on the other hand, so it is a non-polar compound.
      Now, it so happens that a polar liquid will tend to dissolve polar substances but not nonpolar substances, while a nonpolar liquid will tend to dissolve nonpolar substances but not polar ones.
      Thus water, which is polar, will dissolve salt and sugar, which are also polar, but will not dissolve fats or oils (lumped together as "lipids" by chemists), which are nonpolar. Hence the proverbial expression, "Oil and water do not mix."
      On the other hand, methane, a nonpolar compound, will dissolve lipids but will not dissolve salt or sugar. Proteins and nucleic acids are polar compounds and will not dissolve in methane. In fact, it is difficult to conceive of any structure that would jibe with our notions of what a protein or nucleic acid ought to be that would dissolve in methane.
      If we are to consider methane, then, as a background for life, we must change the feature players.

      To do so, let's take a look at protein and nucleic acid and ask ourselves what it is about them that makes them essential for life.
      Well, for one thing, they are giant molecules, capable of almost infinite variety in structure and therefore potentially possessed of the versatility required as the basis of an almost infinitely varying life.
      Is there no other form of molecule that can be as large and complex as proteins and nucleic acids and that can be nonpolar, hence soluble in methane, as well? The most common nonpolar compounds associated with life are the lipids, so we might ask if it is possible for there to exist lipids of giant molecular size.
      Such giant lipid molecules are not only possible; they actually exist. Brain tissue, in particular, contains giant lipid molecules of complex structure (and of unknown function). There are large "lipoproteins" and "proteolipids" here and there which are made up of both lipid portions and protein portions combined in a single large molecule. Man is but scratching the surface of lipid chemistry; the potentialities of the nonpolar molecule are greater than we have, until recent decades, realized.
      Remember, too, that the biochemical evolution of earth's life has centered about the polar medium of water. Had life developed in a nonpolar medium, such as that of methane, the same evolutionary forces might have endlessly proliferated lipid molecules into complex and delicately unstable forms that might then perform the functions we ordinarily associate with proteins and nucleic acids.
      Working still further down on the temperature scale, we encounter the only common substances with a liquid range at temperatures below that of liquid methane. These are hydrogen, helium, and neon. Again, eliminating helium and neon, we are left with hydrogen, the most common substance of all. (Some astronomers think that Jupiter may be four-fifths hydrogen, with the rest mostly helium — in which case good-by ammonia oceans after all.)
      Hydrogen is liquid between temperatures of -253° C. (-423° F.) and -259° C. (-434° F.), and no amount of pressure will raise its boiling point higher than -240° C. (-400° F.). This range is only twenty to thirty Centigrade degrees over absolute zero, so that hydrogen forms a conceivable background for the coldest level of life. Hydrogen is nonpolar, and again it would be some sort of lipid that would represent the featured player.

      So far the entire discussion has turned on planets colder than the earth. What about planets warmer?
      To begin with, we must recognize that there is a sharp chemical division among planets. Three types exist in the solar system and presumably in the universe as a whole.
      On cold planets, molecular movements are slow, and even hydrogen and helium (the lightest and therefore the nimblest of all substances) are slow-moving enough to be retained by a planet in the process of formation. Since hydrogen and helium together make up almost all of matter; this means that a large planet would be formed. Jupiter, Saturn, Uranus and Neptune are the examples familiar to us.
      On warmer planets, hydrogen and helium move quickly enough to escape. The more complex atoms, mere impurities in the overriding ocean of hydrogen and helium, are sufficient to form only small planets. The chief hydrogenated compound left behind is water, which is the highest-boiling compound of the methane-ammonia-water trio and which, besides, is most apt to form tight complexes with the silicates making up the solid crust of the planet.
      Worlds like Mars, earth, and Venus result. Here, ammonia and methane forms of life are impossible. Firstly, the temperatures are high enough to keep those compounds gaseous. Secondly, even if such planets went through a super-ice-age, long aeons after formation, in which temperatures dropped low enough to liquefy ammonia or methane, that would not help. There would be no ammonia or methane in quantities sufficient to support a world-girdling life form.
      Imagine, next a world still warmer than our medium trio: a world hot enough to lose even water. The familiar example is Mercury. It is a solid body of rock with little, if anything, in the way of hydrogen or hydrogen-containing compounds.
      Does this eliminate any conceivable form of life that we can pin down to existing chemical mechanisms?
      Not necessarily.
      There are nonhydrogenous liquids, with ranges of temperature higher than that of water. The most common of these, on a cosmic scale, has a liquid range from 113° C. (235° F.) to 445° C. (833° F.); this would fit nicely into the temperature of Mercury's sunside.

      But what kind of featured players could be expected against such a background?
      So far all the complex molecular structures we have considered have been ordinary organic molecules; giant molecules, that is, made up chiefly of carbon and hydrogen, with oxygen and nitrogen as major "impurities" and sulfur and phosphorus as minor ones. The carbon and hydrogen alone would make up a nonpolar molecule; the oxygen and nitrogen add the polar qualities.
      In a watery background (oxygen-hydrogen) one would expect the oxygen atoms of tissue components to outnumber the nitrogen atoms, and on earth this is actually so. Against an ammonia background, I imagine nitrogen atoms would heavily outnumber oxygen atoms. The two subspecies of proteins and nucleic acids that result might be differentiated by an O or an N in parentheses, indicating which species of atom was the more numerous.
      The lipids, featured against the methane and hydrogen backgrounds, are poor in both oxygen and nitrogen and are almost entirely carbon and hydrogen, which is why they are nonpolar.
      But in a hot world like Mercury, none of these types of compounds could exist. No organic compound of the types most familiar to us, except for the very simplest, could long survive liquid sulfur temperatures. In fact, earthly proteins could not survive a temperature of 60° C. for more than a few minutes.
      How then to stabilize organic compounds? The first thought might be to substitute some other element for hydrogen, since hydrogen would, in any case, be in extremely short supply on hot worlds.

      So let's consider hydrogen. The hydrogen atom is the smallest of all atoms and it can be squeezed into a molecular structure in places where other atoms will not fit. Any carbon chain, however intricate, can be plastered round and about with small hydrogen atoms to form "hydrocarbons." Any other atom, but one, would be too large.
      And which is the "but one?" Well, an atom with chemical properties resembling those of hydrogen (at least as far as the capacity for taking part in particular molecular combinations is concerned) and one which is almost as small as the hydrogen atom, is that of fluorine. Unfortunately, fluorine is so active that chemists have always found it hard to deal with and have naturally turned to the investigation of tamer atomic species.
      This changed during World War II. It was then necessary to work with uranium hexafluoride, for that was the only method of getting uranium into a compound that could be made gaseous without trouble. Uranium research had to continue (you know why), so fluorine had to be worked with, willy-nilly.
      As a result, a whole group of "fluorocarbons," complex molecules made up of carbon and fluorine rather than carbon and hydrogen, were developed, and the basis laid for a kind of fluoro-organic chemistry.
      To be sure, fluorocarbons are far more inert than the corresponding hydrocarbons (in fact, their peculiar value to industry lies in their inertness) and they do not seem to be in the least adaptable to the flexibility and versatility required by life forms.
      However, the fluorocarbons so far developed are analogous to polyethylene or polystyrene among the hydro-organics. If we were to judge the potentialities of hydro-organics only from polyethylene, I doubt that we would easily conceive of proteins.
      No one has yet, as far as I know, dealt with the problem of fluoroproteins or has even thought of dealing with it — but why not consider it? We can be quite certain that they would not be as active as ordinary proteins at ordinary temperatures. But on a Mercury-type planet, they would be at higher temperatures, and where hydro-organics would be destroyed altogether, fluoro-organcs might well become just active enough to support life, particularly the fluoro-organics that life forms are likely to develop.

      Such fluoro-organic-in-sulfur life depends, of course, on the assumption that on hot planets, fuorine, carbon and sulfur would be present in enough quantities to make reasonably probable the development of life forms by random reaction over the life of a solar system. Each of these elements is moderately common in the universe, so the assumption is not an altogether bad one. But, just to be on the safe side, let's consider possible alternatives.

      Suppose we abandon carbon as the major component of the giant molecules of life. Are there any other elements which have the almost unique property of carbon — that of being able to form long atomic chains and rings — so that giant molecules reflecting life's versatility can exist?
      The atoms that come nearest to carbon in this respect are boron and silicon, boron lying just to the left of carbon on the periodic table (as usually presented) and silicon just beneath it. Of the two, however, boron is a rather rare element. Its participation in random reactions to produce life would be at so slow a rate, because of its low concentration in the planetary crust, that a boron-based life formed within a mere five billion years is of vanishingly small probability.

      That leaves us with silicon, and there, at least, we are on firm ground. Mercury, or any hot planet, may be short on carbon, hydrogen and fluorine, but it must be loaded with silicon and oxygen, for these are the major components of rocks. A hot planet which begins by lacking silicon and oxygen as well, just couldn't exist because there would be nothing left in enough quantity to make up more than a scattering of nickel-iron meteorites.
      Silicon can form compounds analogous to the carbon chains. Hydrogen atoms tied to a silicon chain, rather than to a carbon chain, form the "silanes." Unfortunately, the silanes are less stable than the corresponding hydrocarbons and are even less likely to exist at high temperatures in the complex arrangements required of molecules making up living tissue.
      Yet it remains a fact that silicon does indeed form complex chains in rocks and that those chains can easily withstand temperatures up to white heat. Here, however, we are not dealing with chains composed of silicon atoms only (Si-Si-Si-Si-Si) but of chains of silicon atoms alternating with oxygen atoms (Si-O-Si-O-Si).
      It so happens that each silicon atom can latch on to four oxygen atoms, so you must imagine oxygen atoms attached to each silicon atom above and below, with these oxygen atoms being attached to other silicon atoms also, and so on. The result is a three-dimensional network, and an extremely stable one.
      But once you begin with a silicon-oxygen chain, what if the silicon atom's capacity for hooking on to two additional atoms is filled not by more oxygen atoms but by carbon atoms, with, of course, hydrogen atoms attached? Such hybrid molecules, both silicon- and carbon-based, are the "silicones." These, too, have been developed chiefly during World War II and since, and are remarkable for their great stability and inertness.
      Again, given greater complexity and high temperature, silicones might exhibit the activity and versatility necessary for life. Another possibility: Perhaps silicones may exist in which the carbon groups have fluorine atoms attached, rather than hydrogen atoms. Fluorosilicones would be the logical name for these, though, as far as I know — and I stand very ready to be corrected — none such have yet been studied.
      Might there possibly be silicone or fluorosilicone life forms in which simple forms of this class of compound (which can remain liquid up to high temperatures) might be the background of life and complex forms the principal character?

      There, then, is my list of life chemistries, spanning the temperature range from near red heat down to near absolute zero:
  1. fluorosilicone in fluorosilicone
  2. fluorocarbon in sulfur
  3. *nucleic acid/protein (O) in water
  4. nucleic acid/protein (N) in ammonia
  5. lipid in methane
  6. lipid in hydrogen
Of this half dozen, the third only is life-as-we-know-it. Lest you miss it, I've marked it with an asterisk.
      This, of course, does not exhaust the imagination, for science-fiction writers have postulated metal beings living on nuclear energy, vaporous beings living in gases, energy beings living in stars, mental beings living in space, indescribable beings living in hyperspace, and so on.
      It does, however, seem to include the most likely forms that life can take as a purely chemical phenomenon based on the common atoms of the universe.

From Not As We Know It by Isaac Asimov (1961)

To understand why dwarfs and trolls don't like each other you have to go back a long way.

They get along like chalk and cheese. Very like chalk and cheese, really. One is organic, the other isn't, and also smells a bit cheesy.

Dwarfs make a living by smashing up rocks with valuable minerals in them and the silicon-based lifeform known as trolls are, basically, rocks with valuable minerals in them. In the wild they also spend most of the daylight hours dormant, and that's not a situation a rock containing valuable minerals needs to be in when there are dwarfs around.

And dwarfs hate trolls because, after you've just found an interesting seam of valuable minerals, you don't like rocks that suddenly stand up and tear your arm off because you've just stuck a pick-axe in their ear.

From MEN AT ARMS by Terry Pratchett (1993)

Ain't Gonna Look Like Mr. Spock

But even if you handwave that away and declare that there are lots of different species of aliens, there is plenty of room for imagination. Especially in the alien's anatomy. Just here on Terra, we can find jellyfish, tarantulas, viruses, and giraffes. Face it, if these fellow Earth-creatures don't resemble us, a totally alien race from another planet ain't gonna look like Mr. Spock. Personally if I open an SF novel only to discover yet another cat-like alien I may need a nausea bag (RocketCat clears his throat then gives me his best "I'm Looking At A Hypocrite" look).

There might be creeping jellies, giant crystals, intelligent plants, mobile fungoids, energy creatures, fusion plasma beings dancing in solar coronas, liquid or gaseous life, swarming hive intelligences, superintelligent shades of the colour blue, and natural "electronic" life forms in pools of liquid helium. They might not be made of meat. They might not even be composed of matter as we know it, like the Cheela from Dr. Robert Forward's Dragon's Egg who are made of neutronium and white dwarf star matter.

And don't forget the inflatable aliens from John Brunner's The Crucible of Time. Or the bizarre one from Damon Knight's Stranger Station.

Some extraterrestrial creatures inhabit the depths of space itself. In Sir Arthur C. Clarke's Childhood's End was a creature that lived in deep space among asteroid belts. It resembled a huge eye, about twenty feet in diameter. Its survival depended upon the range and resolving power of its eye. Large creatures include the living O'Neil colonies in John Varley's Gaean trilogy and the living planet from Stanislaw Lem's Solaris. Biggest of all is the intelligent nebula from Fred Hoyle's The Black Cloud. Well, actually Olaf Stapedon's intelligent galaxies in Star Maker are bigger, but let's not get carried away.


"Within range of our sensors, there is no life [...]. At least, no life as we know it."

Spock, Star Trek: The Original Series, "The Devil in the Dark"

These are really alien aliens. They may have:

If the aliens in question have two or more of the above traits, you're usually dealing with a Starfish Alien. However they are still "people" in the sense of having:
  • Some kind of language, not necessarily verbal, we can learn to interpret (or maybe not, but we can at least recognize it as a language).
  • Culture
  • Their own belief systems, however unusual.
  • A mind-set that admits to things like logic and intuition; not necessarily those things by our definitions, but things like them.
  • At least some resemblance to living things with which we are familiar. They eat, sleep, reproduce, etc.; they are clearly organic beings, or else Mechanical Lifeforms.

Sometimes, however, they are too alien and their language, mind-set and culture remain incomprehensible to humans. Often (particularly if the beings can't communicate easily with humans) they will be presumed to be evil by the human protagonists without any actual proof. But in accordance with We Come in Peace — Shoot to Kill, starfish aliens who run across innocent, open-minded humans are themselves known to do beyond-horrible things to them, then excuse themselves later with an explanation that they were only trying to communicate with or greet us in the way they know how. Usually, their language and communication are so different from ours that if there is to be any communication between our species and theirs, it must be done by technological means of translation or them taking on a form humans can interact with.

Given the long, strange history of life on Earth (a given house includes such a bewildering variety of life as humans, houseplants, pets, spiders, molds, bacteria, etc.), it's likely if we ever actually encounter alien life it might fit in this category. Species that evolve naturally would have adapted to solve similar basic problems: obtaining food/necessities, negotiating natural disaster, adapting to new circumstances, avoiding contamination by pathogens and parasites, competing with other species, competing with themselves, and so forth. So we would expect to find at least a few familiar aspects to their psychology as opposed to sheer indecipherable mystery... if they evolved in similar conditions as us.

These are much more common in animation, video games, and literature than they are in live-action media, due to the likelihood of Special Effects Failure. They are typically located towards the "hard" end of the Sci-Fi Hardness Scale, though when their biology becomes sufficiently improbable, they may soften it instead. When a story is told from the point of view of Starfish Aliens, and other decidedly non human creatures, it's Xenofiction.

Super Trope to Octopoid Aliens. The inverse of Human Aliens or Rubber-Forehead Aliens. Aliens that don't look like humans, but still have basically the same body type are Humanoid Aliens, or Intelligent Gerbils, if they're obviously based off a particular Earth animal. Insectoid Aliens effectively split the difference.

Prone to enter Grotesque Gallery. May speak a Starfish Language. See also Bizarre Alien Biology, Starfish Robots, and Our Monsters Are Weird. Compare Eldritch Abomination (both tropes have some overlap). The Trope Namer is H.P. Lovecraft's At the Mountains of Madness, written in 1931, where the Old Ones are described as "starfish aliens."

(ed note: see TV Trope page for list of examples)


The Elder Things (also known as the Old Ones and Elder Ones) are fictional extraterrestrials in the Cthulhu Mythos. The beings first appeared in H. P. Lovecraft's novella, "At the Mountains of Madness" (published in 1936, but written in 1931), and later appeared, although not named, in the short story "The Dreams in the Witch-House" (1933). Additional references to the Elder Things appear in Lovecraft's short story "The Shadow Out of Time" (1936).


Description of a partial headless body:

Six feet end to end, three and five-tenths feet central diameter, tapering to one foot at each end. Like a barrel with five bulging ridges in place of staves. Lateral breakages, as of thinnish stalks, are at equator in middle of these ridges. In furrows between ridges are curious growths – combs or wings that fold up and spread out like fans. . . which gives almost seven-foot wing spread. Arrangement reminds one of certain monsters of primal myth, especially fabled Elder Things in the Necronomicon.
—H.P. Lovecraft, At the Mountains of Madness

In the Mythos canon, the Elder Things were the first extraterrestrial species to come to the Earth, colonizing the planet about one billion years ago. They stood roughly eight feet tall and had the appearance of a huge, oval-shaped barrel with starfish-like appendages at both ends. The top appendage was a head adorned with five eyes, five eating tubes, and a set of cilia for "seeing" without light. The bottom appendage was five-limbed and was used for walking and other forms of locomotion. The beings also had five leathery, fan-like retractable wings and five sets of branching tentacles that sprouted from their torsos. Both their tentacles and the slits housing their folded wings were spaced at regular intervals about their bodies.

Lovecraft described the Elder Things as vegetable-like or echinoderm-like in shape, having radial symmetry instead of the bilateral symmetry of bipeds. They also differed in that they had a five-lobed brain. The Elder Things exhibited vegetable as well as animal characteristics, and in terms of reproduction, multiplied using spores, although they discouraged increasing their numbers except when colonizing new regions. Though they could make use of both organic and inorganic substances, the Elder Things were carnivorous by preference. They were also amphibious.

The bodies of the Elder Things were incredibly tough, capable of withstanding the pressures of the deepest ocean. Few died except by accident or violence. The beings were also capable of hibernating for vast epochs of time. Nonetheless, unlike many other beings of the Mythos, the Elder Things were made of normal, terrestrial matter.

From the Wikipedia entry for ELDER THING

     Starfish Aliens: Most of them.

     Digisapiences, of course, have no bodies at all.
     The galari are sophont crystal-virus hybrids with inbuilt techlepathy and mechanical psychokinesis.
     The codramaju are pseudo-fungoids which can merge, exchange, and separate bodies and minds at will.
     The kaeth are vaguely draconic pseudosaurians with a metal-rich biology.
     The hydrogen-breathing sssc!haaaouú are fragile collections of membranes that dwell in the upper layers of gas giants.
     The myneni are crystal-based carbohydrosilicate amoeboids with built-in chemosynthetic talents.
     The mezuar are a network of collectively sophont purplish-blue trees. (Yes, as sessile as that implies, although the selyéva are green-blue plantimals – non-sessile photosynthetics – who probably most closely resemble walking broccoli.)
     The esseli have engineered themselves into brains with manipulating tentacles and customized personal auxiliary organs, and don’t even remember what they used to look like. (And the link!n-Rechesh are heading that way.)
     The qucequql are ammonia-metabolising octopi from a world of nitrogenous oceans.
     The múrast would be simple multiheaded snakes, except that they breathe methane, live in oceans of hydrocarbons, and their primary body structures are constructed of ice.
     The ulakha are metal-plated, fast-moving lizardoids who think Venerian conditions are just about right for a planet.
     The linobir resemble furless, leathery-skinned, hexapedal, hermaphrodite bears.
     The shan kari resemble larger versions of Terran mustelids fairly closely, actually, except they prefer to breathe warm methane.
     The mirilasté are legged-serpents with skin we would recognize as essentially plastic, who breathe the most astonishingly noxious fluorine-hydrocarbon soup.
     The ktelaki are furry arachnids with trilateral symmetry and multi-branched legs.
     The seb!nt!at are star-dwelling creatures of plasma and electromagnetic force.
     The celsesh are quadrilaterally-symmetric with a fused-barrel body plan, and sensory organs on stalks in lieu of a head.
     The embatil are worm/tentacle creatures whose life cycle begins with individuals, but which merge into single creatures as they mature – while transforming a ganglionic into a collegiate intelligence.
     The tennoa are chlorine-breathing radial-crabs blessed/cursed with obligate utilitarianism…

     And that’s all before we get to uplifts, neogens, and exotic neomorphic bioshells.

Nowhere in space will we rest our eyes upon the familiar shapes of trees and plants, or any of the animals that share our world. Whatsoever life we meet will be as strange and alien as the nightmare creatures of the ocean abyss, or of the insect empire whose horrors are normally hidden from us by their microscopic scale.

Sir Arthur C. Clarke, 1962
      This guide is meant as an aide for the prospective science fiction writer, game designer or world-builder wishing to incorporate extraterrestrial elements, in order to improve quality and rationality of the created works. It is not so much a “How To”, which would broach multiple sciences and require a profound understanding of each of these, but a “Before You Go On”, things to consider, wrinkles that need ironing out rather than a methodology. Issues that I bring up here do not necessarily make a choice impossible – you must simply figure out a way around them.

     Herein I will be dealing with sapient species, intelligent beings, if you will, since this is where artists’ and writers’ imaginations most often fall short. Here I must distinguish between sentience and sapience – sentient species are aware of their surroundings (which is to say, just about anything more complex than a jellyfish qualifies, even ants), whereas sapient species are capable of reason (humans are the only known organisms that are indisputably sapient). I will do my best to assume a purely physical, rather than cultural or ideological standpoint: alien culture and psychology I may yet examine in the future.

Before Pursuing a Hominid Design:

     Popular media may have convinced you that the only possible means to sapience is assuming a hominid form, or that it is at least the most likely form for intelligent life. Star Trek’s Klingons are a good example of this: canonically they developed from arthropods and so should resemble something vaguely like lobsters, yet they’re practically indistinguishable from humans (never mind how their lobster ancestor transformed like this). There is a reason for the prevalence of hominid aliens, but it’s not this: rather, artists do it spare themselves the effort of having to develop infrastructure capable of serving inhuman physiology – using our couches, toilets, armor and weapons would be quite awkward for Mass Effect’s elephantine Elcor, but not for the upright Turians – and even amongst those who might be willing to brace this, many think it more likely that readers will empathize with their creations if they appear recognizably human – this is particularly obvious of District 9’s Prawns and Avatar’s N’avi, both of which were heavily humanized relative to their original designs for this express purpose. Once enough of these came about, newer works simply followed the trend (likely based on such misconceptions). This latter argument has weight to it, but it’s far from absolute – anybody who owns and loves a pet can attest to that (and I daresay some people feel for their pets more than fellow human beings). If this is your reason for sticking to hominid aliens, you should not fear viewer rejection simply because your alien does not fit the established mold: if anything, because the mold is so horrifically prevalent as to become cliché, any attempt to break it becomes novel and unique. You’re more likely to grab attention and attract a reader base by daring to think outside the box, and I’m here to challenge you to do it.

     As for those who are more scientifically oriented, I can assure you that intelligence needn’t be hominid. One look at Earthly fauna confirms it: while many of the smartest species are indeed vaguely human-like primates (which makes sense, in that we evolved from such creatures), there are many that look nothing like us, neither in size nor in shape – elephants, dolphins, parrots, higher canines, corvids (crows, ravens and magpies) and finally cephalopods (squids and octopuses). I doubt that I need to justify the intelligence of the first three, but of the others, I’ll say this. City dogs have learned to use subway trains unattended and have even been observed dividing roles among the pack – one would send out either the smallest and cutest dog to beg bystanders for scraps, or the largest and meanest to scare those passing by into dropping whatever food they might be carrying. Corvids have been observed using sticks as tools and can even put human technology to their advantage: crows will drop nuts too hard to crack onto busy roads for cars to run over them, and some will even make sure that to do this at the pedestrian crossing, where they can come down during a red light to eat said nuts without fear of being run over. Amongst cephalopods, squid can distinguish between visitors and apply lessons taught by their trainers: once taught how to open cubes with differently working locks, they are capable of opening series of these with each inlaid in the other.

     That being said, sapience is not the only prerequisite for developing technology, and even if all of these achieved true sapience, some would find this easier to manage than others: primates, elephants and cephalopods. This is because they have complex manipulators – primates have hands, elephants have trunks, and cephalopods have tentacles – where the others must awkwardly use their mouth and feet. The former could more adroitly use and develop tools, building up on them to conquer their environment if they put their mind to it. Again, Earth proves that the human form (or in this case, the human means) is not the only capable one.

     Moreover, we’re unlikely to see many hominid aliens in the first place. Consider all extant life on Earth: out of all terrestrial vertebrates (that’s some 32000 species), there are only two groups of erect bipeds, humans and penguins: there are other species that can temporarily maintain an erect bipedal stance, but only the aforementioned groups rely on it as their chief form of locomotion. That’s 17 species out of some 32000 (even more if you look at all extinct ones), and even amongst those 17, only one has an adroit manipulator. You might find this a flawed statistical argument, given that we’ve only one sample space, but there’s also a found logical and biological explanation, namely that the human body plan developed due to specific evolutionary conditions. Our primate ancestors were only driven out of the trees by the rapidly drying climate of the Miocene and Pliocene, as newly evolved C4 grasses took over our formerly wooded habitat. The exact reason we became bipedal rather than quadrupedal remains disputed – some hold that it allowed more energy efficient locomotion, others that standing upright presented a greater surface area for cooling and intimidation, others yet that greater height helped us better see over tall grasses – but whatever it may be, this choice kept hands already made adroit from tree-climbing free for other tasks, and we would use them well. With plants richer than grass fewer and farther between came the impetus for better hunting, and by eating more and more energy rich meat instead of tougher plantstuff, less energy needed to be directed to digestion and more could be sent to our growing brains. Each of these was a specific condition, and taking out any one of them might have prevented human evolution, or at least set us down a very different track: the chances of a similar sequence occurring elsewhere are improbable at best. Even if such a sequence did occur, you still won’t get a hominid sapient – not unless your alien had a primate for an ancestor.

     Is this to say that you will not find any other hominids at all? That’s probably untrue – the sheer number of possibly life-bearing planets in the universe allows even this negligible chance to be realized – but our form will always be the exception and never the rule. If you can justify use of such design, preferably in a different manner than just explained, there’s nothing stopping you from implementing it. Just don’t have more than a rare few species be hominid amongst the universe’s sapients. There may be in-universe reasons to explain the contrary – Master of Orion III held that all hominids were engineered by a single precursor species to explain their multitude and similarity, even though they were not nearly as prominent as in other universes, numbering but 3 of the 16 playable races – but for the most part your readers will see this as an attempt to cover lack of creativity, rather than ingenuity.

Critical Points on Designing Your Sapient:

     You may have already deduced these points from the former section, but I will reiterate them here. There are two main things you should have in mind when designing a sapient alien:
  1. A non-sapient “animal” ancestor.
  2. An evolutionary impetus for it to develop sapience.
     The first of these can be a challenge in its own right, as the non-sapient ancestor must have had some role and adaptation to survive in a particular environment, even if this applies no longer, otherwise it would have never come into existence. This is true of humans as well: even before our brains grew to their modern size, on the plains we specialized as endurance predators, chasing prey till they dropped of exhaustion (and as marathon runners demonstrate, we’re still good at it). You must imagine where the pre-sapient resided, and how it was capable of surviving in said environment before gaining sapience. I will not outline all the possibilities therein – your imagination will surely outstrip any attempts of mine to list them – but I will provide a series of guidelines and considerations for envisioning it.

  • Body Plan: it does not serve much purpose to discuss these – the imaginative reader will certainly not be limited by vertebrate-like physiology, or even Earthly physiology. This is for the better, as Earthly biology is not inspirational in this regard – of some forty animal phyla, only two have had major success on land, which is to say there are only two distinct terrestrial body plans – but the less-experienced would do well to study this, particularly non-vertebrate (and even aquatic animal) anatomy, in order to feed their imagination and help them understand the relation of form and function. A complete understanding of your creation’s anatomy is not always necessary, but it is good for determining implications of its various systems – a trachea using life form would not be able to hold its breath, for instance – and you should at least have an idea of how it goes about eating, breathing and reproducing. That being said, some generalizations of form can be made.

    • Speed requires sleek, aerodynamic forms, with landrunners possessing long and muscular legs to cross larger distances with every stride: huge numbers of legs do not preclude speed, but managing it does require that they be specially arranged so as to avoid running into each other.

    • Larger and heavier organisms will opt for columnar legs with few joints (unless they spend most of their time on their bellies or underwater, in which case they have other means of support), while smaller and lighter ones will opt for splayed legs: this is because the former better support weight while the latter can take horizontal forces and moments as might be imposed by wind.

    • Diggers tend towards compact, cylindrical forms so as to best fit through tunnels, and often minimize or lose their limbs.

    • Aerial forms specifically adapt themselves to minimize weight, the less that needs to be carried, and typically require energy-rich diets to manage the heavy upkeep of active flight.

    • Treeclimbers require a means of maintaining grip, which generally implies suction ability or opposable digits, and those that wish to cross from tree to tree without returning to ground will also need good jumping ability or a body part of extensive length to reach across.
  • Skeleton: unless your creature is very small (in which case weight is negligible) or lives in a fluid medium (in which case buoyancy counteracts weight), this is a requirement for it to maintain its shape under the load of its own weight, and indeed against other forces that might be arrayed against it. There are fundamentally three kinds of skeleton: exoskeletons (as in arthropods), where the support structure is external and flesh is contained within, endoskeletons (as in vertebrates), where the support structure is internal and flesh is wrapped around it, and mesoskeletons (as in echidnoderms – starfish, crinoids, sea urchins and sea cucumbers), where flesh is both wrapped around the support structure and contained within it. For mechanical reasons, an exoskeleton of a certain mass will always bear the greatest bending stress and be most resistant to buckling, but the external armor carries a heavy price: the rigid armor dulls external sensation, and though it is difficult to penetrate by clawing or biting, it is extremely sensitive to impact loading and is easily shattered by powerful blows. These might be imaginatively compensated for – arthropods have sensitive hairs to feel through their carapace – but exoskeletons are hence presumed to be more viable for small organisms than large ones, as the former do not move fast or far enough to manage such damaging impacts. Functionally mesoskeletons act much like endoskeletons, albeit the former is somewhat stronger yet with more awkward organ arrangements: these do not provide such armor, but the layers of flesh atop the supports buffer them against impacts.
  • Diet: chances are your sapient is going to be predatory. Herbivorous sapients are not impossible, as elephants prove, but they’re much less likely to occur than others are for two primary reasons: firstly because plants have far lower energy density than meat and are typically harder to digest, requiring that herbivores spend much more time eating and leaving less time for mental pursuits (elephants eat 18 hours a day), and secondly because such lifestyle does not in and of itself provide the same impetus for intelligence, as it’s not required to secure a meal, whereas carnivores need some ability to outwit and catch their prey. Similar arguments all but preclude the existence of autotrophic sapients, ones that can gather energy without needing to eat at all (most likely by photosynthesis or chemosynthesis): they simply could not gather enough energy by such means to support their activity – a single human being requires as much energy as several thousand tons of grass. Omnivores stand the best chance, even better than carnivores, as they have the same impetus to develop sapience, but also have fewer limitations on food sources, and hence can more easily substitute when any run out.
     The technological sapient is under even greater limitations. It must of necessity be social: without regular interactions between individuals, there is no way to transmit information between them, or indeed from generation to generation, and hence no way to accumulate information. You could postulate a species in which the individual inherits information from its parent or acquires it from others biologically, perhaps via genetically encoded memory, but even this would soon be overwhelmed by the increasing efforts required to advance its technology. Only through delegation of effort and resources can continuous achievement be realized: arguably such delegation is the society, or at least its basis. This is not to say that all social species will develop technological capability, but the former is a requirement for the latter: similarly, what is to follow can be applied for non-technological sapients, but cannot be ignored for technological variants.
  • Communication: society and transmission both require a means of communication, preferably one which can address large groups – any complex species can manage this via physical contact, but this only works on an individual basis. Barring more exotic means, your public communication must be vision, smell or sound based, and it should go without saying that your sapient must have the required sense be well developed.

    • Auditory means are already familiar to the reader through human speech, and benefit in that they can transmit information quickly across great distances as well as being difficult to obstruct (particularly infrasound, which can go literally kilometers without much attenuation), but this does not mean that the others are not similarly viable, so long as one takes into account their shortcomings.

    • Visual displays suffer in that they only work in daytime and line of sight, which makes them easily obstructed: indeed, they can only grab another’s attention if said other is already looking in the right direction (which may not be as big a problem for sapients with panoramic or Omnidirectional vision).

    • Olfactory displays carry an inherent delay in communicating across all but the shortest distances, seeing as scent takes significant time to cross them, and may not even be able to reach in all directions depending on the wind (that being said, they could also be fanned deliberately so as to move in only one direction): moreover, smells that can make it across distances will persist and build up as the conversation moves along and others add to it, until they become indistinguishable in the increasingly convoluted mix, which may require careful fanning and designated turns to combat accumulation (though these would take long periods to communicate), or short-lived immediate use smells (though these would be useful only over a small range). Many species might also use a more limited form of olfactory communication via pheromones, by which an individual’s emotional or physical state may be communicated to others.

    Don’t ignore the possibility of multiple types of communication – amongst groups, you want to stand out amidst the crowd, but being conspicuous might not always be desirable (say, while hunting), and entirely novel means not discussed here may come up.
  • Senses: herein are imposed the least obvious constraints. The primary sense need not be vision, but a technological sapient will require at least one shape-determining sense to be well developed – that is, touch, vision or echolocation – in order to make possible the precision required for advanced structures and machines. That is not to say that it must be limited to the senses described here, or even the five human senses: species that spend much time underwater may benefit from electroreception, the ability to sense electrical impulses of fellow organisms conducted in the surrounding fluid, and migratory species may benefit from magnetoreception, the ability to sense magnetic fields and hence utilize their planet’s magnetic field as an internal compass (such species might also be able to sense active electronics, which also produce magnetic fields). However, senses that do not see use fade away, and wane almost to the point of uselessness: in particular deepwater, subterranean and cave-dwelling organisms quickly lose their sight, without light to benefit from it.

    There are also some environments where a sense may not be of much use: sound (and by extension, echolocation) requires a medium to transmit it and so is less useful in a rarefied atmosphere (though not useless, as sound can still be carried through soil and other solids), vision requires a source of illumination to be useful which may be absent or blocked in certain mediums (though this can be overcome in some cases by seeing outside the visible spectrum), smell gives a limited range of perception in an atmosphere saturated with it, touch can discern little in the open air or water (although temperature and pressure senses will still be valuable here), and air and soil don’t conduct electricity well enough for meaningful electroreception.
  • Adroit Manipulators: clearly the aspiring technological sapient will require at least one of these, or the precursor to one. Hands, trunks and tentacles have been brought up, and sufficiently prehensile tails, extensive tongues and flexible lips also qualify; certainly the imaginative reader will not be limited by this. What you should consider is the limitation of each. Wholly muscular structures like trunks, tentacles and tongues lack for rigid components and can hence change their length, stretching and constricting as needed, as well as squeeze through all but the tightest spaces, but are limited in the force they can exert: Earthly muscles can only contract, so structures with this basis can only pull, lacking the ability to exert any significant compressive force and excluding sapients reliant on them from a large number of tasks (particularly hammering, which will have a tremendous implications for their mining, building and construction), while those based on a potentially expanding muscle like the Eponan pentapod would only be able to push, lacking the ability to exert any significant tensile force and similarly excluding sapients from an entirely different set of tasks (particularly pulling ropes, which will also impact their construction). Adding rigid bone-like structures as found in hands and tails gets around the force limitations, as muscles can use these parts as levers both to push and pull, but the structure now has fixed geometry, and some loss in flexibility and range in movement is inevitable.
     For those more dedicated worldbuilders, particularly top-down ones, developing the animal pre-sapient may reflect on the ecology of the world as a whole. Firstly, in creating this creature, you are setting down possible characteristics of an order, class, and even a whole phylum. By stripping away its characteristics to a fundamental few and branching out from there, you may design the other organisms on the planet.
     The impetus is in some ways more complicated. Some scientists insist that sapience requires an evolutionary bottleneck, a constraint that only sapience can escape, and this thinking should be incorporated to some extent in your work: is sapience the only means of meeting the challenges imposed upon the creature? Big brains are costly, and if an easier option presents itself, it will be selected for preferentially. That being said, what we know of the evolution of pre-sapients on Earth suggests it’s not always this complex – cephalopods are thought to have developed their intelligence as a response to losing their shell, and with it their chief defense against predation, and it also serves as a means of hunting more diverse prey, each of which requires unique strategy to take down.

Example Design:

     All these taken together might seem overwhelming, so I’ve provided an example of my own to ease you into the process and demonstrate the contrary.

     Crucial to understanding the sapient is understanding its homeworld, the Super-Earth Meios (pictured here [link]), a terrestrial planet with much higher gravity than the Earth and a surface dominated by ocean, with only the occasional island for relief. One reason for the near landlessness is the soupy atmosphere, which quickly erodes any formations out of existence: volcanic action can outstrip atmospheric destruction for a time, but as soon as the hotspot goes silent, the air will see its works undone.

     Nevertheless, there are occasions where a number of volcanoes form in near proximity and can hence form a considerably larger landmass that can stand a little longer. It was the formation of such a “subcontinent” that allowed the evolution of chiefly terrestrial life, where before the ecology had been primarily aerial and aquatic, and it is from this picture that our pre-sapient emerged.

     It was a ballont, member of a clade of organisms that benefitted from the super-dense atmosphere to achieve lighter-than-air flight, and adaptations that formerly suited it for the air were put to good use on land: where their heavier-than-air steelwing competitors had to contend with moving their gravity-enhanced bulk, the ballonts were already able to counteract their weight via buoyancy, the same mechanism that had enabled their flight, and hence could make do without heavyset legs. In particular, it was an ironbelly ballont (as displayed here [link] ), specialized for chasing steelwings down with powerfully beating tails, using its long tentacles to reach through their exoskeleton for energy-rich flesh underneath, and well-armored on its undersides to keep safe from aquatic threats at low altitude – each of these characteristics would be adapted for its new life on land. So long as it stuck close to the ground, there was no longer any threat coming from underneath it, and so the primary danger came from the sky, causing it to flip orientation so that its shell pointed upwards and its balloons earthwards. Extensions of this would form on the wing-fins and tail, completing its protection, but not solely for this purpose: rather by being semi-rigid as opposed to wholly muscular, these limbs could now push against the ground, allowing them to act as braces against the wind and propel the ballont forward that it may chase down prey with impunity. To this end they took on a sprawling configuration, as they had no need to concern themselves with weight, only inertia (a constant unaffected by gravity). That being said, their ‘feet’ and bottom would remain fleshy, both to feel the earth underneath them as well as to allow better grip and traction.

     The success of this body plan lasted only as long as the subcontinent, and when the volcanoes providing for it puttered out one by one, it was only a matter of time before it began to recede. The terrestrial ecosystem was devastated: with their subsistence rapidly disappearing organisms had to return to the water or air or vanish with the landmass. The pre-sapient could not readily do this: while it had maintained the means of flight in its balloons, in adapting its wing-fins and tail for springing it had given up most of its muscles in favor of fewer but stronger units, and without those it could not regain the flexibility and thrust it needed in the air. As the large organisms it once fed off went away with the subcontinent it had to satisfy itself by diversifying its prey, eating everything it could get, and it is in learning how to hunt such numerous prey items without its former speed or grace that it gained sapience. The final design can be found here: [link].

     Society had already been present in certain ironbellies before they set foot on the subcontinent, when small groups would chase down and corner packs of smaller fliers, but the basis of it lay in the mother-infant connection. Because buoyancy requires significant volume, all ballonts give live birth to one or two well-developed young, that they may be born as large and as well-equipped to fly as possible. However, the ironbelly young is born without a shell, that it may better fit inside the mother, and so it is particularly imperative that she defend it: whenever possible she’ll latch her tentacles to those of her young, making sure it’s always within arms’ (tentacles’) reach, and it is from this tentacle-to-tentacle bond that their tactile personal communication is based, while vocal communication is reserved for gaining attention and addressing groups.

Other Important Misconceptions:

     Chances are your sapient does not exist on its own, but as part of a greater science fiction universe, and now you must now consider its place therein – what it thinks of and how it deals with other such races, and what said others think of it and how they deal with it in turn. Ideally this would require understanding the history, culture and psychology of all involved parties, but even ignoring these in favor of solely physical sciences I can caution against certain pitfalls inspired by popular media:
  • The Single-Biome Planet: barring extraordinary circumstances, few life-bearing planets will fall under this description, and you should not expect your sapient’s homeworld to be one. This is mainly due to two phenomena – the first is variation of temperature with latitude, with areas further away from the equator receiving less sunlight and hence less warmth, and variation of precipitation, brought about by varying temperature, wind direction and topography (with mountains creating rain shadows on their leeward sides) – and is further complicated by the twin effect of atmospheric and oceanic circulation, where fluid currents help to deliver heat across the planet’s surface. You are probably already aware of these, but I’m asking you to apply these lessons: unless your world lacks for ocean or atmosphere, in which case there is nothing to enact the changes of temperature, or these are so prevalent that circulation renders surface conditions all but uniform, your world will have multiple climates.
  • Interspecies Romance: I am not talking about platonic relationships – surely we should be able to enjoy the company of a personable sapient – but sexual ones. They will not be prevalent: for the greater part of our population, nonhominid aliens should elicit no sexual response, any more than do animals, plants or inanimate objects to the general observer, but the existence of paraphillia proves that the human form is not necessary for sexual attraction, and presumably, similar pathologies amongst other sapients will allow some to be attracted to those not of their kind. Not all species will be capable of receptivity – asexual species and some that fertilize externally would have no use for it, and many might only be aroused in designated mating seasons and at no other times – but even amongst those that can, consummating the relationship will be no simple matter. Sexual organs should not be compatible and sexual practice of each species could vary so much between the two as to exclude mutual enjoyment, with some examples possibly posing a danger to one of the partners – many Earthly species have a tendency to devour the male after copulation, and even amongst the comparatively mild mammals and reptiles, quite a few species have spiked penises (including our fellow apes), with the semen of some forming a plug to prevent unequipped males from copulating with claimed females. These might be imaginatively compensated for, but for the most part such relationships will only end in futility.
  • Interspecies Hybrids: it should go without saying that such species will never be capable of bearing progeny with any other, and none of them, not one, will be able to bear children by us. While a number of interspecies hybrids do exist on Earth, these are only between closely related species, typically within the same genus or family, and many are infertile. Alien sapients would have developed independently, likely with their own unique incompatible biochemistry, and a divergent evolutionary history will ensure that even if the former did match, their genes would not: what chances do they have? Unless the species in question share a common ancestor and are separated by only a short evolutionary period, cross-compatibility simply isn’t possible.
  • Interspecies Diet: that is the ability of one race to eat another’s foodstuffs, or indeed any organic matter not from their own world – again, this is unlikely due to divergent biochemistries. It’s not nearly as simple an issue as Mass Effect’s levi versus dextro distinction makes it cut out to be: life forms from different worlds may well be based on the same classes of compounds, yet still find other variants of these expressed by the other to be toxic or indigestible – indeed, all life on Earth is based on proteins, carbohydrates, lipids and nucleic acids, like us, but only a fraction of it is edible. Aliens will find this fraction even smaller, if it exists at all, not even having the benefit of having evolved to eat some of it, though there may be quite a few normally incompatible ‘foods’ that could be processed to yield nutrition. Suffice to say, with few exceptions sapients will not be sharing foodstuff: each will have to produce and bring along its own specific sustenance.
  • Interspecies Intelligibility: chances are remote that each species will be able to simulate all the nuances required in the others’ communication, and there’s a good chance that such nuances may even be beyond one’s perception. This is certainly subjectively true of Earthly languages, with cultures capable of distinguishing phonemes that are synonymous to others, but it’s also objectively true, as we’ve seen in our dealings with the planet’s pre-sapients. On the latter end of the spectrum, elephants and dolphins regularly vocalize with one another, but we only hear the occasional grunt or squeak, in the former case because sound frequency is too low, in the latter case because sound frequency is too high for our ears to pick up. On the former end, apes are certainly capable of perceiving human speech, and with proper training can even comprehend it, but none have yet to vocalize any human words – they simply lack the faculties for it. This may be imaginatively compensated for – a Russian elephant managed to mime human words by manipulating its lip with its trunk – but for the most part it seems sapients will not be picking up each others’ tongues, and where they do, it will be butchered beyond belief. More likely than not, the two will have to agree to a shared language, or rely on translators.

“Two large dark-coloured eyes were regarding me steadfastly. The mass that framed them, the head of the thing, it was rounded, and had, one might say, a face. There was a mouth under the eyes, the lipless brim of which quivered and panted, and dropped saliva. The whole creature heaved and pulsated convulsively. A lank tentacular appendage gripped the edge of the cylinder, another swayed in the air. ... There was something fungoid in the oily brown skin, something in the clumsy deliberation of the tedious movements unspeakably nasty.”
– H.G. Wells, The War of the Worlds (1898)

Pretty disgusting, huh? The classic tales of science fiction are full of Bug-Eyed Monsters (or BEMs as they are affectionately termed by cognoscenti) which invade planets, threaten towns. attack rocket ships, and carry off shapely human females. Hollywood producers apparently are convinced most extraterrestrial (ET) beings fall in one of four zoological categories: (1) Human or humanoid, (2) oversized animals, (3) amorphous blobs and pods, and (4) formless energy beings.

Can’t we do any better than this?

Quite! In fact. anyone with access to a good library can walk in and read all about the biology of one of the most fascinating, richly populated worlds anywhere in the Milky Way: Earth! We inhabit a queer planet with many strange settings and fabulous living creatures, altogether an excellent example of what extraterrestrial life may be all about. To a team of Interstellar Zoologists, researching sentient terrestrial mammals out here in the galactic boondocks, our world is as rare a planetary zoo as any in the Milky Way.

Xenobiologists have formulated a simple rule called the Assumption of Mediocrity, which says, in essence, that Earth should be regarded as “typically exotic.” The unusual solutions devised by evolution on this planet to cope with the problem of survival will find their parallels, though not necessarily their duplicates, among the living species of other worlds. As biologist Allen Broms once remarked, “life elsewhere is likely to consist of odd combinations of familiar bits.”

Strange Life

Life as we know it is based on cells: small, neat packages of living protoplasm containing all of the biological machinery necessary for survival. Human body cells average a few microns in size. (One micron is a millionth of a meter, about a hundredth of the thickness of the page these words are printed on.) The smallest living thing on Earth capable of independent metabolic activity is the PPLO, or “pleuropneumonia-like organism,” which measures 0.1 microns. Microbiologists estimate that the smallest cell that could, in theory, exist would measure about 0.04 microns in diameter. It is amusing to speculate that the alien analogue to a human being, constructed in the same form but using these miniature cells, would weigh a mere 50 milligrams and stand only 5 millimeters tall – hardly the thickness of a pencil. Whether creatures so small could retain a human-level intelligence is anyone’s guess.

Fairly large extraterrestrial lifeforms might well exhibit acellular physiology, or be unicellular. For example, at one stage in their life history, slime molds are tiny one-celled flagellates capable of individual multiplication by simple fission. In the later “plasmodium” stage of development, large clumps of these creatures fuse together and their cell walls dissolve away to produce an amorphous acellular mass of living protoplasm which can grown as large as 25 centimeters or more. Further, the largest known single living cell was the egg of the now-extinct half-ton elephant bird or “roc bird” (Aepyornis maximus). This egg measured about a third of a meter across and weighed 15 kilograms.

The number and kinds of organs in alien creatures may also be highly variable. For example, earthly squids have two different kinds of hearts – one for venous and a separate one for arterial blood – and the common earthworm (Pheretima) has a dozen hearts. Two extinct dinosaur species, Brontosaurus and Diplodocus, had two brains, one in the head and an even larger hunk of neural tissue in the hip region. (The volume of this “sacral enlargement” in Stegosaurus, another fossil animal of grand proportions, was perhaps twenty times larger than the brain in the cranial cavity! And the entire body of an insect is its “lung” – oxygen is carried directly to cells by an intricate network of tracheae or microtubules permeating the entire organism.

Sometimes, organs combine several functions in one – such as the human mouth. ETs need not have the same combinations as we. They may have identical or separate organs for eating, drinking, excreting, breathing, and speaking. The dolphin, for instance. eats through its mouth, breathes through its blowhole, and “speaks” through its “ears.” The land snail’s lung opens into a passageway other than its food canal, and sea cucumbers breathe through their rectums (called “anal respiration”). The cloacae of frogs and many other animals is a single organ which combines excretory and reproduction functions. Brachiopods can only vomit excrement from their “blind intestine” (a kind of alimentary cul-de-sac), and the members of phylum Nematomorpha (long worms) eat solely by direct absorption of nutrients through the skin – for they have no mouths.

How Large?

(ed note: This section can be found here)

Gravity and Life

The respected zoologist D’Arcy Wentworth Thompson once speculated about the effects of gravity on evolution. “Were the force of gravity to be doubled,” Thompson declared, “our bipedal form would be a failure, and the majority of terrestrial animals would resemble short-legged saurians, or else serpents. Birds and insects would suffer likewise, though with some compensation in the increased density of the air. On the other hand, if gravity were halved, we should get a lighter, slenderer, more active type, needing less energy, less heat, less heart, less lungs, less blood. Gravity not only controls the actions but also influences the forms of all save the least of organisms.”

It is true that the maximum weight of living species cannot exceed the crushing strength of bony material. But animals are not designed to stand still – if they were, human legs could be a few millimeters thick. Instead they must bear up under the peak pressures and accelerations encountered during normal running, jumping, and other strenuous survival activities. A horse at rest seems greatly overbuilt; on the racetrack where it may pull to a halt in a second or less, near the breaking point of its bones, the design limits are more fully exploited.

Clearly there are other factors at work besides gravitational loading in fixing maximum size – predator/prey relationships, running speeds, food requirements, oxygen levels, ecological constraints, and so forth. Still we can estimate how gravity might influence evolution, based on Earth’s biological history. The largest land creature alive today is the African elephant, weighing an impressive 6600 kilograms. Tyrannosaurus rex, one of the largest land carnivores, was at least 8000 kg. The Baluchitherium, the largest extinct land mammal, was built like a hornless rhinoceros, and carried a bulk of more than 22,000 kg. The largest land animal ever may have been Brachiosaurus, of which some specimens may have weighed 111,000 kg. but we’ll ignore this majestic brute because he probably had to spend lots of time sitting in swamps resting his tired bulk. We may conservatively guess that the heaviest exclusively land-dwelling creature plausible on a 1-gee planet is around 22,000 kg.

How massive will alien animals be? Simulations of model solar systems by Dr. Stephen H. Dole of the RAND Corporation and others suggest that terrestrial rocky worlds with atmospheres suitable for life should have surface gravities between about 0.2 and 2.0 Earth-gees. Now, if gravity doubles, bone stress won’t increase if a creature’s height is halved while other dimensions remain the same. If maximum height is inversely proportional to gravity, then maximum volume (hence mass) goes inversely as gravity cubed. By this measure the heaviest animal on a 2-gee world is about 2800 kg, while on a 0.2-gee planet (like Saturn’s moon Titan) the most massive beast could conceivably reach nearly three million kilograms – though I’d hate to try to keep it fed! So animals like walruses, small elephants, even 70 kg humanoids are quite possible even on the heaviest of all reasonable Earthlike worlds. No need for “powerfully built, squat creatures, perhaps rather like an armoured pancake on multiple legs ... limited to slow, creeping motions across the surface.”

Of course, gravity will affect design. In any given mass category high-gee animals should have shorter, stockier bones than those evolving in low-gee environments. To provide proper support, bone cross-section must increase directly with weight. Weight is the product of mass and gravity, so bone diameter must be proportional to the square root of gravity.

Let’s apply this to man. The typical human femur, the most perfectly cylindrical and largest single bone in our bodies, measures 3.5 centimeters in diameter. Using the above square-root relation, we find that the thigh-bone should increase to 4.9 cm on a two-gee world or shrink to 1.6 cm on a 0.2-gee planet for identical support of a 70 kg human body mass. Experiments have confirmed that animals reared in high gravity grow thicker bones, stronger hearts, and lose fat, but alien creatures will not appear wildly over- or underbuilt as compared with Earth life of equal mass.

Alien Skeletons

Boneless lifeforms in the sea can grow to enormous sizes. There are other advantages to life without a rigid frame we can hardly appreciate. For instance, an octopus, often called the supreme escape artist, can stretch itself incredibly thin, passing rubberlike through small holes or narrow crevasses and sliding confidently across desktops and the decks of ships.

But a creature of land is a denizen of gravity. Surface life must evolve some means of physical support or be reduced to a groveling mass on the ground. On Earth the most common frameworks are the exoskeleton and the endoskeleton. The former, typified by insects and crustaceans, is a hollow bony tube packed with the creature’s viscera. The latter, which all vertebrates have, is a central spine from which vital organs hang like coats on a hat rack. Exoskeletons are bony material surrounding gut; endoskeletons are bone surrounded by gut.

Which design is better? Bioengineers point out that a tubular column always has greater strength than a solid beam of the same mass. Tubes give twice the resistance to bending and many times the opposition to buckling. Mechanical advantages are best exploited by exoskeletons because of the greater bony surface area to which muscles may be attached.

So why be vertebrate? The answer is that we’ve considered only static strength. Large endoskeletons outperform exoskeletons under dynamic impact loading – like falling out of trees – which is why the largest of all animal species have worn their bones on the inside. Massive alien insectoids are not impossible, just less likely. Falling impacts shouldn’t be as severe on low gravity planets, and large active arthropods might survive in a rich oxygen atmosphere. The greatest carapaced creatures on Earth have ranged in size from a tenth of a meter for the South American tarantula on land up to several meters for certain fossil marine arthropods.

ETs have other choices open to them. One of the most popular alternatives among xenobiologists is called the “basket skeleton” found on this planet in marine echinoderms (sea cucumbers. starfish, sea urchins) and the cormorant (a seabird of the pelican family). Physical stress passes through the body along a kind of bony trellis, an unusual internal arrangement which one wag has facetiously termed “bowels in a birdcage.”

Another possibility is the double spine or multiple endoskeleton. On Earth flatworms and other free-living turbellarians have twin neural channels running the length of their bodies. Alien “ladder skeletons” might improve postural stability and provide greater strength on high-gravity worlds, though turning or twisting motions of the trunk might be restricted even if the multiple support posts are jointed or segmented.

A third alternative is the “hydrostatic skeleton,” surprisingly common on Earth. Animal bodies are kept stiff by pressurized fluid trapped in a sack of tough skin. Mostly only small earthworms and nematodes have this support, but massive sea creatures such as sharks compress their innards to help negotiate sharp turns and even man uses the contents of his abdomen as a hydrostatic skeleton. Large aliens might evolve a liquid skeleton inside taut, fiber-strengthened tubes with extensive reinforcing musculature – purely hydrostatic caterpillars, for example, have about 4000 individual muscles as compared to less than 700 for a human being.

How Many Eyes?

Nature often uses the same solution to a given problem encountered by many independently evolved species. Perhaps one of the most striking instances of this “convergent evolution” is the “camera eye’’ invented separately by at least five major terrestrial animal phyla (chordates, mollusks, annelids, coelenterates and protists). Each have radically different developmental histories. Naturally there are a few discrepancies – for example, light-sensitive cells in molluscan eyeballs point towards the light, the opposite of vertebrates. But the adjustable lens. retina, pigments, focusing muscles, iris diaphragm, transparent cornea and eyelids all are immediately recognizable. Nature is perhaps trying to tell us something: The camera eye is ubiquitous because it’s simply the best design for the job, on this or any other world.

The next most successful – indeed more so if you just count species – is the compound eye of insects and crustaceans. Each organ looks like a small multifaceted jewel, actually a tiny bundle of optical tubes that direct light onto a large matrix of individual photosensitive spots on the retina. The image forms a composite mosaic of thousands of little light-dots. (Dragonfly eyes have more than 28,000 facets and can discern motion up to twelve meters away.) The compound eye, however, has such poor resolving power that an insect poring over this page of print would be quite unable to make out the individual letters, so large ETs will find the system unattractive. It seems best for smaller creatures – if a flea had a spherical lens eyeball like that of humans, the pupil would be so minute that diffraction effects would utterly ruin the image.

Other visual techniques of limited importance on Earth may be emphasized on other planets. For instance, alien species may have “pinhole camera” eyes like the chambered nautilus, a beautifully simple system consisting of an open optical pit without lenses, exceptionally useful in water. In the “scanning eye” of the snail, light penetrates a simple crystalline lens and is scanned by a single retinal nerve sensor moving across the visual field, slowly building up an image of the environment. The principle of the optical reflector telescope has never been developed for direct imaging on this world, though many species use a biological mirror assembly to increase camera eye sensitivity (the tapetum of the common tabby cat) or to attract prey using deep-sea “searchlights” in conjunction with bioluminescence (the retractable reflectors of the luminous squid).

How many eyes are best? Nature usually economizes, so a single receptor organ is good enough for nondirectional sensing. Most large organisms have but one organ of smell and one of taste. On the other hand, directional senses can make good use of the benefits of stereo. Triangulation and depth perception require at least two physically separated receptors, and there seems little to be gained by going to more than a single pair. As astronomer Carl Sagan once pointed out, “Three eyes represent not nearly the same improvement over-two that two represent over one.”

Nevertheless a few animal species do have more than one pair of imaging eyes. Zoologist Norman J. Berrill of McGill University in Montreal describes the dinnertime antics of the spider, which has four pairs of eyes: “The rear pair serve to watch behind for either food or danger. The other three pairs work together but in succession. If something comes within the range of vision of one of the outermost pair, the head turns until the object is brought into the field of the two pairs of eyes in the middle, and the spider then advances. When the object is brought into focus of the forward pair, the spider jumps to attack.” The ultimate limit is probably reached by the scallop, whose literally hundreds of tiny, beautifully constructed nonimaging “eyes” are spread around the circumference of its mantle like running lights on an ocean liner.

What about eyes on stalks? Most xenobiologists regard this as a rather unlikely adaptation for thinking animals. Eyestalks require a hydraulic support system inefficient except in small animals. Eyes are vital senses for large organisms, yet stalks could be lopped off by predators with a single stroke of claw or pincer, permanently depriving the owner of sight. Periscoping eyes unprotected by bone are also more prone to common injury – in an accident, stalks could be bumped, slammed or squashed all too easily.

Alien Senses

Vision, of course, is simply the detection of one narrow set of wavelengths of light within the entire electromagnetic spectrum. One alternative to “visual” sight is infrared (IR) vision, or seeing with heat waves. The rattlesnake is quite good at this – the creature has two imaging eyeballs operating in the visible, and two conical pits on either side of the head which permit binocular IR sensing of temperature differences as little as 0.002 °C. The theory of optics predicts that alien infrared eyeballs with resolution close to that of the human eye could have apertures as small as 4 centimeters at 93,000 Angstroms (the peak wavelength of black body radiation emitted by a warm human body). This compares well with the size of the eye of the Indian elephant (4.1 cm), the horse (5 cm), the blue whale (14.5 cm), and the largest cephalopods (up to 37 cm).

Radio vision is another possibility, although there are two major evolutionary problems with this. First, it is difficult (though not impossible) to imagine planetary surface conditions in which the illumination in the radio band is equal to or greater than the brightness in the visible, thus giving radio vision the competitive edge. Second, radio sensors would have to be on the order of 10-1000 meters wide to achieve human-eye acuity, though this resolution may not be absolutely necessary.

Assuming life evolves primarily on planetary surfaces and under air, other forms of vision – very low frequency, ultraviolet, and x-ray – are unlikely because these wavelengths are strongly absorbed during the passage through atmosphere or ocean. Static electric field sensing has been documented in numerous species, notably sharks and electric fishes, and sensitivity to magnetic fields has been found in snails, pigeons; dolphins, bees. and many other animals. The acoustical, tactile, and chemical spectra of sensation have also been well exploited by life on Earth.

One possible extraterrestrial sense often overlooked is the ability to detect radioactivity. On a world with highly concentrated radionuclide ores near the surface, or on a planet in the throes of a global nuclear holocaust, biological Geiger counters would give warning to steer clear of large tracts of radiation hazards. The “radioactive sense” was once artificially bestowed on a small group of laboratory animals by wiring portable Geiger counters directly to the fear center of feline brains. When confronted with a pile of radioactive materials in one comer of their cages, each cat shied away.

The key to alien senses is survival – any environmental information that would permit an animal to better compete for the limited resources available is a valid candidate for sensing. For example, we could imagine a sophisticated meteorological sensorium evolving on a world cursed with highly volatile, perpetually inclement weather. Humidity and barometric sensors would be essential, as would anemometers to calibrate wind velocity. The ability to sense changes in atmospheric composition, such as the carbon dioxide detectors possessed by honeybees and fire ants, would be useful. Atmospheric turbidity, closely related to developing weather patterns, greatly influences the degree of skylight polarization – sensors responsive to the intensity and distribution of polarized light might permit their owner to seek shelter from the elements before disaster struck. The seeming ability of many animals to sense an earthquake or tornado before it arrives may relate to their perception of very low frequency infrasonics or minute electrical field variations immediately preceding the event. And the allegation that elephants can sense water located a meter or so beneath the surface of apparently dry riverbeds is unproven scientifically, yet the fact remains that such biological dowsers would be tar more likely to survive on a drought-stricken planet.

How Many Legs?

On strictly mechanical grounds, three points are needed geometrically to define a surface plane – two points make only a line. ETs trying to stand up on just one or two levers will promptly fall flat on their faces. We bipedal humans manage to remain erect only because our large feet provide additional points of contact with the ground, but without toes or feet a minimum of three legs is necessary.

Are tripedal aliens possible? Traditional biologists say no. A walking three-legger must lift at least one limb off the ground, at which instant it loses its planar support base, a situation statically unstable and dynamically precarious. Four legs seem better from an engineering point of view, as the creature can remain balanced when a leg is in motion. Ancestral fishes only have fins in pairs, so mustn’t all limbs evolve in pairs as well?

Xenobiologists remain unconvinced. Most running bipeds and quadrupeds keep two or fewer limbs on the ground during locomotion, so three-point dynamic stability is probably unnecessary. Land life need not always evolve from pair-finned fishes – descendants of, say, a starfish might be odd-leggers. Most persuasive, however, is the simple fact that tripeds exist on Earth! The extinct Tyrannosaurus rex and a few large contemporary creatures such as the kangaroo run bipedally but stand tripedally. The tails of these animals are as strong and thick as the forelegs and are regularly used for postural support. Indeed, when kangaroos fight, they rear up on their tails, freeing both legs to deliver crushing kicks to opponents.

More legs than four are plausible even for massive, intelligent animals. Odd appendages are often used for highly specialized purposes, as witness the prehensile tail of monkeys and the dexterous trunk of elephants. The key to higher multipedia is neural control. The nervous circuitry for an extra limb is far less than that required to add, say, another eye. Muscles need thousands of new neurons, but eyeballs need millions. About one-third of the mammalian brain is committed to sensory functions, whereas only a small slice handles motor control, ETs are much more likely to have extra arms than extra eyes or ears.

Dr. Bonnie Dalzell, a writer-paleontologist who helped Larry Niven work out some of his fictional aliens, insists that vertebrates on Earth have four limbs solely because of the common descent from fishes adapted to free-swimming conditions in large open oceans. These fish needed only two independent sets of diving planes to make a go of it in the sea. Perhaps if we evolved instead from Euthacanthus, a Devonian Period fish boasting no fewer than seven pairs of fins, we might be hexapodal or more-podal today ourselves.

Dr. Dalzell expects to find intelligent six-leggers on worlds with small, shallow oceans. There, bottom-dwelling fishes would become the predominant coastal and freshwater lifeforms early in evolutionary history. If the planet has a very seasonal climate, perhaps accompanied by large-scale periodic evaporations of lakes and seas, few fish species could evolve into good swimmers as on Earth. Marine creatures with many pairs of fins would have the advantage, ultimately inheriting the land and producing a rich ecology of multipodal animal life.

There are many advantages to six-legged living. On high-gravity worlds hexapedia is a good way to distribute mechanical stresses and help reduce the danger of bone breakage. Injury or loss of a limb is more catastrophic for four-leggers than for six-leggers (who have “spares”). Hexapods also have better balance since, unlike quadrupeds, they can keep a stable support tripod on the ground even when running at high speeds. And it shouldn’t be too hard to coordinate all those legs. Says Dalzell: “Earthly insects with three pairs of legs are hardly noted for their well-developed mental powers, but most of them walk just fine.”

Of course, legs are not the only game in town. The potential of rotary motion (to pick one possibility of many) cries out for fulfillment. A few years ago biologists made the amazing discovery that the tails of tiny bacteria are driven by minute ionic motors complete with rotors, stators, bushings and freely-rotating drive shafts spinning up to 60 cycles per second. The rapid back-and-forth wiggling of flagella we see under the microscope is actually a complicated helical twisting movement more akin to a propeller screw than to a simple fishy undulation. This finding contradicts the long-standing dictum that living organisms may not contain detached, self-rotating parts.

Rotary motion may be possible for large animals too. Picture a small Earthlike world with little tectonic activity and broad, flat continental shelves flooded to a depth of five or ten meters during global warm spells. A creature not unlike the molluscan cuttlefish Sepia hovers near the bottom, stalking small fish, shrimps, and crabs, sometimes jetting about by expelling water rapidly from several exit portals like many other cephalopods. Occasionally sand particles jam in a portal, causing irritation. The animal responds by encasing them in a perfectly smooth spherical pearl, much like those of the modem oyster.

Millions of years later an Ice Age arrives. The retreating shoreline leaves behind vast tracts of smooth hard continental shelf. Forced into ever more turbid, colder, shallower waters, we might imagine our cuttlefish eventually abandoning the sea for land, evolving into a “caster creature.” Its jet ports now permanently plugged by large pearly structures almost from birth, these animals might develop the ability to roll along the graded continental raceways. Speed is controlled by internal sphincters aided by heat sensors for guided braking on gentle downhill stretches and a “low-gear” muscular assist for steep climbs. Tentacle arms like ski poles provide additional stability on fast runs along the coastline.

Air Power

How big could flying ETs evolve? On Earth the albatross is pretty close to the maximum. This 10 kilogram bird reaches wingspans up to four meters and needs a lengthy runway to achieve takeoff speed of 20 kph. This minimum velocity is called the “stall speed” and is partly determined by air density. Venusian pigeons could remain airborne at speeds ten times slower than their Earthly cousins, whereas Martian birds of similar size and shape would have to fly ten times faster to stay aloft.

The main factor fixing avian size is atmospheric pressure, not gravity as some erroneously believe. On high-pressure worlds, alien bird creatures can have surprisingly small wings and large masses. An extraterrestrial with the mass of a man could fly with the wings of an albatross in air just five times thicker than Earth’s, and a Venusian albatross could make do with stubby wings smaller than the page on which these words are printed.

Planetary surface gravity has less effect on size in part because it varies far less than air density from world to world. For the same ease of flight a pigeon on a 2-gee planet with Earthlike air must increase total wing area by only 75 percent; on a bantam-weight 0.2-gee world, wing surface may decrease 75 percent. Gravity also influences stall speed. An albatross on a 2-gee planet needs a 40-percent runway extension; on a 0.2-gee world it can get by with 55 percent less. Massive extraterrestrial avians are more likely on puny planets with dense atmospheres.

How many wings are best? Most common among terrestrial species is a single pair which generate lift by actively beating the air something like the blades of a helicopter rotor. Less common is the “airplane” system, with one pair producing passive lift (like the wings of an airplane) and a second pair taking the more active role (like propellers). Adding yet more wings would serve no useful purpose, hence are unlikely to evolve. Only a very few insect species on Earth retain vestigial traces of an ancestral third wing pair, and these are degenerate and useless for flight.

Alien air travelers may have no wings at all! There are many alternatives that have never been fully exploited by evolution on this planet. Consider, for example, the principles of the rocket, the glider, and the balloon.

A high-gravity world with abundant seas and a warm, thick oxygen atmosphere might produce a “rocket fish” predator, patrolling the coastal shallows and preying on bird-sized torpid insect life thickly swarming high up. Much like the toy plastic projectiles that shoot the length of a playing field when fully charged with water and compressed air, the rocket fish bolts from the sea skyward and mouthes its dinner on the fly. Such an animal must have a sturdy posterior pressure canister that can be discharged rapidly through a rigid bony nozzle, rechargeable in minutes using powerful sphincter muscles, internal gas generation, or osmosis. Earthly precedents include the jet propulsion of squids and octopuses, the pressurized chemical sprays of warrior termites, and the boiling liquid jet of the bombardier beetle.

A lightweight planet with high winds might be ideal for the evolution of sentient “parachute beasts,” large aerial aliens able to navigate the airways of their world by manipulating sturdy chutes or simple gliding surfaces. Vultures can sail for hours with little effort using strong mountain updrafts to gain altitude, but other worlds may be even better suited for this mode of flight. Further terrestrial precedent includes the aerial dispersal of spider young – spiderlings crawl to the tip of a blade of grass, raise their tiny abdomens and let fly a thin silken thread, then hop aboard as a gust of wind catches the gossamer strands and whisks them away into the sky.

The idea of balloonlike living organisms is an old one both in science and science fiction. Bonnie Dalzell designed an “airship beast” for the Pick-a-Planet exhibit at the Smithsonian’s National Air and Space Museum. These creatures supposedly inhabit a world with cold winters, heavy gravity and a thick atmosphere. Twice a year the herbivorous hundred-kilogram blimps inflate their many lifting bags with metabolically generated hydrogen gas and drift to the opposite hemisphere to avoid the seasonal chill. Strong winds are an advantage, but predators are numerous and many noble aeronauts are lost during the migrations when a chance bolt of lightning strikes and ignites their flammable bodies. On Earth the Portuguese man-of-war, the chambered nautilus, and swim bladders in fishes provide precedent for a balloon lifestyle in a fluid medium.

Sail power has also been largely neglected in biology for animal locomotion. One of the few examples on this planet is Velella, a small, baggy, disk-shaped sea creature whose sail-like dorsal fin permits it to drift slowly with the wind. Another example is, surprisingly, the whale. These majestic cetaceans sometimes “stand on their heads” exposing only their giant broadleaf tails above water, catching gusts of wind and playfully “sailing” for hundreds of meters before coming up for air.

More than forty years ago Olaf Stapledon speculated on the possibility of a true biological sailboat. Let us imagine a cephalopod with a heavy concave shell living in the bays and estuaries of some alien world. Over the years the species gradually acquires the ability to float boatlike on the inverted shell as an aid in migration. These creatures drift with the shore currents, feeding on surface algae and nibbling the tops of seaweed stalks. In time the shell could become better adapted for navigation, perhaps with a streamlined undercarriage, allowing the ET to better chart its course between known patches of food and to escape its predators. Eventually it gains still more speed with a crude sail, a thin membrane growing from a shank of cartilage in the animal’s belly. With further evolution the membrane becomes retractable, even delicately manipulatable by fine muscles. At last the emergence of a brain and sensory organs strictly comparable to those of higher mollusks on Earth makes possible a kind of living clipper ship complete with masthead (forward sensors), jib, mainsail, riggings (extensible tendon), and a rudder.

Every habitable planet has millions of living species and billions of extinct ones, and there are many trillions of useful planets in the universe. This adds up to an incredible diversity of life. Christian Huygens wrote in The Celestial Worlds Discover’d (1698) that “Nature seems to court variety in her Works, and may have made them widely different from ours either in their matter or manner of Growth, in their outward Shape, or in their inward Contexture; she may have made them such as neither our Understanding nor Imagination can conceive.” Whether Huygens’s prophecy is true is something we can determine only by traveling to faraway worlds and sampling extraterrestrial ecologies at close hand. Perhaps, someday soon, we will make this epic journey.


Norman J. Berrill, Worlds Without End: A Reflection on Planets, Life and Time, Macmillan, New York, 1964.

R. McNeill Alexander, G. Goldspink, eds., Mechanics and Energetics of Animal Locomotion, John Wiley & Sons, New York, 1977.

Bonnie Dalzell, “Exotic Bestiary for Vicarious Space Voyagers,” Smithsonian 5 (October 1974):84-91.

Doris Jonas, David Jonas, Other Senses, Other Worlds, Stein and Day, New York, 1976.

Robert A. Freitas Jr., “Xenobiology,” Analog 101(30 March 1981):30-41.

Olaf Stapledon, Star Maker, Methuen, 1937. Reprinted: Penguin Books, Baltimore, Maryland, 1972.

From EXTRATERRESTRIAL ZOOLOGY by Robert A. Freitas Jr. (1981)


Before space flight it was often predicted that other planets would appeal strictly to the intellect. Even on Earthlike worlds, the course of biochemical evolution must be so different from the Terrestrial—since chance would determine which of many possible pathways was taken—that men could not live without special equipment. And as for intelligent beings, were we not arrogant to imagine that they would be so akin to us psychologically and culturally that we would find any common ground with them? The findings of the earliest extra-Solar expeditions seemed to confirm science in this abnegation of anthropomorphism.

Today the popular impression has swung to the opposite pole. We realize the galaxy is full of planets which, however exotic in detail, are as hospitable to us as ever Earth was. And we have all met beings who, no matter how unhuman their appearance, talk and act like one of our stereotypes. The Warrior, the Philosopher, the Merchant, the Old Space Ranger, we know in a hundred variant fleshly garments. We do business, quarrel, explore, and seek amusement with them as we might with any of our own breed. So is there not something fundamental in the pattern of Terrestrial biology and in Technic civilization itself?

No. As usual, the truth lies somewhere between the extremes. The vast majority of planets are in fact lethal environments for man. But on this account we normally pass them by, and so they do not obtrude very much on our awareness. Of those which possess free oxygen and liquid water, more than half are useless, or deadly, to us, for one reason or another. Yet evolution is not a random process. Natural selection, operating within the constraints of physical law, gives it a certain direction. Furthermore, so huge is the galaxy that the random variations which do occur closely duplicate each other on millions of worlds. Thus we have no lack of New Earths.

Likewise with the psychology of intelligent species. Most sophonts indeed possess basic instincts which diverge more or less from man's. With those of radically alien motivations we have little contact. Those we encounter on a regular basis are necessarily those whose bent is akin to ours; and again, given billions of planets, this bent is sure to be found among millions of races.

Of course, we should not be misled by superficial resemblances. The nonhuman remains nonhuman. He can only show us those facets of himself which we can understand. Thus he often seems to be a two-dimensional, even comic personality. But remember, we have the corresponding effect on him. It is just as well that the average human does not know on how many planets he is the standard subject of the bawdy joke.

Even so, most races have at least as much contrast between individuals—not to mention cultures—as Homo Sapiens does. Hence there is a degree of overlap. Often a man gets along better with some nonhuman being than he does with many of his fellowmen. "Sure," said a prospector on Quetzalcoatl, speaking of his partner, "he looks like a cross between a cabbage and a derrick. Sure, he belches H2S and sleeps in a mud wallow, and his idea of fun is to spend six straight hours discussin' the whichness of the wherefore. But I can trust him—hell, I'd even leave him alone with my wife!"

—Noah Arkwright
An Introduction to Sophontology

From INTRODUCTION: A SUN INVISIBLE by Poul Anderson (1966)

     "As I remember," he said, collecting his thoughts rapidly, "the biologists asked themselves the question, 'If we had no preconceived ideas, and were starting with a blank sheet of paper-how would we design an intelligent organism?"'
     "I'm not much of an artist," Floyd apologized, after he had managed to borrow paper and pencil, "but the general conclusion was something like this."
     He sketched quickly, and when he had finished Mr. Kelly said, "Ugh!"

     "Well," chuckled Floyd, "beauty lies in the eye of the beholder. And talking of eyes, there would be four of them, to provide all-round vision. They have to be at the highest part of the body, for good visibility—so."
     He had drawn an egg-shaped torso surmounted by a small, conical head that was fused into it with no trace of a neck. Roughly sketched arms and legs were affixed at the usual places.
     "Getting rid of the neck removes a fundamental weakness, we only need it because our eyes have a limited field of view, and we have to turn our heads to compensate."
     "Why not a fifth eye on top, for upward vision?" asked Kaminski, in a tone of voice which showed what he thought of the whole concept.
     "Too vulnerable to falling objects. As it is, the four eyes would be recessed, and the head would probably be covered with a hard protective layer. For the brain would be somewhere in this general region—you want the shortest possible nerve connections to the eyes, because they are the most important sense organs."
     "Can you be sure of that?"
     "No—but it seems probable. Light is the fastest, longest-ranging carrier of information. Any sentient creature would surely take advantage of it. On our planet, eyes have evolved quite independently, over and over again, in completely separate species, and the end results have been almost identical."
     "I agree," said Whitehead. "Look at the eye of an octopus—it's uncannily human. Yet we aren't even remote cousins."

     "But where's the thing's nose and mouth?" asked Mrs. Kelly.
     "Ah," said Floyd mischievously, "that was one of the most interesting conclusions of the study. It pointed out the utter absurdity of our present arrangements. Fancy combining gullet and windpipe in one tube and then running that through the narrow flexible column of the neck! It's a marvel we don't all choke to death every time we eat or drink, since food and air go down the same way."
     Mrs. Kelly, who had been sipping at a highball, rather hastily put it down on the buffet table behind her.
     "The oxygen and food intakes should be quite independent, and in the logical places. Here."
     Floyd sketched in what appeared to be, from their position, two oversized nipples.
     "The nostrils," he explained. "Where you want them—beside the lungs. There would be at least two, well apart for safety."
     "And the mouth?"
     "Obviously—at the front door of the stomach. Here."
     The ellipse that Floyd sketched was too big to be a navel, though it was in the right place, and he quickly destroyed any lingering resemblance by insetting it with teeth.
     "As a matter of fact," he added, "I doubt if a really advanced creature would have teeth. We're rapidly losing ours, and it's much too primitive to waste energy grinding and tearing tissues when we have machines that will do the job more efficiently."
     At this point, the Vice-President unobtrusively abandoned the canape he had been nibbling with relish.
     "No," continued Floyd remorselessly. "Their food intake would probably be entirely liquid, and their whole digestive apparatus far more efficient and compact than our primitive plumbing."

     "I'm much too terrified to ask," said Vice-President Kelly, "how they would reproduce. But I'm relieved to see that you've given them two arms and legs, just like us."
     "Well, from an engineering viewpoint it is quite hard to make a major improvement here. Too many limbs get in each other's way; tentacles aren't much good for precision work, though they might be a useful extra. Even five fingers seems about the optimum number; I suspect that hands will look very much the same throughout the universe even if nothing else does."

     "And I suspect," said Kaminski, "that the people who designed our friend here failed to think far enough ahead. What's the purpose of food and oxygen? Why, merely combustion, to produce energy-at a miserable few percent efficiency. This is what our really advanced extraterrestrial will look like. May I?"
     He took the pen and pad from Floyd, and rapidly shaded the egg-shaped body until the air and food intakes were no longer visible. Then, at waist level, he sketched in an electric power point-and ran a long cable to a socket a few feet away.
     There was general laughter, in which Kaminski did not join, though his eyes twinkled.
     "The cyborg-the electromechanical organism. And even he-it is only a stepping stone to the next stage-the purely electronic intelligence, with no flesh and-blood body at all. The robot, if you like-though I prefer to call it the autonomous computer."

From THE LOST WORLDS OF 2001 by Sir Arthur C. Clarke (1972)

Conditions are so difierent on Mars and—to our earth-centered feelings—so inferior from those on earth that scientists are confident no intelligent life exists there. If life on Mars exists at all (the probability of which is small, but not zero) it probably resembles only the simplest and most primitive terrestrial plant life.

Still, even granted that the likelihood of complex life is virtually nonexistent; we can still play games and let our fancy roam. Let us suppose that we are told flatly: “There is intelligent life on Mars, roughly man-shaped in form.” What reasonable picture can we draw on the basis of what we now know of Mars—bearing always in mind that the conclusions We reach are not to be taken seriously, but only as an exercise in fantasy?

In the first place, Mars is a small world with a gravitational force only two-fifths that of earth. If the Martian is a boned creature, those bones can be considerably slenderer than ours and still support a similar mass of material (an inevitable mechanical consequence of decreased weight). Therefore, even if the torso itself were of human bulk, the legs and arms of the Martian would seem grotesquely thin to us.

Objects fall more slowly in a weak gravitational field and thus the Martians could afford to have slower reflexes. Therefore, they would seem rather slow and sleepy to us (and they might be longer-lived because of their less intense fight with gravitation). Since things are less top-heavy in a low-gravity world, the Martian would probably be taller than earth people. The Martian backbone need not be so rigid as ours and might have two or three elbowlike joints, making stooping from his (possible) eight-foot height more convenient.

The Martian surface has been revealed by the Mars-probe, Mariner IV, to be heavily pockmarked with craters, but the irregularities they introduce are probably not marked to a creature on the surface. Between and Within the craters, much of the surface is probably sandy desert. Yellow clouds obscuring the surface are occasionally detected and, in the 1920s, the astronomer E. M. Antoniadi interpreted these as dust storms. To travel over shifting sands, the Martian foot (like that of the earthly camel) would have to be flat and broad. That type of foot, plus the weak gravity, would keep him from sinking into the sand.

As a guess, the feet might be essentially triangular, with three toes set at 120° separation, with webbing between. (No earthly species has any such arrangement, but it is not an impossible one. Extinct flying reptiles, suchlas the pterodactyl, possessed Wings formed out of webbing extending from a single line of bones.) The hands would have the same tripod development, each consisting of three long fingers, equally spaced. If the slender finger bones were numerous, the Martian finger would be the equivalent of a short tentacle. Each might end in a blunt swelling (like that of the earthly lizard called the gecko), where a rich network of nerve endings, as in human fingertips, would make it an excellent organ for touching.

The Martian day and night are about as long as our own, but Mars is half again as far from the sun as we are, and it lacks oceans and a thick atmosphere to serve as heat reservoirs. The Martian surface temperature therefore varies from an occasional 90° Fahrenheit, at the equatorial noon, down to a couple of hundred degrees below zero, by the end of the frigid night. The Martian would require an insulating coating. Such insulation might be possible with a double skin; the outer one, tough. horny, and water impervious, like that of an earthly reptile; the inner one, soft, pliable, and richly set with blood vessels, like that of an earthly man. Between the two skins would be an air space which the Martian could inflate or deflate.

At night the air space would be full and the Martian would appear balloonlike: The trapped air would serve as an insulator, protecting the warmth of the body proper. In the warm daytime, the Martian would deflate, making it easier for his body to lose heat. During deflation, the outer skin would come together in neat, vertical accordian pleats. The Martian atmosphere, according to Mariner IV data, is extremely thin, perhaps a hundredth the density of our own and consisting almost entirely of carbon dioxide. Thus, the Martian will not breathe and will not have a nose, though he will have a strongly muscled slit—in his neck, perhaps— through which he can pump up or deflate the air space. What oxygen he requires for building his tissue structure must be obtained from the food he eats. It will take energy to obtain that oxygen, and the energy supply for this and other purposes may come directly from the sun. We can picture each Martian equipped with a capelike extension of tissue attached, perhaps to the backbone. Ordinarily, this would be folded close to the body and so would be inconspicuous.

During the day, however, the Martian may spend some hours in sunlight (clouds are infrequent in the thin, dry Martian air) with his cape fully expanded, and resembling a pair of thin, membranous wings reaching several feet to either side. Its rich supply of blood vessels will be exposed to the ultraviolet rays of the sun, and these will be absorbed through the thin, translucent skin. The energy so gained can then be used during the night to enable the necessary chemical reactions to proceed in his body.

Although the sun is at a great distance from Mars, the Martian atmosphere is too thin to absorb much of its ultraviolet, so that the Martian will receive more of these rays than we do. His eyes will be adapted to this, and his chief pair, centered in his face, will be small and slitlike to prevent too much radiation from entering. We can guess at two eyes in front, as in the human being, since two are necessary for stereoscopic vision—a very handy thing to have for estimating distance.

It is very likely that the Martian will also be adapted to underground existence, for conditions are much more equable underground. One might expect therefore that the Martian would also have two large eyes set on either side of his head, for seeing by feeble illumination. Their function would be chiefly to detect light, not to estimate distance, so they can be set at opposite sides of the head, like those of an earthly dolphin (also an intelligent creature) and stereoscopic vision in feeble light can be sacrificed. These eyes might even be sensitive to the infrared so that Martians can see each other by the heat they radiate. These dim-vision eyes would be enormous enough to make the Martian face wider than it is long. In daytime, of course, they would be tightly closed behind tough-skinned lids and would appear as rounded bulges.

The thin atmosphere carries sound poorly, and if the Martian is to take advantage of the sense of hearing, he will have to have large, flaring, trumpetlike ears, rather like those of a jackrabbit, but capable of independent motion, of flaring open and furling shut (during sandstorms, for instance).

Exposed portions of the body, such as the arms, legs, ears, and even portions of the face which are not protected by the outer skin and the airtrap within, could be feathered for warmth in the night.

The food of the Martian would consist chiefly of simple plant life, which would be tough and hardy and which might incorporate silicon compounds in its structure so that it would be gritty indeed. The earthly horse has teeth with elaborate grinding surfaces to handle coarse, gritty grass, but the Martian would have to carry this to a further extreme. The Martian mouth, therefore, might contain siliceous plates behind a rounded opening which could expand and contract like a diaphragm of a camera. Those plates would work almost like a ball mill, grinding up the tough plants.

Water is the great need. The entire eater supply on Mars is equal only to that contained in Lake Erie, according to an estimate cited by astronomer Robert S. Richardson. Consequently, the Martian would hoard the water he consumes, never eliminating it as perspiration or wastes, for instance. Wastes would appear in absolutely dry form and would be delivered perhaps in the consistency, even something of the chemical makeup, of earthly bricks.

The Martian blood would not be used to carry oxygen, and would contain no oxygen-absorbing compound, a type of substance which in earthly creatures is almost invariably strongly colored. Martian blood, therefore, would be colorless. Thus the Martian skin, adapted to ultraviolet and absorbing it as an energy source, would not have to contain pigment to ward it off. The Martian therefore would be creamy in color.

The extensible light-absorbing cape, particularly designed for ultraviolet absorption, might reflect longwave visible light as useless. This reflected light could be yellowish in color. This would cause our Martian to seem to be (when he was busily absorbing energy from solar radiation) a dazzling white creature with golden wings and occasional feathers. So ends our speculation—in a vision of Martian forms not so far removed from the earthman’s fantasies of the look of angels.

From ANATOMY OF A MAN FROM MARS by Isaac Asimov (1965)

      For at least the last three decades, a large number of science fiction writers have been confronted, at one time or another, with the problem of constructing extraterrestrial lifeforms. Naturally the professional chemists and biologists who write science fiction on the side did best, not so much because their professional knowledge led them for long distances on hitherto untrodden paths, but because it made them stop at the right moment.
     As regards those who were primarily writers, the results make one suspect that they at first tried to apply what biology they knew. Since this apparently did not get them very far, they presumably threw overboard whatever it was they had not quite arrived at and wrote things like this: “Surprisingly, the aliens were quite human in shape, the only major differences, or at any event the ones which were easily visible, being a strong tail and a bluish complexion.”
     Or else, if the actual contact with the aliens could be fleeting, they resorted to saying that the forms the Earthmen beheld were so alien, so outside of all terrestrial experience, that it was impossible to describe them.

     However, the occasional science fiction writer of the past was not the only type of creative genius who did, or could have, exerted ingenuity in the building of an extraterrestrial. There were many others who engaged in a very similar line of endeavor for the purpose of representing gods, demons or just outlandish creatures, somewhat along the line of the Midnight Marvels to which I devoted a column some months ago.
     To put it bluntly, nobody showed much imagination and the method was standardized at an early age:
     Combine the features of various kinds of living creatures into something that could be drawn, painted or sculptured and the job was done. Put a woman’s head on a feline body and you had a sphinx. Add the head of a bird to the body of a man and you had ibis-headed Thoth. Take a horse and supply it with the wings of an eagle and Pegasus was ready for flight, though with lateral stability only. Take another horse, cut off its head and graft the upper half of a man’s body to it and the centaur was ready.

     So you obviously cannot produce a biologically possible or even believable creature by the (random or artistic) combination of separate parts. Fine — but how can you go about it? All I can say offhand is that it isn’t easy; so much depends on so many different circumstances.
     There is, in the first place, the planetary environment, consisting of such factors as either much water or very little water; temperature which depends mainly but not only on the distance of the planet from its sun; seasonal changes which depend on the inclination of the axis of rotation of a planet to the plane of its orbit around the sun.
     It depends on the presence or absence of a large moon (or moons) because, with a large and nearby moon, you get pronounced tides, while without a moon, or only very small moons, you only have the solar tide, which is likely to be unimpressive. The relative abundance of the chemical elements in the outer crust and in the atmosphere certainly also plays a role.

     Let us, for a first test, take our two neighbors in the Solar System, Venus inside the Earth’s orbit and Mars outside it.
     When I started reading books on science, as a schoolboy, Venus, in most of them, was firmly declared to be a panthalassa, the technical term for a planet completely covered by water without any land showing. This, after various attempts to be “different,” has recently been revived by Whipple and Menzel as the most likely concept.
     Now such a shoreless ocean — I am avoiding all other consideration and am concentrating on just the one fact that it is an ocean — can harbor virtually everything in abundance. But with limitations; you can’t just mix the fauna of the equatorial Pacific Ocean of today with equatorial seas of the Jurassic and Cretaceous periods and obtain a believable or even possible picture.
     You can have, if you want to, most of the arthropods, lobsters and sea spiders, trilobites and, if you insist, something like a seagoing centipede. But you must specify that there are shallow areas in this ocean if you want to have clams, for they don’t grow too far down. You can have jellyfish in fantastic numbers of species as well as individuals.
     You can have octopi and all sorts of fishes. But you can’t have a turtle, for example, because when, in Earth’s past, some fishes went up on land, they first produced what we now call amphibia — say, primitive salamanders — and the reptiles, the birds and the mammals came afterward. They all are creatures of the land, even though some reptiles, like the turtles and the sea snakes, and some mammals, like the whales and the seals, returned to the ocean at a later date.
     And don’t make anything more intelligent than the most intelligent fish — I don’t know which fish that is or could be — for the open sea is a region of steady movement and no intelligence is needed for that. The exceptions to the statement that this is a region of movement are armored forms like clams, but a perfectly sessile creature which relies on its armor for individual protection and on numerous offspring for survival of the species also is not going to develop intelligence. It doesn’t need any.
     So a shoreless Venusian ocean — I repeat I am concentrating on no other fact than that it is a shoreless ocean — might harbor a very varied life and some forms may be rather pretty. But I challenge anybody to think up an aquatic form of life, especially among the invertebrates, which would look radically different from what we have in our oceans. The multitude of forms on our own planet is so overwhelming that one always gets the impression that anything that can survive with the shape it has is also in existence.

     One thing is absolutely needed in this shoreless ocean if it is to have any life at all. There must be plants, microscopic or otherwise, because animal life alone is an impossibility.
     You know the old tall tale about the man who made a living by having a mouse and cat farm. The cats, of course, ate the mice, and when the cats were big enough, he killed and skinned them, sold the pelts and fed the cat’s bodies to the mice. Even if the mice were carnivorous, this just wouldn’t work. Somewhere at the beginning of such a cycle, there has to be the original food producer, the plant, which makes living (and edible, as a rule) tissue out of dissolved minerals, carbon dioxide and sunlight for energy.

     I might as well, at this point, present two strong hints at caution. If, in that sea, you have a tribe of Kraken, octopi a mile in circumference and the largest thing in the ocean, don’t make them smart. If they are the largest thing in the ocean, immune to all danger except an occasional outburst of the elements, such as a submarine volcano opening up, and, of course, old age, they don’t have to be intelligent. What has been said about oysters a while ago applies also to the invulnerable life-form.
     Likewise, don’t make something one millimeter in diameter into an intelligent life-form. Some time ago, somebody wrote a story in which the main character, who was not a hero, caught what he thought to be a shiny wasp. It stung him so hard that he had to let go — and then noticed to his surprise that the wasp sting made his Geiger counter chatter wildly. The implication was, of course, that this was a tiny spaceship with atomic drive.
     Though I liked the story, I knew that this could never happen. In order to be intelligent enough to even discover atomic energy, a being has to have a rather large number of brain cells. These brain cells must be nourished, which needs organs for eating and digesting food. The digestive tract must be protected by some covering and this package must be moved around in some manner so that it can find food. It must also move around to avoid being eaten, at least until it has attained the intelligence that splits atoms and controls what they do after splitting.
     It has been said and bolstered with many pounds of statistics that, in a modern army, 98 men are needed to ehable two men to shoot at the enemy. This relationship must apply also to the number of cells needed to support the brain cells that do the thinking. Since a cell, in order to function as a cell, must consist of a very large number of molecules and since the size of molecules is a given fact, there must be a minimum size for a functioning cell.
     L. Sprague de Camp, who was to my knowledge the first to present this chain of reasoning (in a two-part article in Astounding, May and June issues of 1939), came to the conclusion that an overall body weight of around 40 pounds would be needed if you want intelligence on the human level.
     It is possible that a few facts permit a little more stretching, so that the minimum weight could be less. But the reasoning itself is valid and the reduction cannot be very much. Whether the first interstellar hero has to establish relations with something weighing 45 or only 30 pounds does not make much of a difference.
     But I did not want to slip out of our solar system yet.

     Now if we look at Mars, we are helped no end by the fact that we know a great deal about it. Here is a small planet with very little water and a thin atmosphere consisting mostly of inert nitrogen. It is generally a cold planet, but during the summer the equatorial regions can attain temperatures between 60 and 70 degrees Fahrenheit at noon. To make our problem still easier, we are virtually certain that we see plant life (this was written in those innocent days of 1956 when they still though Mars had visible signs of plant life, instead of the huge dust storms we now know are the case).
     The dark greenish patches which all bear nice classical names due to Signor Schiaparelli of half a century ago cannot just be mineral discolorations. When covered up by yellow dust from the deserts, they manage to break through again and just during the last close approach of Mars, in 1954, Dr. Earl C. Slipher, working at Bloemfontein, South Africa, found a new one almost the size of Texas under about 15° northern Martian latitude and about 235° Martian longitude, which means about halfway between the northern end of Syrtis major and Trivium Charontis, two well-known Martian markings.

     There has been a lot of discussion recently in learned journals on whether any terrestrial plant could grow on Mars, and if so, which one. Naturally any suggestion made by anybody was countered with heavy arguments by somebody else. But the fact remains that we see something growing on Mars which is, in our terminology, plant life. If we do not understand their biochemistry under the conditions we are forced to assume from astronomical observations, this can only mean one of two things:
     Either we cannot observe all the conditions and something which we have missed, or are bound to miss with present instrumentation, is a perfectly fine explanation; or else we don’t know enough biochemistry and there is a way of living and growing under these conditions.
     The reasoning that forced us to say that there must be plant life in the Venusian oceans, if we want animal life of any kind, almost forces us to say that, since there are plants on Mars, there must be something that we would call animals.
     Some biologists with whom I discussed this stated with professional caution that this reasoning does not necessarily hold true. I don’t agree. Speaking in the largest sense, the animals of Earth, from sow bugs to elephants, are parasitic on plants. Now life, at least on Earth, behaves in such a manner that if there is something to be parasitic on, something else will be happy to take over the role of the parasite.
     Something feeding on these Martian plants must have the power of movement because it needs so much plant tissue for its own sustenance that the rate of the plant growth cannpt furnish the necessary amount. Hence it must be capable of locomotion.
     Whether this supposed Martian plant-eater is built along the lines of a locust, or along the lines of a desert tortoise, or along those of a rabbit is something entirely different again. One can assume that it simply freezes into a deathlike state during the cold Martian rlight and remains in that state until thawed out by the Sun. In. that case, it could be insectlike in organization.
     One can assume with equal justification that the “animal,” at the first sign of cold in the evening, burrows into the ground for a few feet and goes to sleep normally in an environment where the temperature may be quite cold, but where there is very little deviation from whatever temperature it may have. In that case, it could be something comparable to a desert tortoise.
     Or you can make the assumption that it has an internal mechanism like the birds and mammals of Earth, something producing heat. Then it does not have to dig itself in. All it needs is an effective heat insulator around its body, which might be hairlike, or featherlike,' or, if this sounds more “alien,” something like bark or sponge rubber.

     So far, I have mostly talked about extraterrestrial animal life in order to show some of the difficulties. When it comes to an extraterrestrial intelligent lifeform, the difficulties rapidly increase in number and kind.
     It may come as a surprise, but the first tentative recipe for the construction of an intelligent extraterrestrial was written by the Dutch physicist, philosopher and astronomer Christian Huyghens. The title of the book is Kosmotheoros and it appeared posthumously, in 1692, at first in Latin. Nobody seems to know just when Huyghens wrote the major portion of the book.
     He said there that an extraterrestrial must have eyes and ears — that is, senses “and pleasure arising from his senses.” He must know the art of writing to remember things, arithmetic and geometry to understand relationships, hands to make things — and he must be upright.
     It does not become quite clear from Huyghens’ book why he must be upright. It sounds as if Huyghens made this condition to free the forelimbs from the task of locomotion so that there are “hands to make things.”
     The insistence struck me as amusing because Sprague de Camp, in the articles mentioned, also was insistent on that point, but more for mechanical reasons. The brain must be protected against shock as much as possible and the more bone, cartilage and tissue there is between the feet, which take the shocks, and the brain, the better.
     All this is sound logic and it is obvious that the body of the extraterrestrial must be such that it functioned well as an animal body before it grew to be intelligent. Of course, one can postulate that accidental enviromental conditions of the past helped along.
     Around the turn of the century, a number of biologists and zoologists toyed with the idea that Man had evolved in what they called an asylum, an area accidentally free from large predatory animals and with a gentle climate. They obviously did not think much of the human body as a well-functioning animal. We now know that they were wrong and that the idea of the “asylum” is not needed. But it may conceivably have happened somewhere else, for the Galaxy must be full of planets and possibilities.

     There is just one major difficulty in imagining a believable intelligent extraterrestrial — we have never seen one. What I mean by this remark is this:
     We know the organization of living animal tissue on Earth. We know that the organization of the mammal is superior. True, it “wastes” food by making its own heat, but this fact makes it climatically independent. And though a reptile can do quite well in the proper climate, it is very limited. When the air grows too cold, it must be inactive, though it usually survives. When the air grows too hot, it dies of heat stroke, for, lacking a temperatureregulating mechanism, it not only cannot keep warm, it also cannot keep cool.
     Now this vertebrate body, whether mammalian or reptilian, has two pairs of limbs and usually a tail. What we don’t know is Whether it has to be built that way.
     To use a classical example: we don’t know whether the centaur shape is possible or not. On Earth, it doesn’t exist; that much is certain. But is this due to an anatomical necessity for which we don’t know the reason or did it just happen that way here?
     As for comparatively minor matters, we do know that they just happened. Genus Homo is tailless and almost hairless. But it doesn’t have to be hairless and tailless to invent writing, to build and ride cars and to engage in research, politics and crime.
     If we had fur and a tail, our fashions, habits and morals would be different, but if brain and senses and hands were unchanged, we’d still write books and symphonies, build houses, ships and airplanes — and try to build an extraterrestrial.

EXTREME ALIENS know how there are creatures that dwell in the most inaccessible, inhospitable places above, on and under the Earth and in her oceans? I am talking about life-forms you can find in any handbook of zoology, as opposed to those fearsome beings of the Cthulhu Cycle which which we are now so familiar. Well, there are also creatures which exist in the most obscure and random corridors and corners of time, in lost and unthinkable abysses of space, and in certain other twilight places which are most easily explained by referring to them as junctions of forces neither temporal nor spacial, places which by all rights should only exist in the wildest imaginings of theoreticians and mathematicians...

...Suffice to say, then that there are extreme forms of life within and without this universe of ours. And I know it to be so for I have seen or learned of many such forms.

for instance:

...intelligent energies in the heart of a giant alien sun who measure time in ratios of nuclear fission and space in unimaginable degrees of pressure! There are wraithlike biological gasses which issue at the dark of their moon from the fissures of a fungoid world in Hydra, to dance away their brief lives until, exhausted, they die at dawn, scattering the sentient seeds of mushroom minds which will sprout and take root, and whose crevice-deep roots will in turn emit at the dark of the moon euphoric, spore-bearing mists of genesis.

There is a dying purple sun on Andromeda's rim whose rays support life on all seven of its planets. On the fourth planet there are exactly seventeen forms of life, or so it would appear. On closer inspection, however, a zoologist could tell you that these forms are all different phases of only one life-form! Consider the batrachian and lepidopterous cycles of Earth life and this might not seem too astonishing, until I tell you that of these seventeen phases two are as apparently inanimate mineral deposits, six are aquatic, two others amphibious, three land-dwelling cannibals, three more are aerial and the last is to all intents and purposes a plant while all of its preliminary stages (excluding the mineral phases) were animal...


The starcraft gathered the fabric of time and space. Chayn passed stars and groupings of stars, dense clusters of young stars and swirling clouds of dust and gas giving birth to new light in their depths. Black holes tunneled through the space-time structure into elsewhere, glowing ominously as matter spiraled down to annihilation. Chayn could perceive it all, but he focused his attention on the mind fields.

Uncountable multitudes of worlds circles perhaps a third of the stars in his view. Most were lifeless, barren worlds of rock and snow, but even the tiny fraction that had given birth to life emanated a broad mind field that he could sense everywhere. There were worlds of microscopic life and paradises of forests and jungles teaming with dramas of life and death. There were worlds ancient and wise in the ways of evolution, but what Chayn watched for were the sparks of intense awareness, life on levels near his own. Intelligence too far in advance of him were incomprehensible, aware of his passage, but apathetic. Most life forms on his own level were alien, different in inexplicable ways. He felt he could adapt to some of those strange and beautiful worlds if necessary, but he staved his hunger and waited for the worlds of man.

The Watcher told him that man had lived for eons, evolving to the greatness of the stargods, but that man in this galaxy had recently arrived in fleets of starships after sleeps of many millennia. The worlds of man were new here while Earth recycled its continents and evolved new species of life...

...Danger lay immediately ahead, a gulf of darkness between two arms of the galaxy. Chayn approached the starless void with caution. In that incredible abyss four hundred light years across, he could sense another kind of life -- the star travelers. He could sense such small concentrations of explorers only where they stood out like specks of brightness, even the blank minds of those who slept in the frozen oblivion of suspended animation...

One of the star travelers in view piloted a starcraft similar to his own. Two others were primitive vehicles of metal driven by fusion or antimatter-propulsion units to velocities below that of the speed of light ... At first Chayn thought the pilot of the starcraft like his own would seek communication with him, but the entity was highly evolved and looked upon him as a curiosity. Chayn knew himself to be a primitive, more typical of the life forms frozen in their crude ships of metal...

...Chayn's fear intensified as he neared the abyss. Mindspiders lurked in the darkness, many species of them littering the void with invisible webs. Some dangled thin and scraggly. Others spread magnificently, a light year in diameter. Even in that moment, he felt the shock, the utterly brilliant flare of terror of alien minds encountering the web in the far distance. Particles rose to lethal intensities of radiation. Bodies died and the ship heated to incandescence. The mindspiders fed upon disembodied consciousness. Few of the primitives could perceive such danger lurking in the abyss...

...Ahead, he sensed an old, torn web. Even the mindspiders had their predators in their own realm. This one was gone, the web deteriorating.


Hammond's head spun with their tales of spaceman's life, tales of the vast glooms of cosmic clouds that ships rarely dared enter, of wrecks and castaways in the unexplored fringes of the galaxy, of strange races like the thinking rocks of Rigel and the fish-cities of Arcturus' watery worlds and the unearthly tree-wizards of dark Algol.

Edmond Hamilton, THE STAR OF LIFE (1947)

Aliens will not resemble anything we've seen. Considering that octopi, sea cucumbers, and oak trees are all very closely related to us, an alien visitor would look less like us than does a squid. Some fossils in the ancient Burgess shale are so alien that we can't determine which end of the creature is up, and yet these monsters evolved right here on Earth from the same origins as we did.

Johan Forsberg

Bizarre Alien Senses: Well, hell, who doesn’t have some sort of bizarre senses? Especially since it gets very tricky if you count the whole electromagnetic spectrum as one – i.e., “ultravision” and “infravision” are both strict subsets of “vision”. As, for that matter, is sensing gamma rays – and other similar elisions. It’s not like anyone gets to claim the canonical radiation range for “sight”, now is it?

But we’ve got people sensing everything from low infrared to high UV, with bioradio senses, with the ability to detect electromagnetic fields both static and changing, with the ability to feel the curvature of space-time (that would be those bionano vector-control effectors again), with echolocation and/or sonar, the ability to read plasmids by tasting them, and pretty much any other physical effect that you can measure somehow on the macroscale.

(The current eldrae alpha baseline clocks in at 24 recognized senses, by the way, counting the synthetic and transcendent ones, and that’s after considering smell and taste as one: photoception, audition, chemoception/olfaction, static mechanoception, dynamic mechanoception, thermoception, nociception, static electroception, dynamic electroception, proprioception, chronoception, farspeech, spatioception, secondary gestalt, secondary linear, mesh, metadata, worth, mnemonesis, nature, utility, entelechy, obligation, and autosentience.)

Hive Entity

A "hive" intelligence would resemble an intelligent ant-hill, where each ant would be but a cell in the hill's "body". Individual ants may die, but the hill goes on. Examples include the "Boaty Bits" from FARTHEST STAR by Jack Williamson and Frederik Pohl, the "Godtalkers" from THE DRAGON NEVER SLEEPS by Glen Cook, the "Tinker Composite" from THE MIND POOL by Charles Sheffield, the "Mantis" from GREAT SKY RIVER by Gregory Benford, and the Martians from LAST AND FIRST MEN by Olaf Stapedon. If the alien is composed of a hive of several species, it is some times called an "anthology intelligence." Go to The Tough Guide to the Known Galaxy and read the entry "HIVE ENTITY".

A good example of a hive intelligence was in Olaf Stapedon's classic Star Maker. The "cells" composing an individual were free-flying birds linked telepathically. Birds might be born or die, but the flock-individual lived on. A more modest version were the "Tines" in Vernor Vinge's A Fire Upon The Deep. One might even consider an anthill to be a hive organism, an individual who's cells are ants.

HIVE ENTITY. A type of REALLY ALIEN intelligent species - one of the most Really Alien of all - organized along lines rather like the social insects. In a Hive Entity, individuals members of the community count for nothing, and indeed most of them have no individual intelligence to speak of. They are specialized for various functions (particularly warriors), and exist entirely to serve the Hive Entity as a whole.

A Hive Entity's intelligence may reside in specialized "brain" individuals, which have only vestigial legs and even digestive systems, and are themselves entirely dependent on various kinds of "slave" individuals. Or the intelligence may somehow be spread out collectively though the whole Hive Entity, each individually-mindless inhabitant in effect contributing a few neurons to the whole. (Or some combination of these.) Some Hive Entities may not really be intelligent at all, but have evolved the ability to blow up other people's spacecraft the same way that some ants have evolved the ability to keep aphids as cattle.

When encountered in the KNOWN GALAXY, Hive Entities are almost invariably hostile. They apparently have nothing to offer in trade, much less arts or ideas, and you can't even negotiate a peace treaty with them, because there isn't really anyone to negotiate with. In WARFARE they are at once mindlessly ruthless - attacking in endless waves like giant army ants, which they also tend to look like - and malevolently intelligent. Putting no value on their own automaton lives, they obviously have no concept of valuing anyone else's.

In fact, Hive Entities are basically the ultimate totalitarians. It is no surprise that they appeared in written SF, so far as I know, around the mid 20th century CE, the same time that giant ants showed up in HOLLYWOOD SCIFI. Hive Entities were, and are, Nazis, Stalinists, and ChiComs, magnified to the Nth degree and let loose to give better races a harsh lesson in the precious value of individualism.

Which is really too bad. Taken in themselves, Hive Entities are a fascinating concept, precisely because they really are Really Alien. Yet if in fact they are intelligent, they must have ideas of some sort, however hard for them to express in a way we can understand. If the intelligence is spread through the hive community, the time scale of its thinking might be drastically slower than our own, maybe taking weeks to form the equivalent of a sentence. This indeed could make them tricky to deal with at first, since on our time scale they would necessarily act on reflex.

But if we EARTH HUMANS, and similar species, really want to demonstrate individual intelligence, we might actually try figuring the Hive Entities out, and see if we and they might have something to contribute to each other, instead of fighting pretty mindless wars with them. Don't hold your breath, though. It hasn't happened in fifty years, so far as I know.

But maybe the Hive Entities' mental time scale is longer than that.

Social Insects in Science Fiction

I love social insects. Whether they’re ants, bees, termites, wasps, aphids, thrips, or ambrosia beetles, I find them fascinating to learn about. But if the sci-fi books I read as a kid had had their way, I should have run screaming from every ant colony I saw.

From the buggers in Ender’s Game to the Borg in Star Trek to the Vord in Codex Alera to ants and termites themselves from a morph’s-eye view in Animorphs, social insects, and the aliens or artificial intelligences that closely resemble them, are portrayed as “hive minds” with an emotional tone of existential terror. And I’m here to tell you that these portrayals are totally unfair.

What they get right

Here are some features that most portrayals of social insects and their analogues in sci-fi get right. Yes, social insect colonies have queens that are primarily responsible for reproduction. Yes, social insects have very different sensory modalities from ours. We primarily use sight and sound to communicate and navigate the world, while social insects use taste and smell and vibration. Yes, social insects have specialized division of labor to particular tasks, and yes, they are willing to sacrifice themselves in droves to protect the colony. And sometimes, they will enslave social insects from other colonies or even species to serve their own ends (x).

Thus ends what sci-fi portrayals get right.

What they get wrong: Queens

Almost universally in sci-fi, when you kill the queen, the hive disintegrates into chaos. You’ve cut off the head! The central intelligence of the hive is gone! They’re just mindless borg-units with no idea what to do!

Indeed, in some social insects, such as leafcutter ants, if you kill the queen, the whole colony will die – but probably not for the reasons you think. However, it’s more common for social insects to be able to carry on just fine regardless. In most ants and bees, there are “backup” queens that are reared up by the workers in case the current queen should die. And in many social insects, a worker can step up and become a queen in her place.

But here is the most important problem with the sci-fi trope of killing the queen to kill the hive. The queen is not the brain of the hive. She is the ovary.

If you think of a social insect colony as a superorganism, which it’s useful to do in many cases, different groups of insects within the colony act like organs. One caste protects the colony from invaders, which is like an immune system. One caste scouts for new places to forage, which is like a sensory system. Generally, science fiction has a good grip on this idea. Where sci-fi authors fail is that they think the queen is the brain of this superorganism. She is not. She is the reproductive system. The queen does not control what happens in the hive any more than your reproductive system controls what happens in your body. (Which is to say, she has some influence, but she is not the brains of the operation.)

The reason why leafcutter ant colonies die when the queen dies is because the colony has been castrated, not beheaded. Most animals die when they are no longer able to reproduce, even if their brains are still perfectly functional. For castrated colonies with no backup queen or gamergate and no hope of getting one, there is no point in carrying on. Their evolutionary line has ended.

What they get wrong: Swarm intelligence

Here is how social insect hive minds work in science fiction: the queen does the thinking, and the rest of the hive goes along with whatever she thinks.

Now, I’ve already told you that the queen is not the brain of the hive. So where is the brain? Well, that is exactly the point of swarm intelligence. The brain does not reside in one particular animal. It’s an emergent property of many animals working together. A colony is not like your body, where your brain sends an impulse to your mouth telling it to move, and it moves. It’s more like when two big groups of people are walking toward each other, and they spontaneously organize themselves into lanes so no one has a collision (x). There’s no leader telling them to do that, but they do it anyway.

Much of the efficiency of social insect colonies comes from very simple behavioral rules (x). Hymenopterans, the group of insects that includes ants, bees, and wasps, have a behavioral rule: work on a task until it is completed, and when it is done, switch to a different task. If you force solitary bees (yes, most bee species are solitary) to live together, they will automatically arrange themselves into castes, because when one bee sees another bee doing a task like building the nest, its behavioral rule tells it that the task is completed and it needs to switch to a different task, like looking for food.

Individually, a social insect isn’t all that smart, whether it’s a queen, worker, soldier, or drone. But collectively, social insects can do incredibly smart things, like find the most efficient route from the colony to some food (x), or choose the perfect spot to build their hive (x).

What they get wrong: Individuality

The existential terror of the hive mind in science fiction comes from the loss of the self. The idea is that in a social insect colony, there is no individual, but one whole, united to one purpose. No dissent, disagreement, or conflicting interests occur, just total lockstep. I totally get why that’s scary.

The thing is, it’s just not true of real social insects. There is conflict within colonies all the time, up to and including civil war.

A common source of conflict within colonies is worker reproduction. Yes, in most social insects, workers can in fact reproduce, though usually they can only produce males. So why don’t they? Because it’s not in the interest of their fellow workers. Workers are more closely related to their siblings and half-siblings produced by the queen than they are to their nephews, so they pass on more of their genes if they spend resources on raising the queen’s eggs. So, if a worker catches its fellow laying an egg, it will eat the egg. Not exactly “all for one and one for all,” is it?

Worker insects may also fight in wars of succession. If there is more than one queen in a species where queens do not tolerate each other (yes, there are species where multiple queens get along together just fine), such as monogynous fire ants, the workers will ally themselves with one queen or another and engage in very deadly civil war.

Finally, in some species, the queen needs to bully the workers into doing their jobs, and the dominant workers need to bully subordinate workers into doing their jobs (x). Yes, sometimes workers try to laze around and mooch.

Surprisingly human

Here’s what I find weird about depictions of social insects in science fiction. They are portrayed as utterly alien, Other, and horrifying. Yet humans and social insects are very, very similar. The famous sociobiologists E.O. Wilson and Bernard Crespi have both described humans as chimpanzees that took on the lifestyle of ants.

I think what fascinates people, including me, about ants, bees, and their ilk is that you watch, say, a hundred ants working together to tear up a leaf into tiny bits and carry it back to their colony, or a hundred bees all appearing out of seemingly nowhere to sacrifice themselves en masse to stop a bear from eating their hive, and it looks like magic. It really does look like some kind of overmind is controlling their collective actions.

But imagine you’re an alien who comes to Earth, and you know nothing about humans or the way we communicate. Wouldn’t we look exactly the same to them as ants and bees look to us? Wouldn’t they look at us sacrificing our lives by the thousands in wars, or working together to build cities from nothing, and think, Wow, how do they coordinate themselves in such huge numbers, why do they give up their lives to defend their borderlines, I guess there must be some kind of mega-brain they all share that tells them what to do, and they just march in lockstep and do it.

If there’s anything I’ve learned from the study of both social insects and humans, it’s that any system that looks monolithic and simple from a distance is in fact fractured, messy, and complicated when you look at it up close.

Social insects aren’t scary mindless robot-aliens. They’re a lot like you and me. As much as I was terrified as a kid by the Animorphs book where an ant morphs into Cassie and screams in pure existential horror at its sudden individuality, I actually think an ant would adjust very easily to being a human, and that a human would adjust very easily to being an ant — much more easily, in fact, than humans adjusted to morphing, say, sharks, in the very same book series.

From Social Insects in Science Fiction by Poetry (2016)

Winged Aliens

Aliens with wings is a popular trope. The dream of flying like a bird is probably been around since the dawn of recorded history.

But you can forget about angel-like humanoids with wings. Why? Physics.

You see, wings need to flap with enough power to lift the person. The power comes from muscles, lots of muscles. So much muscle in fact that in birds they need a special bone for the wing muscles to attach to. This is called the keel or carina bone. The muscles are what we call the meat of a chicken or turkey breast, and the keel is the breastbone.

I trust you can spy the problem. A humanoid with wings is going to have a deformed chest that looks like the prow of a huge boat. And female humanoids with wings will not have mammary glands. Not on their chest at any rate. That segment of the science fiction audience with the personalties of adolescent boys will be angry at the lack of cheescake fanservice. Once again the fans will be outraged at scientific accuracy. And they will vote with their wallet.

As you can see while it is not actually impossible to have humanoid winged creatures, they are going to be more towards the "noid" and less towards the "human" part of the spectrum. Which will put them right in the uncanny valley, inspiring revulsion instead of attraction. They ain't gonna look like angels.

A more minor problem is the fact that on a bird, the wings are basically its arms. On humans, arms are attached to the shoulder blades. Which means a winged humanoid with both arms and wings is going to need four shoulder blades, not the customary two. Which probably means the wings will be attached to the small of the back, not the shoulders.

Also the neck should be long and articulated so when flying (and basically in a prone position) it can bend the head so it can see where it is going, instead of being forced to look at the ground.


     They were near the door when a shadow fell over them. They whirled and stared upward. Yukiko's indrawn breath hissed from their receivers.
     Aloft hovered one of the great ornithoids. Sunlight struck through its outermost pinions, turning them golden. Otherwise it showed stormcloud-dark. Down the wind stooped a second.
     The sight was terrifying. Only later did the humans realize it was magnificent. Those wings spanned six meters. A muzzle full of sharp white fangs gaped before them. Two legs the length and well-nigh the thickness of a man's arms reached crooked talons between them. At their angles grew claws. In thrust after thrust, they hurled the creature at torpedo speed. Air whistled and thundered.
     Their guns leaped into the men's hands. "Don't shoot!" Yukiko's cry came as if from very far away.
     The splendid monster was almost upon them. Fire speared from Webner's weapon. At the same instant, the animal braked—a turning of quills, a crack and gust in their faces—and rushed back upward, two meters short of impact.
     Turekian's gaze stamped a picture on his brain which he would study over and over and over. The unknown was feathered, surely warm-blooded, but no bird. A keelbone like a ship's prow jutted beneath a strong neck. The head was blunt-nosed, lacked external ears; fantastically, Turekian saw that the predator mouth had lips. Tongue and palate were purple. Two big golden eyes stabbed at him, burned at him. A crest of black-tipped white plumage rose stiffly above, a control surface and protection for the backward-bulging skull. The fan-shaped tail bore the same colors. The body was mahogany, the naked legs and claws yellow.
     Webner's shot hit amidst the left-side quills. Smoke streamed after the flameburst. The creature uttered a high-pitched yell, lurched, and threshed in retreat. The damage wasn't permanent, had likely caused no pain, but now that wing was only half-useful.
     Turekian thus had time to see three slits in parallel on the body. He had time to think there must be three more on the other side. They weirdly resembled gills. As the wings lifted, he saw them drawn wide, a triple yawn; as the downstroke began, he glimpsed them being forced shut.

     "I shot at a dangerous animal. Didn't you see those talons, those fangs? And a buffet from a wing that big—ignoring the claws on it—could break your neck."
     Webner's gaze sought Yukiko's. He mainly addressed her: "Granted, they must be domesticated. I suspect they're used in the hunt, flown at game like hawks though working in packs like hounds. Conceivably the pair we encountered were, ah, sicced onto us from afar. But that they themselves are sophonts—out of the question."
     Her murmur was uneven. "How can you be sure?"
     Webner leaned back, bridged his fingers, and grew calmer while he lectured: "You realize the basic principle. All organisms make biological sense in their particular environments, or they become extinct. Reasoners are no exception—and are, furthermore, descended from nonreasoners which adapted to environments that had never been artificially modified.
     "On nonterrestroid worlds, they can be quite outré by our standards, since they developed under unearthly conditions. On an essentially terrestroid planet, evolution basically parallels our own because it must. True, you get considerable variation. Like, say, hexapodal vertebrates liberating the forelimbs to grow hands and becoming centauroids, as on Woden. That's because the ancestral chordates were hexapods. On this world, you can see for yourself the higher animals are four-limbed.
     "A brain capable of designing artifacts such as we observe here is useless without some equivalent of hands. Nature would never produce it. Therefore the inhabitants are bound to be bipeds, however different from us in detail. A foot which must double as a hand, and vice versa, would be too grossly inefficient in either function. Natural selection would weed out any mutants of that tendency, fast, long before intelligence could evolve.
     "What do those ornithoids have in the way of hands?" He smiled his tight little smile.
     "The claws on their wings?" Yukiko asked shyly.
     "'Fraid not," Turekian said. "I got a fair look. They can grasp, sort of, but aren't built for manipulation."
     "You saw how the fledgling uses them to cling to the parent," Webner stated. "Perhaps it climbs trees also. Earth has a bird with similar structures, the hoactzin. It loses them in adulthood. Here they may become extra weapons for the mature animal."
     "The feet." Turekian scowled. "Two opposable digits flanking three straight ones. Could serve as hands."
     "Then how does the creature get about on the ground?" Webner retorted. "Can't forge a tool in midair, you know, let alone dig ore and erect stone houses."
     He wagged a finger. "Another, more fundamental point," he went on. "Flyers are too limited in mass. True, the gravity's weaker than on Earth, but air pressure's lower. Thus admissible wing loadings are about the same. The biggest birds that ever lumbered into Terrestrial skies weighed some fifteen kilos. Nothing larger could get aloft. Metabolism simply can't supply the power required. We established aboard ship, from specimens, that local biochemistry is close kin to our type. Hence it is not possible for those ornithoids to outweigh a maximal vulture. They're big, yes, and formidable. Nevertheless, that size has to be mostly feathers, hollow bones—spidery, kitelike skeletons anchoring thin flesh.
     "Aram, you hefted several items today, such as a stone pot. Or consider one of the buckets, presumably used to bring water up from the river. What would you say the greatest weight is?"
     Turekian scratched in his beard. "Maybe twenty kilos," he answered reluctantly.
     "There! No flyer could lift that. It was always superstition about eagles stealing lambs or babies. They weren't able to. The ornithoids are similarly handicapped. Who'd make utensils he can't carry?"
     "M-m-m," Turekian growled rather than hummed. Webner pressed the attack:
     "The mass of any flyer on a terrestroid planet is insufficient to include a big enough brain for true intelligence. The purely animal functions require virtually all those cells. Birds have at least lightened their burden, permitting a little more brain, by changing jaws to beaks. So have those ornithoids you called 'watchfalcons.' The big fellows have not."
     He hesitated. "In fact," he said slowly, "I doubt if they can even be considered bright animals. They're likely stupid . . . and vicious. If we're set on again, we need have no compunctions about destroying them."

     Maybe his irritation with the pilot spoke for Webner: "How often must I explain there is no such risk, yet? Instead, here's a chance to learn. What happens next could give us invaluable clues to understanding the whole ethos. We stay." To Turekian: "Forget about that alleged metal. Could be protective collars, I suppose. But take the supercharger off your imagination."
     The other man froze where he stood.
     "Aram." Yukiko seized his arm. He stared beyond her. "What's wrong?"
     He shook himself. "Supercharger," he mumbled. "By God, yes."
     Abruptly, in a bellow: "We're leaving! This second! They are the dwellers, and they've gathered the whole countryside against us!"

     When you know what to expect, a little, you can lay plans. We next sought the folk of Ythri, as the planet is called by its most advanced culture, a thousand kilometers from the triumph which surely prevailed in the mountains. Approached with patience, caution, and symbolisms appropriate to their psyches, they welcomed us rapturously. Before we left, they'd thought of sufficient inducements to trade that I'm sure they'll have spacecraft of their own in a few generations.
     Still, they are as fundamentally territorial as man is fundamentally sexual, and we'd better bear that in mind.
     The reason lies in their evolution. It does for every drive in every animal everywhere. The Ythrian is carnivorous, aside from various sweet fruits. Carnivores require larger regions per individual than herbivores or omnivores do, in spite of the fact that meat has more calories per kilo than most vegetable matter. Consider how each antelope needs a certain amount of space, and how many antelope are needed to maintain a pride of lions. Xenologists have written thousands of papers on the correlations between diet and genotypical personality in sophonts.
     I have my doubts about the value of those papers. At least, they missed the possibility of a race like the Ythrians, whose extreme territoriality and individualism—with the consequences to governments, mores, arts, faiths, and souls—come from the extreme appetite of the body.
     They mass as high as thirty kilos; yet they can lift an equal weight into the air or, unhampered, fly like demons. Hence they maintain civilization without the need to crowd together in cities. Their townspeople are mostly wing-clipped criminals and slaves. Today their wiser heads hope robots will end the need for that.
     Hands? The original talons, modified for manipulating. Feet? Those claws on the wings, a juvenile feature which persisted and developed, just as man's large head and sparse hair derive from the juvenile or fetal ape. The forepart of the wing skeleton consists of humerus, radius, and ulnar, much as in true birds. These lock together in flight. Aground, when the wing is folded downward, they produce a "knee" joint. Bones grow from their base to make the claw-foot. Three fused digits, immensely lengthened, sweep backward to be the alatan which braces the rest of that tremendous wing and can, when desired, give additional support on the surface. To rise, the Ythrians usually do a handstand during the initial upstroke. It takes less than a second.
     Oh, yes, they are slow and awkward afoot. They manage, though. Big and beweaponed, instantly ready to mount the wind, they need fear no beast of prey.
     You ask where the power comes from to swing this hugeness through the sky. The oxidation of food, what else? Hence the demand of each household for a great hunting or ranching demesne. The limiting factor is the oxygen supply. A molecule in the blood can carry more than hemoglobin does, but the gas must be furnished. Turekian first realized how that happens. The Ythrian has lungs, a passive system resembling ours. In addition he has his supercharger, evolved from the gills of an amphibianlike ancestor. Worked in bellows fashion by the flight muscles, connecting directly with the bloodstream, those air-intake organs let him burn his fuel as fast as necessary.

From WINGS OF VICTORY by Poul Anderson (1972)

A shape blotted out the sun. They bounded to their feet.

That which was descending passed the disc, and light blazed off the gold-bronze pinions of a six-meter wingspan. Air whistled and thundered. Fraina cried out. Mikkal poised his javelin. "Don't!" Ivar shouted. "Ya-lawa! He's Ythrian!"

"O-o-oh, ye-e-es," Mikkal said softly. He lowered the spear though he kept it ready. Fraina gripped Ivar's arm and leaned hard against him.

The being landed. Ivar had met Ythrians before, at the University and elsewhere. But his astonishment at this arrival was such that he gaped as if he were seeing one for the first time.

Grounded, the newcomer used those tremendous wings, folded downward, for legs, claws at the bend of them spreading out to serve as feet, the long rear-directed bones lending extra support when at rest. That brought his height to some 135 centimeters, mid-breast on Ivar, farther up on the tinerans; for his mass was a good 25 kilos. Beneath a prowlike keelbone were lean yellow-skinned arms whose hands, evolved from talons, each bore three sharp-clawed fingers flanked by two thumbs, and a dewclaw on the inner wrist. Above were a strong neck and a large head proudly held. The skull bulged backward to contain the brain, for there was scant brow, the face curving down in a ridged muzzle to a mouth whose sensitive lips contrasted curiously with the carnivore fangs behind. A stiff feather-crest rose over head and neck, white edged with black like the fan-shaped tail. Otherwise, apart from feet, arms, and huge eyes which burned gold and never seemed to waver or blink, the body was covered with plumage of lustrous brown.

He wore an apron whose pockets, loops, and straps supported what little equipment he needed. Knife, canteen, and pistol were the only conspicuous items. He could live off the country better than any human…

…The Anglic which replied was sufficiently fluent that one couldn't be sure how much of the humming accent and sibilant overtones were due to Ythrian vocal organs, how much simply to this being an offplanet dialect the speaker had learned. "Thanks, greetings, and fair winds wished for you. I hight Erannath, of the Stormgate choth upon Avalon. Let me quench thirst and we can talk if you desire."

As awkward on the ground as he was graceful aloft, he stumped to the pool. When he bent over to drink, Ivar glimpsed the gill-like antlibranchs, three on either side of his body. They were closed now, but in flight the muscles would work them like bellows, forcing extra oxygen into the bloodstream to power the lifting of the great weight. That meant high fuel consumption too, he remembered. No wonder Erannath traveled alone, if he had no vehicle. This land couldn't support two of him inside a practical radius of operations…

…Mikkal settled himself back in the shade where he had been. "Might I ask what brings you, stranger?"

"Circumstances," Erannath replied. His race tended to be curt. A large part of their own communication lay in nuances indicated by the play of marvelously controllable quills

…Expressions they could not read rippled across the feathers…

…"A sophont," Mikkal said redundantly. He proceeded: "More bright and tough than most. Maybe more than us. Could be we're stronger, we humans, simply because we outnumber them, and that simply because of having gotten the jump on them in space travel and, hm, needing less room per person to live in."

"A bird?"

"No," Ivar told her. "They're feathered, yes, warm-blooded, two sexes. However, you noticed he doesn't have a beak, and females give live birth. No lactation—no milk, I mean; the lips're for getting the blood out of prey."

From THE DAY OF THEIR RETURN by Poul Anderson (1974)

A canned lecture was barely under way. A human xenologist stood in the screen and intoned:

"Warm-blooded, feathered, and flying, the Ythrians are not birds; they bring their young forth viviparously after a gestation of four and a half months; they do not have beaks, but lips and teeth. Nor are they mammals; they grow no hair and secrete no milk; those lips have developed for parents to feed infants by regurgitation. And while the antlibranchs might suggest fish gills, they are not meant for water but for—"…

…He reactivated the screen. It showed an Ythrian walking on the feet that grew from his wings: a comparatively slow, jerky gait, no good for real distances. The being stopped, lowered hands to ground, and stood on them. He lifted his wings, and suddenly he was splendid.

Beneath, on either side, were slits in column. As the wings rose, the feathery operculum-like flaps which protected them were drawn back. The slits widened until, at full extension, they gaped like purple mouths. The view became a closeup. Thin-skinned tissues, intricately wrinkled, lay behind a curtain of cilia which must be for screening out dust.

When the wings lowered, the slits were forced shut again, bellows fashion. The lecturer's voice said: "This is what allows so heavy a body, under Terra-type weight and gas density, to fly. Ythrians attain more than twice the mass of the largest possible airborne creature on similar planets elsewhere. The antlibranchs, pumped by the wing-strokes, take in oxygen under pressure to feed it directly to the bloodstream. Thus they supplement lungs which themselves more or less resemble those of ordinary land animals. The Ythrian acquires the power needed to get aloft and, indeed, fly with rapidity and grace."

The view drew back. The creature in the holograph flapped strongly and rocketed upward.

"Of course," the dry voice said, "this energy must come from a correspondingly accelerated metabolism. Unless prevented from flying, the Ythrian is a voracious eater. Aside from certain sweet fruits, he is strictly carnivorous. His appetite has doubtless reinforced the usual carnivore tendency to live in small, well-separated groups, each occupying a wide territory which instinct makes it defend against all intruders.

"In fact, the Ythrian can best be understood in terms of what we know or conjecture about the evolution of his race."…

…"We believe that homeothermic—roughly speaking, warm-blooded—life on Ythri did not come from a reptilian or reptiloid form, but directly from an amphibian, conceivably even from something corresponding to a lungfish. At any rate, it retained a kind of gill. Those species which were most successful on land eventually lost this feature. More primitive animals kept it. Among these was that small, probably swamp-dwelling thing which became the ancestor of the sophont. Taking to the treetops, it may have developed a membrane on which to glide from bough to bough. This finally turned into a wing. Meanwhile the gills were modified for aerial use, into superchargers."

"As usual," Wa Chaou observed. "The failures at one stage beget the successes of the next."

"Of course, the Ythrian can soar and even hover," the speaker said, "but it is the tremendous wing area which makes this possible, and the antlibranchs are what make it possible to operate those wings.

"Otherwise the pre-Ythrian must have appeared fairly similar to Terran birds." Pictures of various hypothetical extinct creatures went by. "It developed an analogous water-hoarding system—no separate urination—which saved weight as well as compensating for evaporative losses from the antlibranchs. It likewise developed light bones, though these are more intricate than avian bones, built of a marvelously strong two-phase material whose organic component is not collagen but a substance carrying out the functions of Terra-mammalian marrow. The animal did not, however, further ease its burdens by trading teeth for a beak. Many Ythrian ornithoids have done so, for example the uhoth, hawklike in appearance, doglike in service. But the pre-sophont remained an unspecialized dweller in wet jungles.

"The fact that the young were born tiny and helpless—since the female could not fly long distances while carrying a heavy fetus—is probably responsible for the retention and elaboration of the digits on the wings. The cub could cling to either parent in turn while these cruised after food; before it was able to fly, it could save itself from enemies by clambering up a tree. Meanwhile the feet acquired more and more ability to seize prey and manipulate objects.

"Incidentally, the short gestation period does not mean that the Ythrian is born with a poorly developed nervous system. The rapid metabolism of flight affects the rate of fetal cell division. This process concentrates on laying down a body pattern rather than on increasing the size. Nevertheless, an infant Ythrian needs more care, and more food, than an infant human. The parents must cooperate in providing this as well as in carrying their young about. Here we may have the root cause of the sexual equality or near equality found in all Ythrian cultures.

"Likewise, a rapid succession of infants would be impossible to keep alive under primitive conditions. This may be a reason why the female only ovulates at intervals of a year—Ythri's is about half of Terra's—and not for about two years after giving birth. Sexuality does not come overtly into play except at these times. Then it is almost uncontrollably strong in male and female alike. This may well have given the territorial instinct a cultural reinforcement after intelligence evolved. Parents wish to keep their nubile daughters isolated from chance-met males while in heat. Furthermore, husband and wife do not wish to waste a rich, rare experience on any outsider.

"The sexual cycle is not totally rigid. In particular, grief often brings on estrus. Doubtless this was originally a provision of nature for rapid replacement of losses. It seems to have brought about a partial fusion of Eros and Thanatos (sex+death motif. In Freudian psychology these are opposed, apparently in Ythrian psychology they are fused) in the Ythrian psyche which makes much of the race's art, and doubtless thought, incomprehensible to man. An occasional female can ovulate at will, though this is considered an abnormality; in olden days she would be killed, now she is generally shunned, out of dread of her power. A favorite villain in Ythrian story is the male who, by hypnosis or otherwise, can induce the state. Of course, the most important manifestation of a degree of flexibility is the fact that Ythrians have successfully adapted their reproductive pattern, like everything else, to a variety of colonized planets."…

…"But to return to evolution," the lecturer was saying. "It seems that a major part of Ythri underwent something like the great Pliocene drought in Terra's Africa. The ornithoids were forced out of dwindling forests onto growing savannahs. There they evolved from carrion eaters to big-game hunters in a manner analogous to pre-man. The original feet became hands, which eventually started making tools. To support the body and provide locomotion on the ground, the original elbow claws turned into feet, the wings that bore them became convertible to legs of a sort.

"Still, the intelligent Ythrian remained a pure carnivore, and one which was awkward on land. Typically, primitive hunters struck from above, with spears, arrows, axes. Thus only a few were needed to bring down the largest beasts. There was no necessity to cooperate in digging pits for elephants or standing shoulder to shoulder against a charging lion. Society remained divided into families or clans, which seldom fought wars but which, on the other hand, did not have much contact of any sort.

"The revolution which ended the Stone Age did not involve agriculture from the beginning, as in the case of man. It came from the systematic herding, at last the domestication, of big ground animals like the maukh, smaller ones like the long-haired mayaw. This stimulated the invention of skids, wheels, and the like, enabling the Ythrian to get about more readily on the surface. Agriculture was invented as an ancillary to ranching, an efficient means of providing fodder. The food surplus allowed leisure for travel, trade, and widespread cultural intercourse. Hence larger, complex social units arose.

"They cannot be called civilizations in a strict sense, because Ythri has never known true cities. The mobility of being winged left no necessity for crowding together in order to maintain close relationships. Granted, sedentary centers did appear—for mining, metallurgy, and other industry; for trade and religion; for defense in case the group was defeated by another in aerial battle. But these have always been small and their populations mostly floating. Apart from their barons and garrisons, their permanent inhabitants were formerly, for the main part, wing-clipped slaves—today, automated machines. Clipping was an easy method of making a person controllable; yet since the feathers could grow back, the common practice of promising manumission after a certain period of diligent service tended to make prisoners docile. Hence slavery became so basic to pre-industrial Ythrian society that to this day it has not entirely disappeared."

From THE PEOPLE OF THE WIND by Poul Anderson (1973)

Between these investigations, he caught momentary glimpses of the city, and realized how difficult—and dangerous—it would be for him to travel around in it. Streets were practically non-existent, and there seemed to be no surface transport. This was the home of creatures who could fly, and who had no fear of gravity. It was nothing to come without warning upon a vertiginous drop of several hundred metres, or to find that the only entrance into a room was an opening high up in the wall. In a hundred ways, Jan began to realize that the psychology of a race with wings must be fundamentally different from that of earthbound creatures.

It was strange to see the Overlords flying like great birds among the towers of their city, their pinions moving with slow, powerful beats. And there was a scientific problem here.

This was a large planet—larger than Earth. Yet its gravity was low, and Jan wondered why it had so dense an atmosphere. He questioned Vindarten on this, and discovered, as he had half expected, that this was not the original planet of the Overlords. They had evolved on a much smaller world and then conquered this one, changing not only its atmosphere but even its gravity.

The architecture of the Overlords was bleakly functional; Jan saw no ornaments, nothing that did not serve a purpose, even though that purpose was often beyond his understanding. If a man from mediaval times could have seen this red-lit city, and the beings moving through it, he would certainly have believed himself in Hell. Even Jan, for all his curiosity and scientific detachment, sometimes found himself on the verge of unreasoning terror. The absence of a single familiar reference point can be utterly unnerving even to the coolest and clearest minds.

From CHILDHOOD'S END by Arthur C. Clarke (1953)

The city marched up out of the crimson haze, ever more awful, the bulk of it swelling to blot out half the red sky with gleaming black metal, the titanic machines that crowned it frowning down with the threat of unknown death. A palpable atmosphere of dread and horror hung over that unearthly metropolis, a sense of evil power and hostile strength, of ancient wisdom and monstrous science, for it had endured since the Earth was new.

The four ragged creatures on the raft gazed on those marching walls with a hopeless horror. Their minds sank prostrate with realization that unless their puny efforts could free the girl imprisoned there, the makers of this pile of black metal had also shaped the doom of mankind.

The city seemed dead at first, a somber necropolis, too old for any life. But presently they saw movement along the walls. A black spider-ship spread titanic vanes, and rose silently from a high platform to vanish in the red sky eastward.

"We must cover ourselves," said Jay Kalam. "They might be watching."

He had them screen the raft with broken branches, to look like driftwood. And the river carried them on toward the mighty wall. They were gazing upward in awe­struck silence when Hal Samdu cried: "See them moving! Above the wall!"

And the others could presently distinguish the creatures that moved—still tiny with many miles of distance—the ancient masters of this aged planet!

John Star had glimpsed one of the Medusas on Mars, that thing in the gondola swung from the black flier, whose weapon had struck him down. A swollen, greenish surface, wetly heaving; a huge, ovoid eye, luminous and purple. But these were the first he had fully seen.

They drifted above the wall like little green balloons. Their eyes were tiny dark points in their bulging sides—each had four eyes, spaced at equal distances about its circumference. From the lower, circular edge, like the ropes that would have suspended the car of a balloon, hung a fringe of black and whiplike tentacles.

John Star could see the superficial likeness, the dome shape, the fringing tentacles, that had earned them the name Medusae.

In the distance they did not look impressive. There was about them a certain grotesq«eness, a slow awkwardness. They didn't look intelligent. Yet in the way they moved, floating apparently at will above the black wall, was a power and mystery that made for respect. And in the knowledge that they were the builders of this black metropolis was room for awe and terror.

Scrambling over the immense bearing of the shaft, they found a little circular hole in the roof of the tank—it must have been left for attention to the bearings. They climbed through it, Giles Habibula sticking until the others pulled him out, and so at last, on top of the reservoir, they were fairly within the city.

They stood on the lower edge of a conical black metal roof, a dizzy drop of two thousand feet below them, and the slope too steep for comfort.

Standing there on that perilous brink, John Star felt a staggering impact of nightmare strangeness and bewildering confusion. Buildings, towers, stacks, tanks, machines, all loomed up about him, a black fantastic forest against the lurid sky, appallingly colossal. The tallest structures reached, he soberly estimated, two miles high.

If this black metropolis of the monstrous Medusae had order or plan, he did not grasp it. The black wall had seemed to enclose a regular polygon. But within all was strange, astounding, incomprehensible, to the point of stunning dismay.

There were no streets, but merely yawning cavernous abysms between mountainous black structures. The Medusae had no need of streets. They didn't walk, they floated! Doors opened upon sheer space, at any level from the surface to ten thousand feet.

The stupendous ebon buildings had no regular height or plan, some were square, some cylindrical or domed, some terraced, some—like the reservoir upon which they stood—sheerly vertical. All among them were bewildering machines of unguessable function—save that a few were apparently aerial or interstellar fliers, moored on landing stages—but all black, ugly, colossal; dread instrumentalities of a science older than the life of Earth.

From THE LEGION OF SPACE by Jack Williamson (1934)

Wheeled Aliens

Aliens with wheels are a difficult concept. There are problems with making worthwhile wheels using biology, and even more problems finding a plausible sequence where such a thing could be created by evolution.

In the real world the closest thing to an animal with wheels is the spinning flagellum of certain microscopic bacteria.

Wheeled aliens make an appearance in the satirical "Retief" story Retief's War, the g'Kek of Brightness Reef (looking like "a squid in a wheelchair" that suffer from arthritic axles when elderly), and in the Polarians of the Cluster novels (technically the Polarians do not use wheels, they roll around on large spheres).

A milder version is rolling aliens. They are generally shaped like a sphere or a disc harrow, the entire alien rolls instead of just part of the alien. There is a spherical alien in Arena and Tuf Voyaging, a cylindrical alien in Stadium Beyond the Stars, a disembodied wheel in A Star Called Cyrene, and disc harrow aliens called the Slash of the Cluster novels

     "Ah, a bit more than nothing," Turekian said. "A tiny bit. I just wish you were less convinced your science has the last word on all the possibilities. Things I've seen—"
     "I've heard your song before," Webner scoffed. "In a jungle on some exotic world you met animals with wheels."
     "Never said that. Hm-m-m … make a good yarn, wouldn't it?"
     "No. Because it's an absurdity. Simply ask yourself how nourishment would pass from the axle bone to the cells of the disc. In like manner—"

From WINGS OF VICTORY by Poul Anderson (1972)

     The boy slowed. An alien was squatting in the path. A Polarian.
     They drew up before the strange creature. It was a teardrop-shaped thing with a massive spherical wheel on the bottom and a limber tentacle or trunk at the top. When that tentacle reached straight up, it would be as high as Flint, and the body's mass was similar to his. But the Polarian had no eyes, ears, nose, or other appendages.
     The Shaman claimed they were similar to human beings because they liked similar gravity, breathed the same air—though they had no lungs—and had a similar body chemistry. Their brains were as massive and versatile as man's, and they were normally inoffensive. But they looked quite different, and such details as how they ate, reproduced, and eliminated were mysteries.
     But Flint had promised himself to treat the next alien he met with special courtesy. He and the boy halted politely. "Greetings, explorer," Flint said.
     The creature's body glowed with simulated pleasure. It put its stalk down to the ground. In this position it looked more than ever like a dinosaur dropping. Flint stifled a laugh.
     A little ball in the tip of the trunk spun rapidly. "Greetings, native," the ground said.
     Flint was not surprised. He had been familiar with the mechanism from infancy. The little ball vibrated against the ground—or any available surface—to produce intelligible sounds. As the Polarian had no mouth, it could not talk as humans did.

     Now he had done it! He had never suspected the creature would accept! Well, it couldn't be helped. "It is an emergency. We shall be hurrying."
     "I shall not impede you," the Polarian replied.
     Fat chance! But Flint smiled graciously. He gestured to the boy. "Show the way."
     The runner was off, sensing a race. This was firm, level ground, excellent for making time. Flint followed, stretching his legs.
     But Tsopi followed right along, rolling smoothly on her ball-wheel. She was at no disadvantage. Polarians could move rapidly and effortlessly when the terrain was right; their wheel was efficient. Flint had not before appreciated how efficient. On occasion he had wondered how the aliens kept themselves upright. The Shaman had remarked that a man on a unicycle performed the same feat. But there were no unicycles on Outworld.

     How did no-handed creatures manage to build such edifices? Again his memory provided the answer: Polarians were adept at circular manipulation of objects and concepts. They did not carry building blocks into place, they rolled building spheres into place. Where men laid bricks, Polarians rolled stones. Where men hammered nails, Polarians squeezed glue. The end result was rather similar, as though civilization shaped itself into certain configurations regardless of the sapient species invoking it. Here there were no square skyscrapers, but domed dunes serving the same purpose.
     They passed down a smooth ramp, where on Earth there would have been stairs. Of course; ramps were better for wheels, stairs for legs. Ramps were everywhere, contributing to the fluidity of the architectural design.
     They had to roll single file, for efficient progress through the throng. Tsopi's trail just ahead of him was sweet; she had a tantalizingly feminine taste.
     Taste? Flint concentrated, and it came: Polarians laid down taste trails with their wheels, much as humans laid down scent. No, more than that: These were actual, conscious signatures of passage, like the trails of Earthly snails. He remembered the first snail he had seen, beside the huge water of the ocean inlet, under the odd blue sky of Earth. Today he didn't even notice the color of the sky of a given planet; sky was sky color, right for its world. But this taste; every Polarian was really a super-bloodhound, sniffing out every other, all the time. It was the natural way. In fact, it was already difficult to imagine how it could be otherwise.

From CLUSTER by Piers Anthony (1977)

     The monster charged, when Herald was off-balanced from his effort. And suddenly he realized another point of affinity: the monster was like a Slash, his own kind! A Slash was a tubular creature with disks around its girth that it used for slicing out pathways, cutting up food, and dismembering enemies. It also had laser lenses for longer-range action. In his natural body, Herald could have met this creature on even terms, perhaps more than even terms. A Slash was smaller, but the lasers could score with devastating effect before the disks struck. But this Solarian host was a poor excuse for a combat creature.

From KIRLIAN QUEST by Piers Anthony (1978)

Tentacle Aliens

A tentacle is a "flexible, mobile, elongated organ present in some species of animals, most of them invertebrates" (technical term cephalopod limb). Since they are uncommon in familiar earthly animals, they became a popular characteristic on unearthly science fiction aliens.

This old trope dates back to prehistory, when the first man was freaked out when they discovered the octopus. In science fiction it dates back to at least 1898 with H. G. Well's War of the Worlds.

It lingers on in popular media. TV Tropes notes how be-tentacled creatures commonly use their tentacles in unique ways for combat (such as the old tentacle rope trick, that never gets old). As are tropes about the unexpected vulnerabilities of tentacles, such as the "knotty tentacle" trope.

According to Dr. Hans P. Moravec of the Carnegie-Mellon University, most land animals on Terra are "fractal."

Once upon a time animals were shaped like sticks (worms), and couldn't manipulate or even locomote very well. Then the sticks grew smaller sticks (arms and legs) and locomotion was much improved, and manipulation a little. Then the smaller sticks grew yet smaller sticks (fingers), and hands were invented, and manipulation got better.

Generalize the concept. I visualize a robot that looks like a tree, with a big stem, repeatedly branching into thinner, shorter and more numerous twigs, finally ending up in jillions of near-microscopic cilia. Each intermediate branch would have three or four degrees of freedom, an azimuth-elevation mount at its base, and an axial rotation joint at the top, where it connects to the next level of smaller twigs, and possibly also a length altering telescoping joint. To a large extent fewer degrees of freedom per level can be traded off for more levels. Each branch would also incorporate force sensing. Though each branch would be a rigid "mechanical" object, the overall structure would have an "organic" flexibility because of the great multitude of degrees of freedom.

So species that use tentacles figured out how to turn an arm or leg into a manipulative organ without needing to grow fingers.

Mechanically a tentacle is a "Muscular Hydrostat", consisting mainly of muscles with no skeletal support (an arm with no bones). It relies on the fact that water is effectively incompressible at physiological pressures, and the fact that muscles are mostly composed of water (i.e., it is hydraulic machinery). If the structure used pockets of water in separate compartments instead of watery muscles it would be a hydrostatic skeleton, but I digress.

Common examples of muscular hydrostats include octopus tentacles, elephant trunks, the entire body of a worm, and the human tongue.


Tentacles are mostly solid muscle.

Just like in animals with skeletons or exoskeletons, tentacle muscles can only provide force by contracting, expanding doesn't do diddly squat. So just like in conventional animal limbs all tentacle muscles are arranged in antagonistic pairs. If one muscle pulls to the left it is paired with an antagonist muscle that pulls to the right. As one muscle in the pair contracts the other relaxes.

The muscle fibers are oriented in three different directions:

Tentacle Muscles
Perpendicular to the long axis
LongitudinalParallel to the long axis
HelicalWrapped obliquely around the long axis

The closer the longitudinal muscles are located to the tentacle skin, the more elaborate bending movements are possible. Octopus arms, elephant snouts, and other manipulators all have this arrangement. You only see centrally located longitudinal muscles in limbs that just protrude in and out, like snake and anteater tongues.

Muscles perpendicular to the long axis can be in a circular, radial, or transverse pattern. Radial and transverse muscles are anchored to the external connective tissue by threads called "trabeculae" which penetrate the longitudinal and helical muscles that are in the way. Transverse muscles are in sheets that alternate between horizontal and vertical (the "down" direction is towards the side of the tentacle with suckers, technical term is "oral side").

Perpendicular Muscles
CircularRings around long axissquid tentacle
mammal tongue
RadialRadiating from center in a disk shapechambered nautilus tentacle
elephant trunk
TransverseAlternating between horizontal and verticaloctopus tentacle
human tongue

Helical or oblique fibers wrap around the long axis like candy cane stripes. They are usually in two or more layers of opposite chirality (left hand/right hand). The external and medial helixes are at an angle of 50 to 60° to the long axis, internal are at 40 to 50°. The role of the internal helical muscles is unclear.


Like all hydraulic machinery, the operating principle is the incompressibility of water, that is, if you push water into one end of a tube water will come spraying out of the other end. The important point is "incompressible" means the volume of water always stays the same. If you reduce a volume of water's dimension in one direction it will have to expand in at least one other dimension.

So, for instance, if the muscles squeeze the tentacle to reduce its diameter (height and width dimensions), the tentacle will elongate along the long axis (length dimension). Because the volume of tentacle has to always stay the same.

Elongation and Shortening

This is when the tentacle grows or shrinks along the long axis. Like when you stick out your tongue.

When the perpendicular (or helical) muscles contract (decreasing the tentacle's diameter) it elongates along the long axis (increasing the length). When the longitudinal muscles contract the tentacle shortens along the long axis (shortening the length) while simultaneously expanding hight and width (increasing the diameter).

So in this case the perpendicular muscles are operating antagonistic to the longitudinal muscles.

Some frogs can elongate their tongues up to 180% of its resting length. Due to hydraulics, the more the tongue is capable of elongating, the less force it can hit an object with.


Bending the tentacle is done by using the longitudinal muscle to reduce the length of the tentacle while other muscles act to prevent the length reduction on one side of the tentacle. This causes a bend on the opposite side of the tentacle.

Octopi apparently contract all of the longitudinal muscles while strategically using the perpendicular muscles to maintain a constant diameter at specific points.

Some tentacle robot limb designs have no perpendicular muscles. Instead they expand (using inflatable tubes instead of muscles) only some of the longitudinal muscle. The robot tentacle bends on the side of the uninflated tubes.


This is twisting the tentacle on the long axis, like it was a drill bit. It is done by contracting one of the two sets of hexlical muscles.


It is possible to make the tentacle rigid. The details are elusive but I would presume it can be done by contracting all the muscles at once.



Moving the tentacle to increase the distance between tentacle tip and tentacle base. The two basic types of reaches are:

Uncurling Reach: where the tentacle starts out rolled up in a spiral and rolls out.

Elongating Reach: where the arm starts out straight and grows longer.


Moving the tentacle to decrease the distance between tentacle tip and tentacle base. The three basic types of pulls are:

Continuum Curling Pull: where the arm rolls into a spiral.

Straight-arm Shortening: where the arm is straight and grows shorter.

Bending Pull: where the arm creates an elbow like bending point.


These are behaviors that are a lateral combination of sharp bends, sweeps, wraps, lifts, torsional rotations, drop, etc.

Mechanical Tentacles

Robot researchers have been experimenting with making tentacle-like robot arms and bodies. These use the same muscular geometry as tentacles but usually without the hydraulics. NASA had looked into this concept under the title "serpentuator".

Some use a set of contracting longitudinal and transverse muscles. Other just use pneumatically expanding arrays of longitudinal muscles and no transverse muscles.


Penn State Research Team Develops OctArm Soft Robot Manipulator

Recent interest in expanding the capabilities of robot manipulators has led to significant research in continuum manipulators. The idea behind these robots is to replace the serial chain of rigid links in conventional manipulators with smooth, continuous, and flexible links. Unlike traditional rigid-linked robots, continuum robot manipulators can conform to their surroundings, navigate through unstructured environments, and grasp objects using whole arm manipulation. Soft continuum manipulators can be designed with a large number of actuators to provide hyper-redundant operation that enables dexterous movement and manipulation with robust performance. This improved functionality leads to many applications in industrial, space, and defense robotics.

Previous continuum robots used cable-tendon and pressurized tube actuators with limited performance. Cable-tendons must be tensioned or the cables become snarled or fall off drive pulleys, limiting the robot speed. Pneumatic bellows have low shear stiffness, limiting load capacity. Thus, there exists a need for a highly dexterous, fast, and strong soft robot manipulators.

Dr. Christopher Rahn, Professor of Mechanical Engineering at Penn State along with his students Dustin Dienno and Mike Pritts, and assisted by Dr. Michael Grissom developed the OctArm manipulator using air muscle actuators. These actuators are constructed by covering latex tubing with a double helical weave, plastic mesh sheath to provide the large strength to weight ratio and strain required for soft robot manipulators.

OctArm is divided into three sections. Each section is capable of two axis bending and extension which allows nine degrees of freedom. The manipulators are actuated with pressurized air (Maximum pressure = 120 psi) pressure control valves and polyurethane connective tubing.

The air muscle actuators are optimized to provide the desired wrap angles and workspace. The distal section of each OctArm is designed to have a minimum wrap diameter of 10 cm. The length of each section is chosen so that the manipulator can provide a range of 360 degrees wrap angles to accommodate a wide range of objects sizes. To provide the desired dexterity, OctArm is constructed with high strain extensor actuators extend up to 80%.

To provide two-axis bending and extension, three control channels are used. selected. Six actuators are used in sections one and two and three actuators are used in section three. The six sections have two actuators for each control channel and results in actuators located at a larger radius, corresponding to higher stiffness and load capacity. Secondary layers of mesh sleeving are used to group individual actuators in control channels. Three closely-spaced actuators provide high curvature for the distal sections. The third, visible, mesh layer or fabric skin is designed to protect the manipulator from abrasion and wear.

For the field tests, OctArm was mounted to the second link of a Foster-Miller TALON platform. The control valves and two air tanks provided nine channels of controlled pneumatic pressure. Clemson University provided the control electronics and operation interface for these tests. The OctArm /Talon system underwent extensive field trials in the spring of 2005 at the Southwest Research Institute (SwRI) in San Antonio, Texas.

(ed note: original article has links to some PDF reports)


(ed note: the jennre blog summarizes the plot thusly: "A robot designer struggles to understand why the current generation of robots is so inefficient. Eventually, he realizes that the human form (of current generations of robots) is a skeuomorph."

In the novel, the Empire wants to negotiate a mining treaty with the Martians. Who have tentacles, by the way. The problem is that the key Martian ambassador is partial to a cocktail called a "Three Planets". Only a Martian bartender can make a proper Three Planets, something to do with using tentacles. Our Heroes are contracted to make a robot bartender capable of mixing a proper Three Planets. This is a problem, since if you add three drops of vuzd liquor to the drink it is incipid, but if you add four drops it tastes nasty.

Our Heroes enlist the aid of a Martian Bartender named Guzub.)

"I got one of those new electronic cameras — you know, one thousand exposures per second… So we took pictures of Guzub making a Three Planets, and I could construct this one to do it exactly right down to the thousandth of a second. The proper proportion of vuzd, in case you're interested, works out to three-point-six-five-four-seven-eight-two-three drops. It's done with a flip of the third joint of the tentacle on the down beat.

"It didn't seem right to use Guzub to make a robot that would compete with him and probably drive him out of business, so we've promised him a generous pension from the royalties on usuform barkeeps."

I took one sip and said, "Where's Guzub?... this Three Planets, it's perfect..."

Quinby opened a door. There sat the first original Quinby usuform — no remake of a Robinc model, but a brand-new creation. Quinby said, "Three Planets," and he went into action. He had tentacles, and the motions were exactly like Guzub's except that he himself was the shaker. He poured the liquids into his maw, joggled about, and then poured them out of a hollow hoselike tentacle.

(ed note: "Usuform" means a robot that is designed along functional lines, instead of stupidly forcing the design to look like a mental man)

From Q.U.R. by Anthony Boucher (1943)

Crystal Life

These are critters that look like large quartz crystals, often with flashing lights inside. Most are immobile, some can move. Some crystal life is silicon-based life, other are not.

An odd one was the Monolith Monsters. They were not invading aliens so much as an extraterrestrial chemical reaction. Instant monster: just add water.

In some cases the line between crystal life and electronic life is very blurry. The most obvious basis for such life is that it is based on semiconductor electronic circuits that somehow evolve and become more complicated inside the crystals.


(ed note: Mr. Miller apparently got a good bargain on a crate of exclamation points.)

It wakened a lustrous opalescence in the two great spheres (alien spaceships) that nestled like mighty twin pearls against the dark rock, to create beings of the rock and of the shadow, gliding wraithlike among the shattered boulders!

Painfully I crept through the dense growth of the brink, nearer to those great spheres and their dreadful cargo. Within me my brain whirled and throbbed, my throat froze against the cry of shocked incredulity that rushed to my lips, cold, clammy sweat oozed from gaping pores ! It was beyond all reason — all possibility ! And yet — it was! Now I could see them clearly, rank on rank of them in orderly file, some hundred of them, strewn in great concentric rings about the softly glowing spheres — harsh as the black rock itself, hard, and glittering, and angular — a man’s height and more from summit to base — great, glittering tetrahedra — tetrahedra of terror!

They were tetrahedra, and they were alive — living even as you and I! They stirred restlessly in their great circles, uneasy in the dim light. Here and there little groups formed, and sometimes they clicked together in still other monstrous geometric shapes, yet always they moved with an uncanny stillness, darting with utter sureness among the scattered rocks. And now from the nearer of the twin spheres came another of their kind, yet twice their size, the pearly walls opening and closing as by thought-magic for his passing! He swept forward a little, into the full light of the moon, and the rings followed him, centered about him, until the spheres lay beyond the outermost and the giant tetrahedron faced alone the hosts of his lesser fellows!

Then came their speech — of all things the most mind-wracking! I felt it deep within my brain, before I sensed it externally, a dull, heavy rhythm of insistent throbbing, beating at my temples and throwing up a dull red haze before my staring eyes!

“Yes, I’m Hawkins. The plane is somewhere over there, if it didn’t burn, with all your supplies in it. I was held up crossing the mountains. But tell me, first — those things, there — are they alive?”

“You've wondered that? I suppose anyone would. The Indians make them gods of a kind — realize they’re beyond all experience and tradition. But I'm a biologist. I have had some experience in strange forms of life. They are as much alive as we — perhaps even more than we. After all, if life is energy, why should it not rest where it will? Need we — soft, puny things of carbon and water and a few unstable elements — be the only things to harbor life? But this is no place to moralize — come on!”

And through the curtain where fire of heavens and fire of Earth met in that terrible holocaust, those three saw the curving flames of the twin spheres gape wide, saw huge angular shapes file from the darkness within — shapes never yet associated in the Mind of Man with the meaning of life! Careless of the flame that seethed about them, they glided out over the fusing rock of the valley floor, score on score of them, showing in the fierce glare as mighty, eight-foot tetrahedra of dark, glistening crystal. They were of a purple that seemed to be of the essence of the things themselves, rather than a pigmentation of their surface; and near one apex each had two green-yellow unstaring, unseeing eyes!

Within them one glimpsed a spherical body — purple too — from which ran hundreds of curious filaments to the smooth surfaces. Tetrahedra they were — living tetrahedra of chilling terror that feared neither flame nor lightning and spread destruction on every side!

Sick at heart the three men watched, while the flames died and the winds came and stripped the blanket of dust and ash from the blasted rock. The tetrahedra meanwhile glided about their endless affairs, forming and reforming in geometric pattern. Or they clicked swiftly into many-faceted forms that in turn mounted into monolithic, crystalline monstrosities, then melted with startling suddenness into their original components. These were idle, pointless maneuverings from the human viewpoint, yet fraught with some hidden meaning and purpose as alien to Earth as the things themselves. They suggested the terrible energies that were under their control — energies such as our little science has never hinted at.

Now, in the full light of day, I could see that it was as Professor Hornby had said. The tetrahedra were formed from some hard, crystalline mineral, black almost to invisibility, with a faint wash of rich purple running through it. As they moved, the sun sent up glittering flashes of brilliance from their polished flanks, dancing like little searchlight rays along the shadowed face of the forest. For the tetrahedra were restless, were weaving aimlessly in and out among the boulders in weird arabesques as of some unearthly dance of the crystal folk, were condensing in little groups of half a dozen or less that formed and broke again even as do restless humans, waiting impatiently for some anticipated event.

Apart from the rest, motionless in a sort of circular clearing among the rocks, squatted the giant leader of the tetrahedra. In him the deep violet of the crystal became a rich, plum-like hue, purple flushed with warm red, and the underlying black seemed less harsh. It was warmer and more like the calm velvet of the tropic night. But these are impressions, qualitative terms with which to distinguish him in some way other than by mere size from his fellows. To an observer, the distinction was apparent, but it is not easy to express in everyday terms. It must suffice that he was indefinably different from the others, that he seemed to have character and personality, where the rest were but pyramidal crystals, albeit terribly alive.

And now the giant leader was dinning out his mighty call in long, slow billows of beating sound that seemed to thrust me back, press me into the dark of the forest, away from the alien monsters of the valley! In response came thirty of the lesser tetrahedra, chosen seemingly at random from the scattered ranks, to range themselves at equal intervals about their master, forming a single great circle a dozen yards in diameter.

Again the throbbing call shattered against the cliffs about me, and now all the hordes of the tetrahedra broke into flowing motion, converging in a torrent of glittering purple crystal upon the natural amphitheater, clustering in threes at the spots that their fellows had marked — all but ten, who glided into place before every third group, forming a giant toothed wheel with hub and rim and spokes of living, sentient crystal — crystal with a purpose!

There under that blazing sun they lay, gleaming like giant purple gems against the jetty rock. I thought of the great stone wheel of Stonehenge, and of the other monolithic circles that men have found in England and on the Continent. Strange resemblance, between the pattern of living monsters of another world and the ancient temples of a prehistoric race! And yet, is it too far-fetched to suggest that the superstitious savages should pattern their greatest temples after the unearthly gods of their worship — gods of purple crystal that came and smote and vanished again into the skies, leaving the memory of their inevitable circling, and the thunder of their language in the great drums of worship? May it not be that they have come before, and found Earth unfitted for their usage, and passed on to other worlds? And if they have so come, and found us wanting, what lies beyond that has prevented them from bearing back the tale of their findings, marking Earth as useless for their tetrahedral purposes ? Why have they had to come again and again ?

I COULD see that the groups of three that formed the toothed rim of the giant crystal wheel were tipping inward, bringing their peaks together in a narrow focus, and more, that the ten that were the spokes, the binding members of the wheel, were of the same rich hue as their master. The shadows of the myriad tetrahedra squatted short and black about their shining bases, against the shining rock.

As the sun soared higher, pouring its blazing rays straight down upon the sweltering world, I sensed the beginning of a vague roseate glow at the foci of the circling trios, a glow as of energy, light, focussed by the tetrahedra themselves, yet not of themselves, but sucked from the flood of light that poured upon them from above. For the light that was reflected from their sides gleamed ever bluer, ever colder, as they drank in the warm red rays and spewed them forth again into the seething globes of leashed energy that were forming just beyond their pointing tips!

The rose-glow had deepened to angry vermillion, seemingly caged within the spheres defined by the tips of the tilted tetrahedra. Thirty glowing coals against the black, ninety great angular forms gleaming ghastly blue in the pillaged sunlight, forms that were slowly closing in upon the center, upon their mighty master, bearing him food, energy of the sun for his feasting!

Now the scarlet flame of the prisoned light was mounting swiftly in an awful pinnacle of outrageous color — pure fire torn from the warm rays of the sun — raw energy for the glutting of these tetrahedral demons of another world! It seemed to me that it must needs burst its bounding spheres and fuse all that crystal horde with its unleashed fury of living flame, must win free of the unimaginable forces that held it there between the eager, glittering facets, must burst its unnatural bonds and sweep the valley with a tempest of awful fire that would consign the furnace of the tetrahedra to pitiful insignificance! It did none of these, for the power that had reft it from the golden sunbeams could mould it to the use and will of the tetrahedra, as clay before the potter!

Slowly the great ring contracted, slowly the tetrahedra tipped toward their common center, bearing at their foci the globes of angry flame. Now they stopped, hung for a long moment in preparation. Then in an instant they loosed the cradled energy of the spheres in one mighty blaze of blinding crimson that swept out in a single huge sheet of flame, blanketing all the giant wheel with its glory, then rushing into the blazing vortex of its center. Here, all the freed energy of the flame was flowing into the body of the mighty ruler of the tetrahedra, bathing him in a fury of crimson light that sank into his glowing facets as water into parched sand of the desert, bringing a fresh, new glow of renewed life to his giant frame!

And now, as in recoil, there spouted from his towering peak a fine, thin fountain of pale blue fire, soundless, like the blaze of man-made lightning between two mightily energized electrodes — the blue of electric fire — the seepage of the giant’s feast! Like slaves snatching at the crumbs from their master’s board, the ten lesser tetrahedra crowded close. As their fierce hunger voiced itself in awful, yearning force, the fountain of blue flame split into ten thin tongues, barely visible against the black rock, that bent down into the pinnacles of the ten and poured through them into the crowding rim of the giant wheel, a rim where again the spheres of crimson fire were mounting to their climactic burst!

Again the crimson orbs shattered and swept over the horde in a titanic canopy of flame, and again the giant master drank in its fiery glory! Now the fountain of seepage had become a mighty geyser of sparkling sapphire light that hurtled a hundred feet into the shimmering atmosphere, and, bent by the fierce hungering of the lesser creatures, curved in a glorious parabola above the crystal wheel, down over them and into them, renewing their substance and their life!

For as I watched, each tetrahedron began to swell, visibly, creeping in horrid slow growth to a magnitude very little less than that of their giant leader. And as they mounted in size, the torrent of blue fire paled and died, leaving them glutted and expectant of the final stage!

It came, with startling suddenness! In an instant each of the hundred clustering monsters budded, burst, shattered into four of half its size that cleaved from each corner of the parent tetrahedron. They left an octahedral shape of transparent crystal, colorless and fragile, whence every evidence of life had been withdrawn into the new-born things — a shell that crumpled and fell in fine, sparkling crystal dust to the valley floor. Only the giant ruler lay unchanged beneath the downward slanting rays of the sun. The hundred had become four hundred! The tetrahedra had spawned!

Four hundred of the monstrous things where a hundred had lain the moment before! Drinking in the light of the noonday sun, sucking up its energy to give them substance, these tetrahedral beings from an alien world held it in their power to smother out the slightest opposition by sheer force of ever-mounting numbers! Against a hundred, or four hundred, the armies and the science of mankind might have waged war with some possibility of success, but when each creature of these invulnerable hosts might become four, with the passing of each noon’s sun, surely hope lay dead ! Man was doomed!

“Do you realize that this spawning means that they’re ready to go ahead and burn their way right through everything — make this whole planet a safer and better place for tetrahedra? Doc has figured they’re from Mercury — overcrowded, probably, by this wholesale system of reproduction in job-lots, and hunting for new stamping-grounds. I don’t know what our chances are of bucking them — about a quarter of what they were an hour ago — but they’re mighty slim, armed as we are. You’ve got the other machine-gun?”

I HAD no trouble in finding the Professor. In truth, he found me. He was all but boiling over with excitement, for he had seen something we had not. “Hawkins,” he exclaimed, grabbing my shoulder fiercely, “did you see them spawn? It is remarkable — absolutely unequalled! The speed of it all — and, Hawkins, they do not have to grow before cleaving. I saw two that divided and redivided into three-inch tetrahedra — over a thousand of them ! Think of it — Hawkins, they can overrun our little planet in a few days, once they start! We’re done for!”

Now, their army of destruction assembled, the tetrahedra began their conquest of Earth! In vast waves of horrid destruction with rays of angry yellow flame darting from apexes their flaming floods of energy swept over the jungle, and now not even its damp dark could resist. Mighty forest-giants toppled headlong, by the cleaving yellow flame, to melt into powdery ash before they touched the ground. Giant lianas writhed like tortured serpents as their juices were vaporized by the awful heat, then dropped away in death to lie in long grey coils along the stripped rock of the forest floor — rock that was fast taking on the glassy glare of the little valley, rock fused by heat such as Earth had never known.

Now we could watch their plan of campaign, and our hearts sank in fear for our race, for while half of the tetrahedral army engaged in its holocaust of destruction, the remaining half fed and spawned in the full blaze of the sun. With every day dozens of square miles were added to their hellish domain and thousands of tetrahedra to their unnatural army. For now we could see that more and more of them were taking the second course, were splitting into hosts of tiny, three-inch creatures which, within a few days’ time, had swelled to full size and on the following day could spawn anew!

The yelling circle was thinning fast, yet they had not realized the futility of their attack when suddenly the tetrahedra deserted quiet defense for active combat!

The cause was evident. Five Indians on the upslope had shoved over the cliff a huge rounded boulder that bounded like a live thing among the rocks and crashed fufl into the side of a great eight-foot tetrahedron, splintering its flinty flank and freeing the pent-up energy in a blinding torrent of blue flame that cascaded over the nearby ledges, fusing them into a white-hot, smoking pool of molten lava that glowed evilly in the ill-lit gloom! It was the last straw! The mad attack had become a thing of real menace to the tetrahedra, and they sprang into swift retribution. From their apexes they flashed out the flaming yellow streaks of destruction.

Ever since Marston had first mentioned Professor Hornby’s theory that the things were Mercutians, I had been trying to find some way of verifying it. Now that we were in semi-intimate terms with the tetrahedra, I wondered if I might not get them, somehow, to supply this evidence. I thought of stories I had read of interplanetary communication — of telepathy, of word-association, of sign-language. They had all seemed far-fetched to me, impossible of attainment, but I resolved to try my hand at the last.

There was some rather soft rock in the structure of the watch-tower, and as Valdez had rescued my tool kit from the plane, I had a hammer and chisel. With these, and a faulty memory, I set out to make a rough scale diagram of the inner planets, leaning a bit on the Professor’s theory. I cut circular grooves for the orbits of the four minor planets — Mercury, Venus, Earth, Mars — and dug a deep central pit. In this I set a large nugget of gold, found in the ruins of the fortress, for the Sun, and in the grooves a tiny black pebble for Mercury, a large white one for Venus, and a jade bead from the ruins for Earth. Earth had a very small white moon, in its own deep-cut spiral orbit. Mars was a small chunk of rusty iron with two grains of sand for moons. I had a fair-sized scale, and there was no room for more.

Now I was prepared to attempt communication with the tetrahedra, but I wanted more than one diagram to work with. Consequently I attempted a map of Earth, with hollowed oceans and low mountain-ridges.

A cloud-burst, it would be called in the United States. The heavens opened in the night, and water fell in torrents, streaming from every angle of the rock, standing in pools wherever a hollow offered itself, drenching us and the world through and through. Day came, but there was no sun for the tetrahedra to feed on. Nor were they thinking of feeding, for very definite peril threatened them. To the tetrahedra, water was death!

As I have said, their fires had flaked huge slabs of rock from the walls of the ravine leading from the high-walled valley where they slept, choking its narrow throat with shattered stone. And now that the mountain slopes, shorn of soil and vegetation, were pouring water into its bed, the stream that had carved that ravine found its course dammed — rose against it, poured over it, but not until the valley had become a lake, a lake where only the two pearly spheres floated against the rocky wall, the thousands of tetrahedra gone forever — dissolved!

Water was death to them — dissolution! Only in the shelter of the spheres was there safety, and they were long since crowded. The hordes of the tetrahedral monsters perished miserably in the night, before they could summon the forces that might have spun them a fiery canopy of arching lightnings that would drive the water back in vapor and keep them safely dry beneath. A hundred had come in the twin spheres. A hundred thousand had been born. A bare hundred remained.

(ed note: Our heroes use the the map of the solar system to explain to the tetrahedrons that [a] water is death to tetrahedrons, [b] Earth is 75% water, [c] right now Earth is in the dry season. Implication is that if the cloud-burst that killed 99.9% of their invasion force happens in the dry season, the wet season will be utterly deadly. Perhaps it would make more sense to go invade Mars?)

From TETRAHEDRA OF SPACE by P. Schuyler Miller (1931)

Electronic Life

What if an alien ecosystem is not composed of organic life based on chemistry, but instead on cybernetic life based on electronics?

In science fiction there are two main trends:

  • a high tech organic species creates a robotic species capable of reproduction. The robots start spreading across the galaxy. They may or may not go full Skynet on the parent organic species. And it is possible the organic species created the robot species by accident, usually by created a von Neumann machine which unexpectedly evolves.
  • a sufficiently weird planet manages to naturally create a native species based on electronics.

      One night, during their absence, and close to the front door, something grew. The scientists, after long conference, decided it was a plant but it didn't look like a plant.
     It was a triangular mirror balanced on a cable-like stem as thick as a man's wrist. The "mirror" followed the sun and, at evening or on dull days, folded itself up geometrically into a neat square black box.
     Two days later there was another growth. This was a small brass colored sphere about the size of a walnut perched on the top of a thin black rod about two feet in height.
     An intrigued expert touched it with his hand and was flung untidily to the path. He was not dead but the local hospital had some difficulty bringing him round. A diagnostician pronounced near-lethal electric shock.

     The door of Lipscombe's house had been open and on the path was—It had looked like an oxygen cylinder some six feet in length and supported itself on thin legs like black cables. On top, near the thicker end, something spun rapidly, catching the sunlight. He'd had the curious impression that something was watching through it and the thought radar vision had occurred to him before the soldiers had bustled him out of the room.

     After much security checking they were finally admitted to a pleasantly furnished recreation room where a group of obvious scientists were arguing fiercely.
     "But, my dear fellow, why should such a conclusion strike you as pure fantasy?"
     "Because the conception in itself is preposterous—the term natural electronic life is a sheer absurdity."
     "Why should it be? When one considers the incredible complication of normal organic life why should not a simpler, less complex life form evolve in a different environment. A planet with a highly radio-active crust, a chemical atmosphere and, possibly, rich surface metal deposits and you have the perfect incubator for electronic life to develop. Consider, an almost pure copper deposit, a few drops of acid from the chemical atmosphere, a natural vein of metallic ore and you have not only natural electricity, but the basis for a natural circuit, or, if you prefer it, an electronic nervous system. Have you read what Mayer deduced during his experiments with radio-active crystals, for example?"
     "I have, but the fact that these artificial cells developed apparent reflexes is no basis for presupposing the preposterous."
     "My God, man, you conceded yesterday that we can make organic life cells in a laboratory. You must also concede, therefore, that this same organic life has evolved naturally on this planet. Why, then, knowing also that electrical life has also been constructed in a laboratory will you not admit the possibility of electronic life evolving naturally?"
     "I still find the conception of intelligent life housed in a metallic body and based on series of circuits wildly improbable. The theory of outworld invasion by what you call an electronic life form is, in my opinion, sheer imagination and owes nothing whatever to applied science."

"What good do they think all these troops and weapon experts will do? They're still thinking in terms of an outworld alien invader with super weapons and they're not. Why the hell can't they see that? This is a minor occupation force sitting on its presumed backside in one of the safest conquests it's possible to conceive. They've introduced their own ecology into this environment and, because it's dominant in respect of our own, we're going under. Oh yes, I know it looks horrible and alien to our own but the principle is the same. We have sparrows and they introduce hawks. We have oak trees but along comes a strangling ivy; it's as simple as that. Some of the alleged machines our troops are now reporting may be equivalent of wolves or tigers and not armored vehicles at all. Once or twice they have opened up with something new, but this, I think, is reflex. At times we may be an irritant and the aliens take a smack at us like a dozing man slapping at a fly. They can afford to doze, chuck a few seeds out of the window, let loose some hawks and nature will do the job for them. Not too far in the future they can step out of the front door into world which is seeded, prepared and ready for them. Their peculiar ecology will have removed anything alien which might once have cluttered up the place."

From NO TRUCE WITH TERRA by Philip E. High (1964)

It was a world that had never known a sun. For more than a billion years, it had hovered midway between two galaxies, the prey of their conflicting gravitational pulls. In some future age the balance would be tilted, one way or the other, and it would start to fall across the light-centuries, down toward a warmth alien to all its experience.

Now it was cold beyond imagination; the intergalactic night had drained away such heat as it had once possessed. Yet there were seas there—seas of the only element that can exist in the liquid form at a fraction of a degree above absolute zero. In the shallow oceans of helium that bathed this strange world, electric currents once started could flow forever, with no weakening of power. Here superconductivity was the normal order of things; switching processes could take place billions of times a second, for millions of years, with negligible consumption of energy. It was a computer’s paradise. No world could have been more hostile to life, or more hospitable to intelligence.

And intelligence was there, dwelling in a planet-wide incrustation of crystals and microscopic metal threads. The feeble light of the two contending galaxies—briefly doubled every few centuries by the flicker of a supernova—fell upon a static landscape of sculptured geometrical forms. Nothing moved, for there was no need of movement in a world where thoughts flashed from one hemisphere to the other at the speed of light. Where only information was important, it was a waste of precious energy to transfer bulk matter.

Yet when it was essential, that, too, could be arranged. For some millions of years, the intelligence brooding over this lonely world had become aware of a certain lack of essential data. In a future that, though still remote, it could already foresee, one of those beckoning galaxies would capture it. What it would encounter, when it dived into those swarms of suns, was beyond its power of computation.

So it put forth its will, and myriad crystal lattices reshaped themselves. Atoms of metal flowed across the face of the planet. In the depths of the helium sea, two identical subbrains began to bud and grow…

Once it had made its decision, the mind of the planet worked swiftly; in a few thousand years, the task was done. Without a sound, with scarcely a ripple in the surface of the frictionless sea, the newly created entities lifted from their birthplace and set forth for the distant stars.

(ed note: spoiler alert: the cold intelligence discoveres many civilization in our galaxy, but all of them are organic warm-life like us. The cold intelligence rejects as nonsense the warm-life claim that they created computers, since warm-life is limited and inferior. The cold intelligence figures that warm-life is merely enslaving computers. So the cold intelligence embarks upon a crusade to exterminate all warm-life in our galaxy. )

From CRUSADE by Arthur C. Clarke (1968)

(ed note: Terran civilization has become very high tech. They have lots of self-reproducing self-maintaining sun-powered gadgets. Nuclear war looms. A group of fifty scientist run away to Tau Ceti to make an interstellar colony. Years later they go back to visit Terra. Due to the weird properties of their faster-than-light starship, they arrive at Terra about three billion years after they left.)

(They are rather startled to discover that humans are extinct, and there is now an ecosystem based on cybernetic life.)

Earth rolled into sight. The planetary disc was still edged with blueness darkening toward black. Clouds still trailed fleecy above shining oceans; they gleamed upon the darkness near the terminator as they caught the first light before sunrise. Earth was forever fair.

But the continental shapes were new, speckled with hard points of reflection upon black and ocher where once they had been softly green and brown. There were no polar caps; sea level temperatures ranged from eighty to two hundred degrees Fahrenheit. No free oxygen remained: the atmosphere was nitrogen, its oxides, ammonia, hydrogen sulfide, sulfur dioxide, carbon dioxide, and steam. Spectroscopes had found no trace of chlorophyll or any other complex organic compound. The ground cover, dimly glimpsed through clouds, was metallic.

His name was a set of radio pulses. Converted into equivalent sound waves, it would have been an ugly squawk; so because he, like any consciousness, was the center of his own coordinate system, let him be called Zero.

He was out hunting that day. Energy reserves were low in the cave. That other who may be called One—being the most important dweller in Zero's universe—had not complained. But there was no need to. He also felt a dwindling potential. Accumulators grew abundantly in their neighborhood, but an undue amount of such cells must be processed to recharge One while she was creating. Motiles had more concentrated energy. And, of course, they were more highly organized. Entire parts could be taken from the body of a motile, needing little or no reshaping for One to use. Zero himself, though the demands on his functioning were much less, wanted a more easily assimilated charge than the accumulators provided.

In short, they both needed a change of diet.

The sky was still light when he came on spoor: broken earthcrystals not yet healed, slabs cut from several boles, a trace of lubricant. Tuning his receptors to the highest sensitivity, he checked all the bands commonly made noisy by motiles. He caught a low-amplitude conversation between two persons a hundred miles distant, borne this far by some freak of atmospherics; closer by he sensed the impulses of small scuttering things, not worth chasing; a flier jetted overhead and filled his perception briefly with static. But no vibration of the big one. It must have passed this way days ago and now be out of receptor-shot.

A nearly full moon rose over the hills like a tiny cold lens. Night vapors glowed in masses and streamers against a purple-black sky where stars glittered in the optical spectrum and which hummed and sang in the radio range. The forest sheened with alloy, flashed with icy speckles of silicate. A wind blew through the radiation-absorber plates overhead, setting them to ringing against each other; a burrower whirred, a grubber crunched through lacy crystals, a river brawled chill and loud down a ravine toward the valley below.

Once he tapped lubricant from a cylinder growth and once he thinned his acids with a drink of water. Several times he felt polarization in his energy cells and stopped for a while to let it clear away: he rested.

Swiftly, he prepared himself. First he considered his ordinary weapons. The wire noose would never hold the monster, nor did he think the iron hammer would smash delicate moving parts (it did not seem to have any), or the steel bolts from his crossbow pierce a thin plate to short out a crucial circuit. But the clawed, spearheaded pry bar might be of use. He kept it in one hand while two others unfastened the fourth and laid it with his extra armament in the carrier rack. Thereupon they deftly hooked his cutting torch in its place. No one used this artificial device except for necessary work, or to finish off a big motile whose cells could replace the tremendous energy expended by the flame, or in cases of dire need.

A tree is a tree, anywhere and anywhen, no matter how intricate its branching or how oddly shaped its leaves and blossoms. But what is a—

—thick shaft of gray metal, planted in the sand, central to a labyrinthine skeleton of straight and curved girders, between which run still more enigmatic structures embodying helices and toruses and Möbius strips and less familiar geometrical elements; the entire thing some fifty feet tall; flaunting at the top several hundred thin metal plates whose black sides are turned toward the sun?

There was no soil, only sand, rusty red and yellow. But outside the circle which had been devastated by the boat's jets, Darkington found the earth carpeted with prismatic growths, a few inches high, seemingly rooted in the ground. He broke one off for closer examination and saw tiny crystals, endlessly repeated, in some transparent siliceous material: like snowflakes and spiderwebs of glass. It sparkled so brightly, making so many rainbows, that he couldn't well study the interior. He could barely make out at the center a dark clump of … wires, coils, transistors?

They walked among surrealistic rods and frames and spirals, under ringing sheet metal. The crystals crunched beneath their tread and broke sunlight into hot shards of color. But not many rays pushed through the tangle overhead; shadows were dense and restless. Darkington began to recognize unrelated types of structure. They included long, black, seemingly telescopic rods, fringed with thin plates; glassy spheres attached to intricate grids; cables that looped from girder to girder. Frequently a collapsed object was seen crumbling on the ground.

Frederika looked at several disintegrated specimens, examined others in good shape, and said: "I'd guess the most important material, the commonest, is an aluminum alloy. Though—see here—these fine threads embedded in the core must be copper. And this here is probably manganese steel with a protective coating of … um … something more inert."

Darkington peered at the end of a broken strut through a magnifying glass. "Porous," he said. "Good Lord, are these actually capillaries to transport water?"

It stirred among shadows, behind a squat cylinder topped with the usual black-and-mirror plates. Perhaps three feet long, six or eight inches high … It came out into plain view. Darkington glimpsed a slim body and six short legs of articulated dull metal. A latticework swiveled at the front end like a miniature radio-radar beamcaster. Something glinted beadily beneath, twin lenses? Two thin tentacles held a metal sliver off one of the great stationary structures. They fed it into an orifice, and sparks shot back upward—

The thing stopped in its tracks. The front-end lattice swung toward the humans. Then the thing was off, unbelievably fast. In half a second there was nothing to see.

"It was eating that strut." Frederika walked over to the piece of metal which the runner had dropped. She picked it up and came stiffly back with it. "See, the end has been ground away by a set of coarse emery wheels or something. You couldn't very well eat alloy with teeth like ours. You have to grind it."

Somehow they found themselves pushing on. Once, crossing an open spot where only the crystals stood, they spied something in the air. Through binoculars, it turned out to be a metallic object shaped vaguely like an elongated manta. Apparently it was mostly hollow, upborne by air currents around the fins and propelled at low speed by a gas jet. "Oh, sure," Frederika muttered. "Birds."

(ed note: Zero appears and charges)

Time slowed for Darkington, he had minutes or hours to tug at his gun, hear Frederika call his name, see Kuroki take aim and fire. The shape was mountainous before him. Nine feet tall, he estimated in a far-off portion of his rocking brain, three yards of biped four-armed monstrosity, head horned with radio lattice, eyes that threw back sunlight in a blank glitter, grinder orifice and— The rocket exploded. The thing lurched and half fell. One arm was in ruins.

(ed note: Zero captures the humans. He finds them puzzling.)

The units Zero had captured were making considerable sound-wave radiation. If not simply the result of malfunction in their damaged mechanism, it must be produced by some auxiliary system which they had switched on through interior controls. Zero's sound receptors were not sensitive enough to tell him whether the emission was modulated. Nor did he care. Certain low forms of motile were known to have well-developed sonic parts, but anything so limited in range was useless to him except as a warning of occurrences immediately at hand. A person needed many square miles to support himself. How could there be a community of persons without the effortless ability to talk across trans-horizon distances?

That energy drain left him ravenous. He scouted the forest in a jittery spiral until he found some accumulators of the calathiform sort (cup-shaped; concave). A quick slash with his pry bar exposed their spongy interiors, rich with energy storage cells and mineral salts. They were not very satisfying eaten unprocessed, but he was too empty to care. With urgency blunted, he could search more slowly and thoroughly. Thus he found the traces of a burrow, dug into the sand, and came upon a female digger. She was heavy with a half-completed new specimen and he caught her easily. This too would have been better if treated with heat and acid, but even raw the materials tasted good in his grinder.

From EPILOGUE by Poul Anderson (1962)

Energy Creatures

Energy creatures have a biological basis of patterns of energy with little or no matter involved. In science fiction they are usually fuzzy glowing balls or are totally invisible. Living ball-lightning.

In James Blish's The Star Dwellers, the "angels" are a species of energy creature that inhabit nebulae, and love to curl up in the cozy warmth of a starship's Nernst-effect fusion reactor. They are long-lived, the eldest were born shortly after the birth of the universe about 13 billion years ago. The Starfish from Glen Cook's Starfishers are vast creatures composed of fusion fires and magnetic fields. The human Starfishers protect the Starfish from the "sharks", and in exchange the Starfish give "ambergris nodes" which are the sine qua non of tachyon communication equipment. Magnetic nebula life appears in William Tedford's Nemydia Deep and "magnetovores" (i.e., organisms that consume magnetism) living in the solar corona are in David Brin's Sundiver. There are photovores around the galactic core in Gregory Benford's Sailing Bright Eternity (also described in Benford's article in the August 1995 issue of Fantasy & Science Fiction magazine, A Scientist's Notebook: Life at Galactic Center).

And many more.

The natural habitat of such creatures in science fiction is commonly in the interiors of stars or nebulae. Pulp scifi often have energy creatures native to Sol visiting Mercury, where they are encountered by human astronauts. Electromagnetic creatures in pulp scifi often cause mischief by zipping through telegraph and power lines, and radio beams. Since they presumably eat energy they are immune to most weapons, and have a nasty habit of sucking power plants dry of electricity.

And if you are an old geezer like me, the episode of Jonny Quest called "The Invisible Monster" which featured an energy creature scared the living poop out of you. At least in Jonny Quest polls, that's the ep which wins the "most scary" vote.


"You can't beat the Drej. They're pure energy!"

Korso, Titan A.E.

Creatures that dispense with the need to have a body altogether.

Energy Beings are frequently Sufficiently Advanced or Precursors; in fact, non-physicality is a common prerequisite, though they may take on A Form You Are Comfortable With. Often times this means that when they "Touch" a corporeal being it has interesting side effects. Other times they are the result of when a species Ascend to a Higher Plane of Existence. e Even cheaper and simpler to pull off than Rubber-Forehead Aliens, which explains why Energy Beings and Human Aliens so often dominate the demographics of The Final Frontier. The Angelic Aliens and Starfish Aliens especially may appear in this form.

Never mind that being "made of energy" makes as much sense as being "made of weight". This shows that many still don't know what the word "energy" really means.

It is also considered nonsensical because "energy" beings usually act more like floating clouds of luminescent gas. So a better term to use here might be "gaseous beings". Or, they might just plain be made of stars.

They may often be used as a way to represent gods, angels, the afterlife, and similar subjects without dealing with the religious connotations normally attached to them.

Of course, Energy Beings share many characteristics usually ascribed to concepts such as spirits and souls, often making them an example of Sufficiently Analyzed Magic.

See Made of Magic for a more fantastic version.

See also: Evolutionary Levels and Hollywood Evolution, as well as Ball of Light Transformation.

(ed note: see TV Trope page for list of examples)


(ed note: In this charming story from those innocent days when astronomers thought Mercury was tidally braked to Sol. The scene is an observatory based in the twilight belt, observing the Sun on every conceivable frequency. One fine day there is a record-breaking solar prominence. As the astronomers frantically observe it they notice something ... odd. )

      Though this had happened half a dozen times before, it was always exciting. It meant that we could capture some of the very substance of the sun as it went hurtling past in a great cloud of electrified gas. There was no danger; by the time it reached us it would be far too tenuous to do any damage, and, indeed, it would take sensitive instruments to detect it at all.
     One of those instruments was the Observatory’s radar, which was in continual use to map the invisible ionised layers that surround the sun for millions of miles. This was my department; as soon as there was any hope of picking up the oncoming cloud against the solar background, I aimed my giant radio mirror toward it.
     It came in sharp and clear on the long-range screen—a vast, luminous island still moving outward from the sun at hundreds of miles a second. At this distance it was impossible to see its finer details, for my radar waves were taking minutes to make the round trip and to bring me back the information they were presenting on the screen. Even at its speed of not far short of a million miles an hour, it would be almost two days before the escaping prominence reached the orbit of Mercury and swept past us toward the outer planets. But neither Venus nor Earth would record its passing, for they were nowhere near its line of flight.

     I switched to the short-range scanner, and the image of the cloud expanded so enormously that only its central portion was on the screen. At the same time I began to change frequency, tuning across the spectrum to differentiate among the various levels. The shorter the wave length, the farther you can penetrate into a layer of ionised gas; by this technique I hoped to get a kind of X-ray picture of the cloud’s interior.
     It seemed to change before my eyes as I sliced down through the tenuous outer envelope with its trailing arms, and approached the denser core. ‘Denser’, of course, was a purely relative word; by terrestrial standards even its most closely packed regions were still a fairly good vacuum. I had almost reached the limit of my frequency band, and could shorten the wave length no farther, when I noticed the curious, tight little echo not far from the centre of the screen.
     It was oval, and much more sharp-edged than the knots of gas we had watched adrift in the cloud’s fiery streams. Even in that first glimpse, I knew that here was something very strange and outside all previous records of solar phenomena. I watched it for a dozen scans of the radar beam, then called my assistant away from the radio-spectrograph, with which he was analysing the velocities of the swirling gas as it spun toward us.

     ‘Look, Don,’ I asked him, ‘have you ever seen anything like that?’
     ‘No,’ he answered after a careful examination. ‘What holds it together? It hasn’t changed its shape for the last two minutes.’
     ‘That’s what puzzles me. Whatever it is, it should have started to break up by now, with all that disturbance going on around it. But it seems as stable as ever.’
     ‘How big would you say it is?’ I switched on the calibration grid and took a quick reading.
     ‘It’s about five hundred miles long, and half that in width.’
     ‘Is this the largest picture you can get?’
     ‘I’m afraid so. We’ll have to wait until it’s closer before we can see what makes it tick.’
     Don gave a nervous little laugh.
     ‘This is crazy,’ he said, ‘but do you know something? I feel as if I’m looking at an amoeba under a microscope.’
     I did not answer; for, with what I can only describe as a sensation of intellectual vertigo, exactly the same thought had entered my mind.

     We forgot about the rest of the cloud, but luckily the automatic cameras kept up their work and no important observations were lost. From now on we had eyes only for that sharp-edged lens of gas that was growing minute by minute as it raced towards us. When it was no farther away than is the moon from Earth, it began to show the first signs of its internal structure, revealing a curious mottled appearance that was never quite the same on two successive sweeps of the scanner.
     By now, half the Observatory staff had joined us in the radar room, yet there was complete silence as the oncoming enigma grew swiftly across the screen. It was coming straight toward us; in a few minutes it would hit Mercury somewhere in the centre of the daylight side, and that would be the end of it—whatever it was. From the moment we obtained our first really detailed view until the screen became blank again could not have been more than five minutes; for every one of us, that five minutes will haunt us all our lives.
     We were looking at what seemed to be a translucent oval, its interior laced with a network of almost invisible lines. Where the lines crossed there appeared to be tiny, pulsing nodes of light; we could never be quite sure of their existence because the radar took almost a minute to paint the complete picture on the screen—and between each sweep the object moved several thousand miles. There was no doubt, however, that the network itself existed; the cameras settled any arguments about that.

     So strong was the impression that we were looking at a solid object that I took a few moments off from the radar screen and hastily focused one of the optical telescopes on the sky. Of course, there was nothing to be seen—no sign of anything silhouetted against the sun’s pock-marked disc. This was a case where vision failed completely and only the electrical senses of the radar were of any use. The thing that was coming toward us out of the sun was as transparent as air—and far more tenuous.

     As those last moments ebbed away, I am quite sure that every one of us had reached the same conclusion—and was waiting for someone to say it first. What we were seeing was impossible, yet the evidence was there before our eyes. We were looking at life, where no life could exist…
     The eruption had hurled the thing out of its normal environment, deep down in the flaming atmosphere of the sun. It was a miracle that it had survived its journey through space; already it must be dying, as the forces that controlled its huge, invisible body lost their hold over the electrified gas which was the only substance it possessed.
     Today, now that I have run through those films a hundred times, the idea no longer seems so strange to me. For what is life but organised energy? Does it matter what form that energy takes—whether it is chemical, as we know it on Earth, or purely electrical, as it seemed to be here? Only the pattern is important; the substance itself is of no significance. But at the time I did not think of this; I was conscious only of a vast and overwhelming wonder as I watched this creature of the sun live out the final moments of its existence.
     Was it intelligent? Could it understand the strange doom that had befallen it? There are a thousand such questions that may never be answered. It is hard to see how a creature born in the fires of the sun itself could know anything of the external universe, or could even sense the existence of something as unutterably cold as rigid nongaseous matter. The living island that was falling upon us from space could never have conceived, however intelligent it might be, of the world it was so swiftly approaching.
     Now it filled our sky—and perhaps, in those last few seconds, it knew that something strange was ahead of it. It may have sensed the far-flung magnetic field of Mercury, or felt the tug of our little world’s gravitational pull. For it had begun to change; the luminous lines that must have been what passed for its nervous system were clumping together in new patterns, and I would have given much to know their meaning. It may be that I was looking into the brain of a mindless beast in its last convulsion of fear—or of a godlike being making its peace with the universe.

     Then the radar screen was empty, wiped clean during a single scan of the beam. The creature had fallen below our horizon, and was hidden from us now by the curve of the planet. Far out in the burning dayside of Mercury, in the inferno where only a dozen men have ever ventured and fewer still come back alive, it smashed silently and invisibly against the seas of molten metal, the hills of slowly moving lava. The mere impact could have meant nothing to such an entity; what it could not endure was its first contact with the inconceivable cold of solid matter.
     Yes, cold. It had descended upon the hottest spot in the solar system, where the temperature never falls below seven hundred degrees Fahrenheit and sometimes approaches a thousand. And that was far, far colder to it than the Antarctic winter would be to a naked man.
     We did not see it die, out there in the freezing fire; it was beyond the reach of our instruments now, and none of them recorded its end. Yet every one of us knew when that moment came, and that is why we are not interested when those who have seen only the films and tapes tell us that we were watching some purely natural phenomenon.
     How can one explain what we felt, in that last moment when half our little little world was enmeshed in the dissolving tendrils of that huge but immaterial brain? I can only say that it was a soundless cry of anguish, a death pang that seeped into our minds without passing through the gateways of the senses. Not one of us doubted then, or has ever doubted since, that he had witnessed the passing of a giant.

     We may have been both the first and the last of all men to see so mighty a fall. Whatever they may be, in their unimaginable world within the sun, our paths and theirs may never cross again. It is hard to see how we can ever make contact with them, even if their intelligence matches ours.
     And does it? It may be well for us if we never know the answer. Perhaps they have been living there inside the sun since the universe was born, and have climbed to peaks of wisdom that we shall never scale. The future may be theirs, not ours; already they may be talking across the light-years to their cousins in other stars.
     One day they may discover us, by whatever strange senses they possess, as we circle around their mighty, ancient home, proud of our knowledge and thinking ourselves lords of creation. They may not like what they find, for to them we should be no more than maggots, crawling upon the skins of worlds too cold to cleanse themselves from the corruption of organic life.
     And then, if they have the power, they will do what they consider necessary. The sun will put forth its strength and lick the faces of its children; and thereafter the planets will go their way once more as they were in the beginning—clean and bright… and sterile.

From OUT OF THE SUN by Arthur C. Clarke (1958)

And now, out among the stars, evolution was driving toward new goals. The first explorers of Earth had long since come to the limits of flesh and blood; as soon as their machines were better than their bodies, it was time to move. First their brains, and then their thoughts alone, they transferred into shining new homes of metal and of plastic.

In these, they roamed among the stars. They no longer built spaceships. They were spaceships.

But the age of the Machine-entities swiftly passed. In their ceaseless experimenting, they had learned to store knowledge in the structure of space itself, and to preserve their thoughts for eternity in frozen lattices of light. They could become creatures of radiation, free at last from the tyranny of matter.

Into pure energy, therefore, they presently transformed themselves; and on a thousand worlds, the empty shells they had discarded twitched for a while in a mindless dance of death, then crumbled into rust.

Now they were lords of the galaxy, and beyond the reach of time. They could rove at will among the stars, and sink like a subtle mist through the very interstices of space. But despite their godlike powers, they had not wholly forgotten their origin, in the warm slime of a vanished sea.

And they still watched over the experiments their ancestors had started, so long ago.

From 2001 A SPACE ODYSSEY by Arthur C. Clarke

He had quickly realized that he was a specimen in a cosmic zoo, his cage carefully recreated from the images in old television programmes. And he wondered when his keepers would appear, and in what physical form.

How foolish that expectation had been! He knew now that one might as well hope to see the wind, or speculate about the true shape of fire.

From 2010 ODYSSEY TWO by Arthur C. Clarke (1982)

Ecosystem Classification

In the Traveller role playing game, it broke down animal types into four broad classes: Herbivore, Omnivore, Carnivore, and Scavenger. They were further broken down into sub-types:

  • Herbivore: Animals that eat unresisting food. Plant-eaters, but also whales eating krill and anteaters eating ants.
    • Grazers: Herbivores that devote most of their time to eating. They may be solitary or grouped in herds. Their primary defense is running away very fast. Examples: antelope, moose, whale.
    • Intermittents: Herbivores that do not devote most of their time to eating. They tend to be solitary. They tend to freeze when encountering another animal but will flee if attacked by something larger. Examples: chipmunk and elephant.
    • Filters: Herbivores that pass the environment through their bodies. Grazers move towards food, filters move a flow of water or air through their body in order to gain food. They generally suck, trip, push or pull anything at close range into their digestive sack. They are solitary and tend to be slow-moving. Examples: barnacle.
  • Omnivore: Animals that eat food regardless of its resistance. For instance: bears eat berries as well as small animals.
    • Gatherers: Omnivore that display a greater tendency to herbivorous behavior. They are similar to Intermittents. Examples: raccoon and chimpanzee.
    • Hunters: Omnivore that display a greater tendency to carnivorous behavior. Similar to small or inefficient chasers. Examples: bears and humans.
    • Eaters: Omnivore that does not distinguish its food, it consumes all that it confronts. Examples: a swarm of army ants.
  • Carnivore: Animals that eat violently resisting food by attacking and killing said food.
    • Pouncers: Carnivore that kill their prey by attacking from hiding, or by stalking and springing. Generally solitary since it is hard to coordinate such attacks. If they surprise their prey they will attack, but will sometimes attack even when surprise is lost. If they themselves are surprised they will flee. Examples: cats.
    • Chasers: Carnivore that kill their prey by attacking after a chase. They tend to be pack animals. Examples: wolves.
    • Trappers: Carnivore that passively allow their prey to enter a created trap, whereupon the prey is killed and eaten. They tend to be solitary and slow, but will attack literally anything that enters the trap. Examples: spider and ant lion.
    • Sirens: Similar to Trappers, except it creates some kind of lure to draw prey into the trap. Sometimes the lure is specific to some prey animal, sometimes the lure is universal. Examples: angler fish, Venus fly trap.
    • Killers: Carnivore that devote much attention to killing, a blood lust. They have a raw killing instinct. Attacks are fierce and violent. They do not care how large their opponent is. Examples: shark.
  • Scavenger: Animals that share or steal the prey of others, or that takes the nasty unconsumed left over bits.
    • Intimidators: Scavenger that steal food from other animals by frightening or threatening. They approach another animal's kill and force it away by appearing to be a threat. Examples: coyote.
    • Hijackers: Scavenger that boldly steal food from another animal. Hijackers are stronger or larger than the victim animal, so that it cannot effectively object. Examples: lion, tyrannosaurus rex.
    • Carrion-Eaters: Scavengers that take dead meat when it becomes available, often waiting patiently for all other threats to disperse first. Examples: buzzard.
    • Reducers: Scavengers that act constantly on all available food. They eat the remains of food after all other scavengers are finished with it. They are generally microscopic. Examples: bacteria.

Note that the animal type which an intelligent alien evolved from will give clues as to that alien's psychology.

Here on Terra, Carnivores and Omnivores tend to have their eyes aimed forwards working together, so as to allow binocular vision to gauge the distance to their prey. In self-defense, Herbivores (i.e., the prey) tend to have monocular vision, eyes on the side of their face aimed left and right working separately. This allows them to approximate 360° vision thus reducing the blind spot a carnivores can use for ambush purposes.

Square-Cube Law

Back in the 1950s a popular scifi b-movie trope was giant insects and other monsters. Not to mention Godzilla. These were quite popular at the time. A pity they are yet another iconic scifi trope that science grinds into the dirt while saying You Can't Do That.

The minor problem with Big Bugs is that insect's crude stand-in for actual lungs is utterly incapable of absorbing enough oxygen to keep the blasted critter alive. Not when scaled up to monster size, at any rate.

But the main problem is the pesky Square-Cube Law.

When an object undergoes a proportional increase in size, its new surface area is proportional to the square of the multiplier and its new volume is proportional to the cube of the multiplier. In English: if you enlarge a bug with blue Pym-particles its weight will grow much faster than its skin.

For example, if you double the size (measured by edge length) of a cube, its surface area is quadrupled, and its volume is increased to eight times its original volume.

This creates many problems.


To the mouse and any smaller animal it presents practically no dangers. You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft. A rat is killed, a man is broken, a horse splashes.

For the resistance presented to movement by the air is proportional to the surface of the moving object. Divide an animal’s length, breadth, and height each by ten; its weight is reduced to a thousandth, but its surface only to a hundredth. So the resistance to falling in the case of the small animal is relatively ten times greater than the driving force.

A typical small animal, say a microscopic worm or rotifer, has a smooth skin through which all the oxygen it requires can soak in, a straight gut with sufficient surface to absorb its food, and a single kidney. Increase its dimensions tenfold in every direction, and its weight is increased a thousand times, so that if it is to use its muscles as efficiently as its miniature counterpart, it will need a thousand times as much food and oxygen per day and will excrete a thousand times as much of waste products.

Now if its shape is unaltered its surface will be increased only a hundredfold, and ten times as much oxygen must enter per minute through each square millimetre of skin, ten times as much food through each square millimetre of intestine. When a limit is reached to their absorptive powers their surface has to be increased by some special device. For example, a part of the skin may be drawn out into tufts to make gills or pushed in to make lungs, thus increasing the oxygen-absorbing surface in proportion to the animal’s bulk. A man, for example, has a hundred square yards of lung. Similarly, the gut, instead of being smooth and straight, becomes coiled and develops a velvety surface, and other organs increase in complication. The higher animals are not larger than the lower because they are more complicated. They are more complicated because they are larger.

Comparative anatomy is largely the story of the struggle to increase surface in proportion to volume.Some of the methods of increasing the surface are useful up to a point, but not capable of a very wide adaptation.

For example, while vertebrates carry the oxygen from the gills or lungs all over the body in the blood, insects take air directly to every part of their body by tiny blind tubes called tracheae which open to the surface at many different points. Now, although by their breathing movements they can renew the air in the outer part of the tracheal system, the oxygen has to penetrate the finer branches by means of diffusion. Gases can diffuse easily through very small distances, not many times larger than the average length traveled by a gas molecule between collisions with other molecules.

But when such vast journeys—from the point of view of a molecule—as a quarter of an inch have to be made, the process becomes slow. So the portions of an insect’s body more than a quarter of an inch from the air would always be short of oxygen. In consequence hardly any insects are much more than half an inch thick. Land crabs are built on the same general plan as insects, but are much clumsier. Yet like ourselves they carry oxygen around in their blood, and are therefore able to grow far larger than any insects.

If the insects had hit on a plan for driving air through their tissues instead of letting it soak in, they might well have become as large as lobsters, though other considerations would have prevented them from becoming as large as man.



If an animal were isometrically scaled up by a considerable amount, its relative muscular strength would be severely reduced, since the cross section of its muscles would increase by the square of the scaling factor while its mass would increase by the cube of the scaling factor. As a result of this, cardiovascular and respiratory functions would be severely burdened.

In the case of flying animals, the wing loading would be increased if they were isometrically scaled up, and they would therefore have to fly faster to gain the same amount of lift. Air resistance per unit mass is also higher for smaller animals, which is why a small animal like an ant cannot be seriously injured from impact with the ground after being dropped from any height.

As was elucidated by J. B. S. Haldane, large animals do not look like small animals: an elephant cannot be mistaken for a mouse scaled up in size. This is due to allometric scaling: the bones of an elephant are necessarily proportionately much larger than the bones of a mouse, because they must carry proportionately higher weight. To quote from Haldane's seminal essay On Being the Right Size, "...consider a man 60 feet high...Giant Pope and Giant Pagan in the illustrated Pilgrim's Progress.... These monsters...weighed 1000 times as much as Christian. Every square inch of a giant bone had to support 10 times the weight borne by a square inch of human bone. As the human thigh-bone breaks under about 10 times the human weight, Pope and Pagan would have broken their thighs every time they took a step." Consequently, most animals show allometric scaling with increased size, both among species and within a species. The giant creatures seen in monster movies (e.g., Godzilla or King Kong) are also unrealistic, as their sheer size would force them to collapse.

However, the buoyancy of water negates to some extent the effects of gravity. Therefore, sea creatures can grow to very large sizes without the same musculoskeletal structures that would be required of similarly sized land creatures, and it is no coincidence that the largest animals to ever exist on earth are aquatic animals.

The metabolic rate of animals scales with mathematical principle named Quarter-power scaling according to the metabolic theory of ecology.

From the Wikipedia entry for SQUARE-CUBE LAW

How Large?

How big can ETs be? To answer the question we need to understand something called the Square-Cube Law. This universal geometrical principle, first recognized by Galileo more than three centuries ago, holds that volume always increases faster than surface area as size increases. A solid cubical box whose edge is doubled increases in surface area by a factor of two squared (2x2), or four; whereas volume, hence mass, increases by two cubed (2x2x2), or eight.

It’s easy to apply this to biology. Picture a bony extraterrestrial herbivore placidly grazing in some alien meadow. Suddenly we double its size all over. The animal’s leg bones, now twice as thick, have quadrupled in cross-sectional area; but the creature weighs eight times as much so its bones must sustain double the pressure. It may collapse under normal exertion unless it grows proportionally stouter limbs to handle the added physical stress.

All parts of an animal must be reengineered when size increases. Like bone, muscle strength is determined by cross-sectional area. Humanoids twice as large need quadruply thick biceps: otherwise they’d be pulling eight times the mass with only four times the force. Lungs, kidneys, intestines and other blood filtering organs function according to surface area, so must either increase in mass or become more convoluted at larger body sizes.

The horror movies about giant insects ravaging the countryside are really quite impossible, even on low-gravity worlds. A bug as large as a house would weigh a billion times more than its flea-sized Earthly cousins. Its thin spindly legs would be called upon to sustain stresses thousands of times greater. To walk at all the overgrown arthropod needs muscles proportionally thousands of times thicker; unfortunately, vital tissues already fill the hollow skeleton of the tiny original. It did not collapse under its own weight or was not immobilized by the feebleness of its muscles, an overgrown insect would starve to death because its stomach would be a thousandfold too small to absorb enough food; or it would suffocate because its tracheae could carry only a thousandth as much air as needed.

Sea creatures are free of gravity at neutral buoyancy, but still they’re dogged by the Square-Cube Law. Bodies in motion like to continue in motion – extraterrestrial leviathans larger than whales would experience serious steering, turning and braking difficulties because of their relatively great mass compared to the area of their control surfaces. Cornering too fast might cause stresses in excess of the tensile strength of biological materials and the behemoth would literally snap in two. These problems are familiar to pilots of modem supertankers, huge ships requiring kilometers to turn or stop.

From EXTRATERRESTRIAL ZOOLOGY by Robert A. Freitas Jr. (1981)

Body Type Classification

In his Lensman series, E.E. "Doc" Smith invents an alien body type classification system, though he gives precious few details. In the system, human beings are classified as AAAAAAAAAAAA to twelve places, and aliens have other letter codes depending upon how they vary from humans. The fifth place is for number and type of arms, the sixth is for number and type of legs, and seventh place is skin.

...The thing's bodily structure was RTSL, to four places. No gross digestive tract - atmosphere-nourished or an energy-converter, perhaps. Beyond four places was pretty dim, but Q P arms and legs - Dhilian, eh? - would fit, and so would an R-type hide.

...As she was wafted gently across the intervening space upon a pencil of force, Kathryn took her first good look at the precisionist himself-or herself. She - it - looked something like a Dhilian, she thought at first. There was a squat, powerful, elephantine body with its four stocky legs; the tremendous double shoulders and enormous arms; the domed, almost immobile head. But there the resemblance ended. There was only one head-the thinking head, and that one had no eyes and was not covered with bone. There was no feeding head-the thing could neither eat nor breathe. There was no trunk. And what a skin!

It was worse than a hide, really-worse even than a Martian's. The girl had never seen anything like it. It was incredibly thick, dry, pliable; filled minutely with cells of a liquid-gaseous something which she knew to be a more perfect insulator even than the fibres of the tegument itself.

"R-T-S-L-Q-P." She classified the creature readily enough to six places, then stopped and wrinkled her forehead. "Seventh place-that incredible skin-what? S? R? T? It would have to be R . . .

..."VWZY, to four places." Con concentrated. "Multi-legged. Not exactly carapaceous, but pretty nearly. Spiny, too, I believe. The world was cold, dismal, barren; but not frigid, but he-it-didn't seem exactly like an oxygen-breather - more like what a warm-blooded Palainian would perhaps look like, if you can imagine such a thing. VWZYTXSYZY to ten places.

...Classification, straight Z's to ten or twelve places, she - or it - seemed to be trying to specify. A frigid race of extreme type, adapted to an environment having a temperature of approximately one degree absolute.

...physically, his classification to four places is TUUV; quite a bit like the Nevians, you notice.To ten places it was TUUVWYXXWT.

From Children of the Lens by E.E. "Doc" Smith (1947)

James White adapted the system to his Sector General novels, with the the more reasonable specification that human beings were not the measure of all things, i.e., in the Sector General system humans are classified as DBDG, not AAAAAAAAAAAA.

Conway muted the speaker which carried the conversation between ship and receptionist into the gallery and said, "This is as good a time as any to explain our physiological classification system to you. Briefly, that is, because later there will be special lectures on this subject."

Clearing his throat, he began, "In the four-letter classification system the first letter indicates the level of physical evolution, the second denotes the type and distribution of limbs and sense organs and the other two the combination metabolism and pressure and gravity requirements, which in turn give an indication of the physical mass and form of protective tegument possessed by the being. I must mention here, in case any of you might feel inferior regarding your classification, that the level of physical evolution has no relation to the level of intelligence...

Species with the prefix A, B and C, he went onto explain, were water breathers. On most worlds life had originated in the sea and these beings had developed high intelligence without having to leave it. D through F were warm-blooded oxygen-breathers, into which group fell most of the intelligence races in the galaxy, and the G and K types were also oxygen breathing but insectile. The Ls and Ms were light-gravity, winged beings.

Chlorine-breathing life-forms were contained in the 0 and P groups, and after that came the more exotic, the more highly-evolved physically and the downright weird types. Radiation-eaters, frigid-blooded or crystalline beings, and entities capable of modifying their physical structure at will. Those possessing extra-sensory powers sufficiently well-developed to make walking or manipulatory appendages unnecessary were given the prefix V, regardless of size or shape.

Conway admitted to anomalies in the system, but these could be blamed on the lack of imagination by its originators. One of the species present in the observation gallery was a case in point - the AACP type with its vegetable metabolism. Normally the A prefix denoted a water breather, there being nothing lower in the system than the piscatorial life forms. But the AACPs were vegetables and plants had come before fish.

From Star Surgeon by James White (1963)

Alien Intelligence

The average level of intelligence of an alien species is anybody's guess. However, there are thought experiments suggesting that their intelligence would tend to be about the same as our own. Of course there might be outliers; morons on the planet Spengo and Pakled, super geniuses on Altair IV and Arisia. Or if they hit the Singularity and shoot off the top of the chart, turning into StarGods or something.


What is Intelligence?

Intelligence is much easier to talk about than to define. Webster calls intelligence “the power or act of understanding … the power of meeting any situation, esp. a novel situation, successfully by proper behavior adjustments ; also, the ability to apprehend the interrelationships of presented facts in such a way as to guide action toward a desired goal…"

Obviously, the dictionary does not define “intelligence” in the same way it defines more palpable terms like “height,” “weight,” or even “brain.” And note that the definition of “intelligence” is a subjective and external one. That is, to apply the definition, an outside observer must watch the antics of the creature in question and see if his behavior entitles him to be temed “intelligent.” Evidently there is no absolutely accurate way of measuring intelligence in the same manner that a physician can measure blood pressure or basal metabolic rate. Even an I.Q. test yields only a guide, an approximation.

Note also that the dictionary definition poses three tests for deciding if a creature is intelligent. The creature must “understand,” make “behavior adjustments,” and be able to “apprehend … interrelationships.” Understand, adjust, interrelate: certainly without these abilities a creature cannot be termed intelligent. Yet—every animal has the ability to recognize certain sets of sensory impressions, to interrelate them and adjust its behavior accordingly. The difference between intelligent man and unintelligent amoeba is in the degree of understanding, adjustment and interrelating.

More particularly, it has been said that the real test of intelligence is the ability to handle abstract thought. Animals live in the present, responding to immediate sense impressions. The past is meaningless and the future dimly perceived, if at all. Men live in a continuum of past, present and future. Man is evidently the only creature on Earth that can consciously conceive of a time that is not-yet. A male gibbon can make a sound that means “Keep away from my wife!” Imperative, immediate. A male human being can say, “If you don’t keep away from my wife, I will shoot you.” A choice of conscious actions which will take place in the future.

Intelligence, then, is a relative matter—something that may be judged qualitatively, but defies quantitative measurement.

All of which brings us to a rule-of-thumb test for intelligence: If a race of creatures has the ability to communicate, so that one individual of the species can share a pool of knowledge accumulated by the race as a whole, then we may say that the race is fully intelligent. This test carries within it the implications of abstract thought and communication, the ability to understand, adjust, and interrelate. Moreover, it implies that a truly intelligent race will be constantly adding to its pool of accumulated knowledge, as new individuals create and communicate new ideas. An intelligent race, in other words, will constantly change its environment —sometimes very slowly, sometimes explosively fast. This is what is usually meant when people speak of man’s “progress.”

Intelligent Species of Earth

Is man truly the only intelligent species on this planet? The social insects have accomplished remarkable achievements, and have survived many hundreds of times longer than man’s onemillion- year tenure on Earth. Some of the large sea-going mammals, such as the dolphins and killer whales, have large complex brains and the ability to make linguistic sounds. And there are other primates, particularly the chimpanzee, which show more intellect at birth than do human babies.

The social insects—particularly the ants—are a fascinating example of how complicated this problem of intelligence can be. A single ant is demonstrably dull. It can learn its way through a maze, but quickly forgets. It has not even the lowly intelligence of a mouse. Yet colonies of ants behave in a highly-organized fashion. There is division of labor, engineering and architecture, “nursing” of young, exploration, some degree of communication. The concept of group intelligence has often been raised in connection with the ants and other social insects.

Is an ant colony an intelligent, sentient creature composed of many unintelligent individuals? This is a bit hard for most human beings to accept, and yet an unbiased observer might point out that the human being is an intelligent, sentient creature composed of many unintelligent individual cells. A single brain cell is certainly not intelligent, yet it belongs to a system that is.

Let us apply our rule-of-thumb test to the ants, after making a slight mental adjustment that allows us to consider both an individual and a colony as a single creature. Have the ants been able to communicate and establish a pool of knowledge that can be shared by succeeding generations of colonies (and/or individuals)? The best answer that scientists can give today is: No. Apparently, everything the ants do, they do by instinct. They do not learn to speak, they are born with an instinctive communicative ability, and no ant can rise beyond the limitations of its instincts. Ants can perform prodigies of labor, but everything they do can be explained in terms of physical adaptions.

The final test of the ants’ intelligence is to compare their progress over the past million years with man’s. A million years ago, man-like primates were shambling through African forests. A hundred thousand years ago, human hunters were slaughtering every edible land animal on this planet. Ten thousand years ago, men invented agriculture and began to build cities. Today, humanity holds the power of the atom and has already begun to explore interplanetary space. And in all that time, the ants have changed their ways not at all.

Much the same argument can be applied to the dolphins. Recently there has been great interest in the relative intelligence of dolphins, and a Nobel Prizewinning scientist, Leo Szilard, has even written a science-fiction story that hinges on the credibility of intelligent, speaking dolphins. The main interest in this case is the large brain of the dolphin; it is larger than the human brain (about 1600-1700 grams compared to an average of 1400 for man), and structurally just as complex. Dolphins have been trained to imitate human voices. They evidently communicate among themselves in a primitive fashion. They are surprisingly bright, agile, and beautifully adapted to sea-faring. There are many, many stories, dating back to antiquity, of dolphins helping to save floundering human swimmers by buoying them up on their backs and carrying them to shore.

Admittedly, the scientific study of dolphins is just beginning. But to date, there is no really impressive evidence for an intelligence comparable to man’s. Despite their large and complex brains, the dolphins have shown an intelligence little better than a dog’s, and not as high as a chimpanzee’s. Their vocal abilities are on a par with a trained parrot’s, and stem mainly from their use of a vocal “sonar” in underwater navigation. However, man is at a distinct handicap in assessing dolphin intelligence, simply because we cannot watch the dolphins in their natural environment. It is only in the past ten years or so that we have built swimming tanks lai'ge enough to allow aquatic mammals to be studied.

In the open ocean, the dolphins are free-roaming hunters. They are playful and plentiful—two indications of at least some intelligence. Their cousins, the vicious, slightly larger killer whales, actually hunt in packs and show much ingenuity in attacking practically every type of sea-going mammal, including the gigantic blue whale. But, again, the killer whales have not shown a higher degree of intelligence than a pack of terrestrial wolves. If the dolphins or killer whales are as intelligent as man, their environment is so different from ours that we have, at present, no adequate method of gauging their talents. This is an important fact to keep in mind when considering the intelligence of creatures from alien planets.

Of all the potentially-intelligent animals of Earth, the chimpanzee is the closest to man, the most easily studied, and easily the brightest. In fact, for the first year or so after birth chimps actually learn more quickly than human babies. A one-year-old chimp can do a variety of tricks and even rasp out a few human words, if properly trained. The chimpanzee matures much more quickly than a human baby, both physically and mentally. But there is a cross-over point. Sometime during the second year, the human baby begins to learn how to speak. He starts to tap that reservoir of accumulated racial knowledge. The chimp, meanwhile, seems to get tired of doing tricks—the whole business of performing and saying words apparently becomes pointless to him. His “intellectual” development ends. In short, the human baby goes on to become human; the chimpanzee, despite a heroic effort, cannot be anything other than an ape.

The major reason for this is, of course, the relative sizes of the human and chimpanzee brain; man’s brain is some 3.5 times larger (1400 grams compared to about 400).

But there are other reasons also. In fact, many physical anthropologists now believe that man’s brain was the last part of him to become human. To bear this out they have unearthed evidence that points to the conclusion that man developed physically into human form before he developed mentally into Homo sapiens, thinking man.

Man's Five Gifts

The anthropologist Carleton S. Coon pointed out that man has five distinct features—five gifts—that distinguish him from the other primates: First, man stands erect and walks on two feet. This leaves him prone to backaches, due to the forced curvature of his primate spine. Man’s foot has fused into an arched load-carrier capable of supporting his body weight with no help whatsoever from the arms. The arch occasionally collapses and leaves man flat-footed. But even so, man’s overworked feet and aching back allow his hands complete freedom from the chore of locomotion.

Man’s remarkable hands are his second gift, and perhaps his most important. Once man becomes an erect biped, his hands are free to get into mischief—and also to pick fruit, to fold in prayer, to fashion a tool. Archeological evidence has shown that very primitive proto-men, creatures that were still mostly ape and had quite small brains, actually used tools. Anthropologists are now largely agreed that tools made man, not vice versa. Without his grasping hands, man could never have become a toolmaker. Human hands are more flexible, grasp better, and are capable of much more delicate manipulations than any of the primate apes’ (or, indeed, the grasping organs of any animal on Earth).

The ability to use his hands to grasp objects no doubt had much to do with man’s third gift—fine-focusing, stereoscopic, color vision. All the primates have good eyes, but man’s seem to be the best of the lot. Part of man’s visual acuity is probably due to the need to inspect very closely objects that his hands have picked up. A million years ago, man’s ancestors picked fruits and examined them carefully. Today, man uses the eyes he has developed to examine stars and atoms—and astronomers still insist that no camera made can equal the human eye’s ability to distinguish fine detail.

The first three of man’s five gifts were physical. The other two are mental. With feet capable of fleet running, hands free to seize the environment, the eyes sharp enough to spot a meal on the hoof from a distance of miles, man began to develop his ability to think. The anthropological evidence shows that man’s brain began to increase in size only after the rest of body became human. But once man’s brain did increase to its present size, it became his fourth gift.

Why and how man developed his brain is still a mystery. The plain truth is that man’s brain and his intelligence—is at least a full order of magnitude greater than that of his nearest rivals, the chimps. The dolphins, as we have seen, have larger brains. But, lacking hands, lacking the challenges and stimuli of a terrestrial environment, they have never developed their brains to the pitch that man has. The ants, clever and highly-regimented though they are, simply lack the brain size to break free of their instincts.

The fifth and final gift stems directly from man’s brain, and puts the final touch on his development of intelligence: it is his ability to speak. Not merely to make noises, as birds and monkeys and dolphins do. Not merely to communicate to few limited present-tense imperatives, as the ants and bees and some primate apes do. Man can speak. He can tell about the past, he can speculate about the future, he can unload his fears into the ear of a psychiatrist or a priest, he can recite poetry, he can argue physics, and metaphysics, he can add to—and draw from—an accumulation of knowledge that goes back to before the taming of fire.

The impact of this ability to speak must have been infinitely more meaningful to man’s development than its paler counterpart of historic times—the invention of printing. The ability to speak far beyond the range of the unaided voice, through books, ushered in the scientific age. Without printing, we would still be in medieval darkness. Without speech itself, we would be a little better than the chimpanzees.

Thus, while we cannot fully define man’s intelligence, we can describe its attributes and its sources. Now we are ready to look out into space and see if these qualities can be found elsewhere.


Improving lives doesn’t.

Among the baker’s dozen of known galactic species that crawled their way to sapience, sociopsychologists were astonished to find that every one of them had the same intelligence. The bipeds from Earth, the avian dinosaurs from that one outer rim world, the furry bear-creatures that ate methane, put any together and they score within 10 points of each other on an IQ test. This wasn’t true for any other attribute. (Im)mortality? widely varying. Genders? Different systems. Biochemistry? Carbon through Arsenic. Size, shape? Hell no.

But intelligence? Why that?

It turns out that entry-level sapience evolves as a survival trait. Hunt/find your food, develop technologies to make that easier, maybe do some farming, and so on. After basic establishment of civilization, mortality drops by factors in the hundreds or thousands. Population booms, and you start getting plagues from the species concentrating in cities.

This is where it gets interesting. See, once you have plagues, you need doctors. And once you have doctors, you start thinking about all of the other ways to cheat death. So the plagues are beaten back by vaccinations or antibiotics, and then your civ starts concentrating on welfare and quality-of-life.

Pretty soon, your species is living at the maximum, or nearly, of their theoretically longest lives. For some species, this is an extension from a lifespan of decades to millennia.

This is bad.

At best, evolution stagnates. Your weak and stupid have the same chance of reproduction as anyone else–and they’re certainly not going to die before influencing their environments. Diseases that should have killed are mere annoyances, chomping futilely against a barrier of solid medical science. Predators that once ravaged tribes now are confined in zoos or hunted to extinction.

So no one gets any smarter.

The long and short of it is, after a certain point, intelligence is no longer a tremendous advantage to survival and, subsequently, traditional selection factors are abrogated completely. That is point at which medical science develops, which itself happens only when sapients begin the process of introspection and develop sympathy–that is, shortly after the development of sapience itself.


(ed note: in the novel a cosmic accident raises the IQ of everybody on Terra to about 500, or beyond super-genius. Eventually they send out a faster-than-light starship to survey the stars in the local area)

Lewis spoke slowly in the quiet of the ship: “This makes nineteen planets we’ve visited, fourteen of them with intelligent life.”

Corinth’s memory went back over what he had seen, the mountains and oceans and forests of whole worlds, the life which blossomed in splendor or struggled only to live, and the sentience which had arisen to take blind nature in hand. It had been a fantastic variety of shape and civilization. Leaping, tailed barbarians howled in their swamps; a frail and gentle race, gray like silver-dusted lead, grew their big flowers for some unknown symbolic reason; a world smoked and blazed with the fury of nations locked in an atomic death clutch, pulling down their whole culture in a voluptuous hysteria of hate; beings of centaur shape flew between the planets of their own sun and dreamed of reaching the stars; the hydrogen-breathing monsters dwelling on a frigid, poisonous giant of a planet had evolved three separate species, so vast were the distances between; the world-civilization of a biped folk who looked almost human had become so completely and inflexibly organized that individuality was lost, consciousness itself was dimming toward extinction as antlike routine took the place of thought; a small snouted race had developed specialized plants which furnished all their needs for the taking, and settled down into a tropical paradise of idleness; one nation, of the many on a ringed world, had scorned wealth and power as motivations and given themselves to a passionate artistry. Oh, they had been many and strange, there was no imagining what diversity the universe had evolved, but even now Corinth could see the pattern.

Lewis elaborated it for him: “Some of those races were much older than ours, I’m sure. And yet, Pete, none of them is appreciably more intelligent than man was before the change. You see what it indicates?”

“Well, nineteen planets—and the stars in this galaxy alone number on the order of a hundred billion, and theory says most of them have planets—what kind of a sample is that?”

“Use your head, man! It’s a safe bet that under normal evolutionary conditions a race only gets so intelligent and then stops. None of those stars have been in the inhibitor field, you know. (the "inhibitor field" is the cosmic accident that raised humanity's IQ)

“It ties in; it makes good sense. Modern man is not essentially different from the earliest Homo sapiens, either. The basic ability of an intelligent species is that of adapting environment to suit its own needs, rather than adapting itself to environment. Thus, in effect, the thinking race can maintain fairly constant conditions. It’s as true for an Eskimo in his igloo as it is for a New Yorker in his air-conditioned apartment; but machine technology, once the race stumbles on to it, makes the physical surroundings still more constant. Agriculture and medicine stabilize the biological environment. In short—once a race reaches the intelligence formerly represented by an average I.Q. of 100 to, say, 150, it doesn’t need to become smarter than it is.

Corinth nodded. “Eventually surrogate brains are developed, too, to handle problems the unaided mind couldn’t deal with,” he said. “Computers, for instance; though writing is really the same principle.

From BRAIN WAVE by Poul Anderson (1954)

Alien Communication

There are some notes on talking to aliens here.

In the real world, communication with hypothetical extraterrestrials is such a huge problem that it may never be properly solved. Researchers are having enough problems trying to talk to porpoises, and they are from our own planet. Alien thought processes might be forever inscrutable. There is a good list of examples of inscrutable alien languages on TV Tropes.

In C. J. Cherryh's Chanur novels, the methane-breathing Tc'a species are almost impossible to be communicated with, since their brains are multi-part and their speech decodes as complex matrices of intertwined meanings. In Piers Anthony's KIRLIAN QUEST, the Slash use modulated laser beams. As did the deep space beings in Jack Williamson's TRAPPED IN SPACE. In Charles Sheffield's PROTEUS novels, the Logeinan life form uses an area of skin that has changing color dots. As does the intelligent squid in Arthur C. Clarke's The Shining Ones.

And just imagine the headaches of trying to communicate with a species that uses various scents and smells instead of sound. Or radio waves. Or modulated laser beams. Or rapid changes in skin color. Or all four combined.

...the vast majority of sentients (alien races) cannot directly communicate with each other.

Some species operate on different time lines, or are out of phase with the four dimensions we can perceive, are too small or too large, or perhaps, if they had to acknowledge us, they would have to kill us.

So even when an atomic matrix life form that feeds off the microwave hum left over from the Big Bang and excretes time lives in the same solar system with your typical silicon-based life form that eats rocks and excretes hydrogen, communication between them may be close to impossible.

Luckily it's not really a big deal, because they usually don't have anything to talk about. Or so it appears, right up until said atomic matrix life form begins a simple operation to make the local sun go nova in order to harvest neutrinos, and to their surprise, are vigorously opposed by those gritty little creatures clinging to their large orbiting rocks, who have had to start throwing anti-matter around to get their attention, and things usually deteriorate from there.

From Buck Godot: The Gallimaufry by Phil Foglio

"This man Boyce," said Karellen. "Tell me all about him." The Supervisor did not use those actual words, of course, and the thoughts he really expressed were far more subtle. A human listener would have heard a short burst of rapidly modulated sound, not unlike a high-speed Morse sender in action. Though many samples of Overlord language had been recorded, they all defied analysis because of their extreme complexity. The speed of transmission made it certain that no Interpreter, even if he had mastered the elements of the language, could ever keep up with the Overlords in their normal conversation.

From Childhood's End by Arthur C. Clarke (1954)

Alien Psychology

The psychology of an alien species is any body's guess. It could be so alien as to be forever beyond our understanding. It could be quite human. Or somewhere in-between.

There is a sophisticated alien psychology generation system in the role-playing game GURPS: Uplift, and a good tutorial on TV Tropes.

Some clues to an alien species psychology might be found in their ecosystem classification. For instance, herbivores might be skittish, only comfortable in groups, and tend to flee if they feel threatened.

In James P. Hogan's The Gentle Giants of Ganymede, on the giant's planet the herbivores evolved a third circulatory system full of toxins which made their flesh poisonous to carnivores. It was so effective that carnivores became extinct. The herbivores evolved to look like animal illustrations from a nursery or kindergarten story book, all cute, plump and cuddly. The result was that the giant psychology has no confrontation, pride, or sense of danger.

Larry Niven's Puppeteers evolved from herbivores. They are the cowards of the universe, their leader is called "The Hindmost" because it is the furthest from any danger. In Puppeteer society, courage is seen as a mental illness. Puppeteers are pragmatic to a fault. Human traits such as wishful thinking and superstition are nonexistent. This means there is no level of danger that they'd consider to be an acceptable risk, the only acceptable level is 0%. They are willing to go to any lengths to protect themselves from perceived danger and provide a safer environment for themselves.

In Niven and Pournelle's classic novel Footfall, the alien Fithp are herd creatures. They do not understand how or why you would possibly initiate diplomacy before first fighting to see which party was dominant. When a Fithp is defeated, it surrenders, and thereafter becomes totally devoted and subservient to its conqueror.

Fithp are horrified when they defeat humans in battle, the humans surrender, then the humans suddenly break their surrender and counter-attack. To the Fithp, this is mad-dog behavior, and the humans are treated as such.

However aliens can have such a bizarre psychology as to be forever beyond comprehension, as in Terry Carr's "The Dance of Changer and the Three"

The Dance of the Changer and Three

(ed note: on the mineral-rich planet, the miners make contact with the alien energy-creatures native to the place (the Loarra). They receive permission to mine. After mining for four years, the Loarra show up and kill everybody outside of the mountain and destroy all the mining equipment)

After a while I sent out a fourth “eye.” One of the Loarra came over, flitted around it like a firefly, blinked through the spectrum, and settled down to hover in front for talking. It was Pura Pur who was a thousand million billion life cycles removed from the Pur we know and love, of course, but nonetheless still pretty much Pur.

I sent out a sequence of lights and movements that translated, roughly, as, “What the hell did you do that for?”

And Pur glowed pale yellow for several seconds, then gave me an answer that doesn’t translate. Or, if it does, the translation is just “Because.”

Then I asked the question again, in different terms, and she gave me the same answer in different terms. I asked a third time, and a fourth, and she came back with the same thing. She seemed to be enjoying the variations on the Dance; maybe she thought we were playing.

Well … We’d already sent out our distress call by then, so all we could do was wait for a relief ship and hope they wouldn’t attack again before the ship came, because we didn’t have a chance of fighting them, we were miners, not a military expedition. God knows what any military expedition could have done against energy things, anyway. While we were waiting, I kept sending out the “eyes,” and I kept talking to one Loarra after another. It took three weeks for the ship to get there, and I must have talked to over a hundred of them in that time, and the sum total of what I was told was this:

Their reason for wiping out the mining operation was untranslatable. No, they weren’t mad. No, they didn’t want us to go away. Yes, we were welcome to the stuff we were taking out of the depths of the Loarran ocean.

And, most importantly: No, they couldn’t tell me whether or not they were likely ever to repeat their attack.


(ed note: Chee Lan is a Cynthian, an alien resembling an angora cat. Adzel is a Wodenite, resembling a cross between centaur and a dragon. The team is trying to figure out the psychology of the Shenna of Dathyna.)

He drained his beer. Soothed thereby, he lit his pipe, settled back, and rumbled, "We got our experience and information. Also we got analogues for help. I don't think any sophonts could be total unique, in this big a universe. So we can draw on our understanding about other races.

"Like you, Chee Lan, for instance: we know you is a carnivore—but a small one—and this means you got instincts for being tough and aggressive within reason. You, Adzel, is a big omnivore, so big your ancestors didn't never need to carry chips on their shoulders, nor fish either; your breed tends more to be peaceful, but hellish independent too, in a quiet way; somebody tries for dictating your life, you don't kill him like Chee would, no, you plain don't listen at him. And we humans, we is omnivores too, but our primate ancestors went hunting in packs, and they got built in a year-around sex drive; from these two roots springs everything what makes a man a human being. Hokay? I admit this is too generalistic, but still, if we could fit what we know about the Shenna in one broad pattern—"

On Dathyna, the predicament was worse. The solar bombardment was always greater than Earth receives. At the irregular peaks of activity, it was very much greater. Magnetic field and atmosphere could not ward off everything. Belike, mutations which occurred during an earlier maximum led to the improbable result of talking, dreaming, tool-making herbivores. If so, a cruel natural selection was likewise involved: for the history of such a planet must needs be one of ecological catastrophes.

The next radiation blizzard held off long enough for the race to attain full intelligence; to develop its technology; to discover the scientific method; to create a worldwide society which was about to embark for the stars, had perhaps already done it a time or two. Then the sun burned high again.

Snows melted, oceans rose, coasts and low valleys were inundated. The tropics were scorched to savanna or desert. All that could be survived. Indeed, quite probably its harsh stimulus was what produced the last technological creativity, the planetary union, the reaching into space.

But again the assault intensified. This second phase was less an increase of electromagnetic energy, heat and light, than it was a whole new set of processes, triggered when a certain threshold was passed within the waxing star. Protons were hurled forth; electrons; mesons; X-ray quanta. The magnetosphere glowed with synchrotron radiation, the upper atmosphere with secondaries. Many life forms must have died within a year or two. Others, interdependent, followed them. The ecological pyramid crumbled. Mutation went over the world like a scythe, and everything collapsed.

No matter how far it had progressed, civilization was not autonomous. It could not synthesize all its necessities. Croplands became dustbowls, orchards stood leafless, sea plants decayed into scum, forests parched and burned, new diseases arose. Step by step, population shrank, enterprises were abandoned for lack of personnel and resources, knowledge was forgotten, the area of the possible shrank. A species more fierce by nature might have made a stronger effort to surmount its troubles—or might not—but in any event, the Dathynans were not equal to the task. More and more of those who remained sank gradually into barbarism.

And then, among the barbarians, appeared a new mutation.

A favorable mutation.

Herbivores cannot soon become carnivores, not even when they can process meat to make it edible. But they can shed the instincts which make them herd together in groups too large for a devastated country to support. They can acquire an instinct to hunt the animals that supplement their diet—to defend, with absolute fanaticism, a territory that will keep them and theirs alive—to move if that region is no longer habitable, and seize the next piece of land—to perfect the weapons, organization, institutions, myths, religions, and symbols necessary—

—to become killer herbivores.

And they will go farther along that line than the carnivora, whose fang-and-claw ancestors evolved limits on aggressiveness lest the species dangerously deplete itself. They might even go farther than the omnivora, who, while not so formidable in body and hence with less original reason to restrain their pugnacity, have borne arms of some kind since the first proto-intelligence developed in them, and may thus have weeded the worst berserker tendencies out of their own stock.

Granted, this is a very rough rule-of-thumb statement with many an exception. But the idea will perhaps be clarified if we compare the peaceful lion with the wild boar who may or may not go looking for a battle and him in turn with the rhinoceros or Cape buffalo.

The parent stock on Dathyna had no chance. It could fight bravely, but not collectively to much effect. If victorious in a given clash, it rarely thought about pursuing; if defeated, it scattered. Its civilization was tottering already, its people demoralized, its politico-economic structure reduced to a kind of feudalism. If any groups escaped to space, they never came back looking for revenge.

A gang of Shenna would invade an area, seize the buildings, kill and eat those Old Dathynans whom they did not castrate and enslave. No doubt the conquerors afterward made treaties with surrounding domains, who were pathetically eager to believe the aliens were now satisfied. Not many years passed, however, before a new land-hungry generation of Shenna quarreled with their fathers and left to seek their fortunes.

The conquest was no result of an overall plan. Rather, the Shenna took Dathyna in the course of several centuries because they were better fitted. In an economy of scarcity, where an individual needed hectares to support himself, aggressiveness paid off; it was how you acquired those hectares in the first place and retained them later. No doubt the sexual difference, unusual among sophonts, was another mutation which, being useful too, became linked. Given a high casualty rate among the Shenn males, the warriors, reproduction was maximized by providing each with several females. Hunting and fighting were the principal jobs; females, who must conserve the young, could not take part in this; accordingly, they lost a certain amount of intelligence and initiative. (Remember that the original Shenn population was very small, and did not increase fast for quite a while. Thus genetic drift operated powerfully. Some fairly irrelevant characteristics like the male mane became established in that way—plus some other traits that might actually be disadvantageous, though not crippling.)

At length the parricidal race had overrun the planet. Conditions began to improve as radiation slacked off, new life forms developed, old ones returned from enclaves of survival. It would be long before Dathyna had her original fertility back. But she could again bear a machine culture. From relics, from books, from traditions, conceivably from a few last slaves of the first species, the Shenna began rebuilding what they had helped destroy.

But here the peculiar set of drives which had served them well during the evil millennia played them false. How shall there be community, as is required for a high technology, if each male is to live alone with his harem, challenging any other who dares enter his realm?

The answer is that the facts were never that simple. There was as much variation from Shenn to Shenn as there is from man to man. The less successful had always tended to attach themselves to the great, rather than go into exile. From this developed the extended household—a number of polygynous families in strict hierarchy under a patriarch with absolute authority—that was the "fundamental" unit of Shenn society, as the tribe is of human, the matrilineal clan of Cynthian, or the migratory band of Wodenite society.

The creation of larger groups out of the basic one is difficult on any planet. The results are all too likely to be pathological organizations, preserved more and more as time goes on by nothing except naked force, until finally they disintegrate. Consider, for example, nations, empires, and world associations on Earth. But it need not always be thus.

The Shenna were reasoning creatures. They could grasp the necessity for cooperation intellectually, as most species can. If they were not emotionally capable of a planet-wide government, they were of an interbaronial confederacy.

Especially when they saw their way clear to an attack—the Minotaur's charge—upon the stars!

From SATAN'S WORLD by Poul Anderson (1968)

Alien Lebensraum

Desirable Real Estate

Many aliens could prefer to live on planets that human beings would find to be miserable hell-holes.

Many point to the ecosystems at the Galapagos black smokers as proof that life is possible in underground oceans on, say, Europa. However, if this is true, the implication is that such life will be far more common than terrestrial life. After all, there are several such moons in our solar system, and only one Terra (Europa, Enceladus, Ganymede and Titan). If there are four such moons, then throughout the universe iceball life will outnumber liquid water life four to one, on average. Such life turns up in The Killing Star by Charles Pelligrino and George Zebrowski.

But most humans would rather not live in sub-zero water under kilometers of solid ice.

However, if the aliens like to live on the same kinds of planets that Terrans do, the way to bet is that eventually there will be war. The only wild card is if one or both species like living in mobile asteroid habitats (Macrolife). Or if a species is a primitive civilization with the misfortune to live in an Elder God Galaxy, and has to keep a real low profile in order to survive.

As a side note, one can use the time between apes and angels for the "average lifespan of a technological civilization". Insert this into the Drake equation along with a few other guesses and you can calculate the average distance between alien civilization homeworlds. (and of course the distance between Terra and the closest aliens).

I say "homeworlds" because they might have colonized nearby stars to form an empire. In this case the homeworld will probably be in the center of the empire's sphere of influence. Therefore the closest aliens will be the average distance between minus the radius of their empire. Go to The Tough Guide to the Known Galaxy and read the entry "HOMEWORLD".

If you already have an idea of how close you want civilizations to be spaced, you work the Drake equation backwards. Keep altering the values until you get the spacing you want. But now you have to live with the consequences of those various values, and their implications.

It will be even worse if the average lifespan of a technological civilization is shorter than expected, due to premature death by nuclear holocaust or unexpected apotheosis by a Vingian Singularity.

HOMEWORLD. This may have any of three related but distinct meanings.

1) Someone's native PLANET; where they were born, or at least their permanent residence address.

2) The capital Planet of an EMPIRE, especially if the Empire builders started out there.

3) The Planet where an intelligent race originated. In this sense, the Homeworld of all EARTH HUMANS is of course Earth, even if they have lived for generations on a COLONY.

Gas Giant Dweller

Gas giant planets are very unattractive places for humans to colonize. Blasted things do not have a real surface, the atmosphere just gradually thickens into a slush, which gradually thickens into metallic hydrogen or something.

Somewhat arbitrarily the "surface" of a gas giant (zero point for altitudes) is set when the pressure reaches 1 bar (average sea-level pressure on Terra). The vague "top" of Jupiter's atmosphere is roughly 5,000 kilometers above the surface. The "cloud-top" level of Jupiter's atmosphere is where the pressure is about 0.1 bar (50 km above surface). Confusingly astronomers decided the base of the atmosphere (base of the troposphere) is not at the "surface", it is below that where the pressure reaches 10 bar (90 km below surface). The atmosphere starts turning into a slushy gas at about 13 bar (95 km below surface). And it turns into a slushy liquid at about 5,000 bar (at 1,000 km below surface, and 1,700° C).

But long before you get to the slushly liquid state the pressure will grow high enough to make your spacecraft implode and the temperature will melt the ship. Presumably any native life form on such a planet will either perpetually float in the upper atmosphere, or be very crush-proof heat-resistant slush swimmers.

Jupiter's Atmosphere
from datum
+5,000Top: exosphere
Atmosphere fades into
interplanetary space
+1,0001 nbarBottom: exosphere
Top: thermosphere
+3201 μbarBottom: thermosphere
Top: stratosphere
+500.1112-161°Bottom: stratosphere
Top: troposphere (tropopause)
"cloud top" (start of haze layer)
+46?Top: Sinker zone
0.6145Top: Ammonia cirrus cloud level
0.9150Bottom: Ammonia cirrus cloud level
01.0165-108°Datum (ave. Terran sea-level)
Top: Ammonia-sulfur cloud level
3He scoop mining level
2.0200-73°Bottom: Ammonia-sulfur cloud level
3.0Top: Water clouds level
7.0Bottom: Water cloud level
-70?7.3?30027°Optimum photosynthesis
Center of sinker zone
Floater feeding zone?
-9010.034067°Bottom: Troposphere
-95?12.9Top: Hydrogen becomes slushy gas
-185?250?500?230°?Bottom: Sinker zone
Top: Organisms incinerated
(pyrolytic depth)
280900627°Aluminum melts
-1,0005,0002,0001,727°Titanium melts
Top: Hydrogen becomes slushy fluid
Pressure of Mariana Trench
2,000,00010,0009,700°Top: Metallic hydrogen

Values with question marks were calculated with linear interpolation.

Carl Sagan and E. E. Salpeter postulated floating organisms could exist in the temperate regions of Jupiter's atmosphere in a 1975 paper. An entire ecosystem, with aerial plankton grazed on by sky whales, who were preyed on in turn by flying sharks. This was later featured in Sagan's documentary series Cosmos.

In Sagan and Salpeter's paper, "sinkers" were aerial plants that were born in the upper troposphere and gradually fell to their death in the inferno of Jupiter's lower atmosphere. Along the way they grew by photosynthesis using blue light and abundant atmospheric methane, water, and ammonia. They also reproduced by emitting tiny spores, stimulated by moving from region of depleted resources into a region of abundant nutrients. The spores were carried up to the upper troposphere by atmospheric turbulence, where the cycle of life starts anew. The paper calculates that a sinker has a size of about 30μM (about the size of a small terrestrial protozoa) and will take about two months to fall from the birth altitude to the incineration altitude. Later Sagan upped the size estimate to up to the size of a toy balloon.

Alternatively sinkers can grow by becoming a colony creature. The component creatures reproduce and the colony grows. When it sinks too close to incineration depth, the colony disperses into individuals. These are small enough to rise to safe altitudes by atmospheric turbulence. Paper estimates a colony can contain about 10,000 if colony and individuals do not exceed max size.

"Floaters" are herbivores. They feed on the sinkers, and use the extra metabolic energy to maintain float bladders. This allows them to avoid falling to a fiery death. One way to float is to pump their bladder such that it contains close to pure hydrogen, instead of the hydrogen-helium mixture composing the Jovian atmosphere. The other is to use metabolic energy to heat the atmosphere inside the bladder (since hot hydrogen-helium is lighter than cool hydrogen-helium). Heating will require a larger bladder than pumping helium. The paper calculates that it is possible to have floaters with sizes measured in kilometers.

And where you find herbivores you generally also find carnivores preying upon them. The "hunters" kill and eat floaters, using the more concentrated food energy to allow stalking and chasing. Hunters are also after their prey's store of purified hydrogen inside their float bladders.

There is a second class of (thermoresistant) floaters called "scavengers", living just above the hot zone and eating the steady fall of incinerated sinkers, or the incinerated bodies of dead floaters and hunters.

Sir Arthur C. Clarke expanded upon this theme in "A Meeting With Medusa" and in 2010: Odyssey Two. These stories featured creatures that were sort of a cross between a titanic jellyfish and a zeppelin. A similar ecosystem is in Ben Bova's novel Jupiter. There are also "sky whales" appearing in Dr. Robert Forward's Saturn Rukh.

As a rule, species that inhabit terrestrial planets (such as our species) do not have much interaction with aliens who live on gas giants. In the general this is because we and they have little or no common frames of reference which makes communication difficult. In the specific it is because we and they do not covet each other's real estate so there is no reason to go to war. In Poul Anderson's galactic novels, the human galactic empire and several gasworlder empires interpenetrated each other and ignored each other.

There are exceptions, such as Kevin J. Anderson's Saga of Seven Suns series. In the first novel, the human empires are unaware of the existence of the Gasworlders ("Hydrogues"). This proves to be unfortunate. When the humans test a device which converts gas giants into blazing suns (including the Hydrogue inhabitants), the remaining Hydrogues in the Hydrogue Empire become very very angry. Hilarity ensues as the diamond-armored Hydrogue dreadnoughts start kicking the living snot out of the human planets.

Another exception is described by Hal Clement here, where humans and jovians interact exactly like they were engaged in a war, but they are not. Humans are scoop-mining Jupiter's atmosphere, and the Jovians become furious at hypersonic scoopships obliterating their orchards, gardens, and flocks; not to mention Jovian citizens. So the Jovians start attacking the human scoopships. Humans will retaliate, and the net result will be very hard to distinguish from actual warfare.

Alternatively, the Jovians might see the scoopships as valuable concentration of metals, and start harvesting the scoopships. In that case the Jovians might limit the number of scoopships they grab, or the humans might get fed up and stop sending them.

Phil Masters, in his article for the game Traveller about his gas giant dwelling Jgd-Ll-Jagd aliens, had this to say: The chief point to note in such systems is that fuel-skimming a Jgd world is extremely unwise; shock waves from the pass will cause severe damage to the beings and their environment, and their response is certain to involve high-energy weapons fire. For this reason, Jgd systems are well-marked with navigational beacons. (Traveller tramp merchant ships routinely skim gas giants for free fuel)


(ed note: Howard Falcon is the first man to explore the upper atmosphere of Jupiter, using the spacecraft Kon-Tiki supported by a huge fusion-powered hot-air balloon)

     And then his concern changed to wonder—and to fear. What was developing in his line of flight was not a storm at all. Something enormous—something scores of miles across—was rising through the clouds.
     The reassuring thought that it, too, might be a cloud—a thunderhead boiling up from the lower levels of the atmosphere—lasted only a few seconds. No; this was solid. It shouldered its way through the pink-and-salmon overcast like an iceberg rising from the deeps.
     An iceberg floating on hydrogen? That was impossible, of course; but perhaps it was not too remote an analogy. As soon as he focused the telescope upon the enigma, Falcon saw that it was a whitish, crystalline mass, threaded with streaks of red and brown. It must be, he decided, the same stuff as the “snowflakes” falling around him—a mountain range of wax. And it was not, he soon realized, as solid as he had thought; around the edges it was continually crumbling and reforming…
     “I know what it is,” he radioed Mission Control, which for the last few minutes had been asking anxious questions. “It’s a mass of bubbles—some kind of foam. Hydrocarbon froth. Get the chemists working on … Just a minute!”
     The things moving up and down those waxen slopes were still too far away for Falcon to make out many details, and they must have been very large to be visible at all at such a distance. Almost black, and shaped like arrowheads, they maneuvered by slow undulations of their entire bodies, so that they looked rather like giant manta rays, swimming above some tropical reef.
     Perhaps they were sky-borne cattle, browsing on the cloud pastures of Jupiter, for they seemed to be feeding along the dark, red-brown streaks that ran like dried-up river beds down the flanks of the floating ctiffs. Occasionally, one of them would dive headlong into the mountain of foam and disappear completely from sight.
     Kon-Tiki was moving only slowly with respect to the cloud layer below; it would be at least three hours before she was above those ephemeral hills. She was in a race with the Sun. Falcon hoped that darkness would not fall before he could get a good view of the mantas, as he had christened them, as well as the fragile landscape over which they flapped their way.
     It was a long three hours. During the whole time, he kept the external microphones on full gain, wondering if here was the source of that booming in the night. The mantas were certainly large enough to have produced it; when he could get an accurate measurement, he discovered that they were almost a hundred yards across the wings. That was three times the length of the largest whale—though he doubted if they could weigh more than a few tons.
     “No,” said Falcon, answering Mission Control’s repeated questions about the mantas, “they’re still showing no reaction to me. I don’t think they’re intelligent—they look like harmless vegetarians. And even if they try to chase me, I’m sure they can’t reach my altitude. “
     Yet he was a little disappointed when the mantas showed not the slightest interest in him as he sailed high above their feeding ground. Perhaps they had no way of detecting his presence. When he examined and photographed them through the telescope, he could see no signs of any sense organs. The creatures were simply huge black deltas, rippling over hills and valleys that, in reality, were little more substantial than the clouds of Earth. Though they looked solid, Falcon knew that anyone who stepped on those white mountains would go crashing through them as if they were made of tissue paper.

     It was not easy to see, being only a little darker than the whirling wall of mist that formed its background. Not until he had been staring for several minutes did Falcon realize that he had met it once before.
     The first time it had been crawling across the drifting mountains of foam, and he had mistaken it for a giant, many-trunked tree. Now at last he could appreciate its real size and complexity and could give it a better name to fix its image in his mind. It did not resemble a tree at all, but a jellyfish—a medusa, such as might be met trailing its tentacles as it drifted along the warm eddies of the Gulf Stream.
     This medusa was more than a mile across and its scores of dangling tentacles were hundreds of feet long. They swayed slowly back and forth in perfect unison, taking more than a minute for each complete undulation—almost as if the creature was clumsily rowing itself through the sky.
     The other echoes were more distant medusae. Falcon focused the telescope on half a dozen and could see no variations in shape or size. They all seemed to be of the same species, and he wondered just why they were drifting lazily around in this six-hundred-mile orbit. Perhaps they were feeding upon the aerial plankton sucked in by the whirlpool, as Kon-Tiki itself had been.
     “Do you realize, Howard,” said Dr. Brenner, when he had recovered from his initial astonishment, “that this thing is about a hundred thousand times as large as the biggest whale? And even if it’s only a gasbag, it must still weigh a million tons! I can’t even guess at its metabolism. It must generate megawatts of heat to maintain its buoyancy.”
     “But if it’s just a gasbag, why is it such a damn good radar reflector?”
     “I haven’t the faintest idea. Can you get any closer?”
     For the next two hours Kon-Tiki drifted uneventfully in the gyre of the great whirlpool, while Falcon experimented with filters and camera contrast, trying to get a clear view of the medusa. He began to wonder if its elusive coloration was some kind of camouflage; perhaps, like many animals of Earth, it was trying to lose itself against its background. That was a trick used by both hunters and hunted.
     In which category was the medusa? That was a question he could hardly expect to have answered in the short time that was left to him. Yet just before noon, without the slightest warning, the answer came…
     Like a squadron of antique jet fighters, five mantas came sweeping through the wall of mist that formed the funnel of the vortex. They were flying in a V formation directly toward the pallid gray cloud of the medusa; and there was no doubt, in Falcon’s mind, that they were on the attack. He had been quite wrong to assume that they were harmless vegetarians.
     Yet everything happened at such a leisurely pace that it was like watching a slow-motion film. The mantas undulated along at perhaps thirty miles an hour; it seemed ages before they reached the medusa, which continued to paddle imperturbably along at an even slower speed. Huge though they were, the mantas looked tiny beside the monster they were approaching. When they flapped down on its back, they appeared about as large as birds landing on a whale.
     Could the medusa defend itself, Falcon wondered. He did not see how the attacking mantas could be in danger as long as they avoided those huge clumsy tentacles. And perhaps their host was not even aware of them; they could be insignificant parasites, tolerated as are fleas upon a dog.
     But now it was obvious that the medusa was in distress. With agonizing slowness, it began to tip over like a capsizing ship. After ten minutes it had tilted forty-five degrees; it was also rapidly losing altitude. It was impossible not to feel a sense of pity for the beleaguered monster, and to Falcon the sight brought bitter memories. In a grotesque way, the fall of the medusa was almost a parody of the dying Queen’s last moments (The Queen Elizabeth was the giant zeppelin Howard Falcon captained before the … accident).
     Yet he knew that his sympathies were on the wrong side. High intelligence could develop only among predators—not among the drifting browsers of either sea or air. The mantas were far closer to him than was this monstrous bag of gas. And anyway, who could really sympathize with a creature a hundred thousand times larger than a whale?
     Then he noticed that the medusa’s tactics seemed to be having some effect. The mantas had been disturbed by its slow roll and were flapping heavily away from its back—like gorged vultures interrupted at mealtime. But they did not move very far, continuing to hover a few yards from the still-capsizing monster.
     There was a sudden, blinding flash of light synchronized with a crash of static over the radio. One of the mantas, slowly twisting end over end, was plummeting straight downward. As it fell, a plume of black smoke trailed behind it. The resemblance to an aircraft going down in flames was quite uncanny.
     In unison, the remaining mantas dived steeply away from the medusa, gaining speed by losing altitude. They had, within minutes, vanished back into the wall of cloud from which they had emerged. And the medusa, no longer falling, began to roll back toward the horizontal. Soon it was sailing along once more on an even keel, as if nothing had happened.
     “Beautiful!” said Dr. Brenner, after a moment of stunned silence. “It’s developed electric defenses, like some of our eels and rays. But that must have been about a million volts! Can you see any organs that might produce the discharge? Anything looking like electrodes?”
     “No,” Falcon answered, after switching to the highest power of the telescope. “But here’s something odd. Do you see this pattern? Check back on the earlier images. I’m sure it wasn’t there before.”
     A broad, mottled band had appeared along the side of the medusa. It formed a startlingly regular checkerboard, each square of which was itself speckled in a complex sub-pattern of short horizontal lines. They were spaced at equal distances in a geometrically perfect array of rows and columns.
     “You’re right,” said Dr. Brenner, with something very much like awe in his voice. “That’s just appeared. And I’m afraid to tell you what I think it is.”
     “Well, I have no reputation to lose—at least as a biologist. Shall I give my guess?”
     “Go ahead.”
     “That’s a large meter-band radio array. The sort of thing they used back at the beginning of the twentieth century.”
     “I was afraid you’d say that. Now we know why it gave such a massive echo.”
     “But why has it just appeared?”
     “Probably an aftereffect of the discharge.”
     “I’ve just had another thought,” said Falcon, rather slowly. “Do you suppose it’s listening to us?”
     “On this frequency? I doubt it. Those are meter—no, decameter antennas—judging by their size. Hmm … that’s an idea!”
     Dr. Brenner fell silent, obviously contemplating some new line of thought. Presently he continued: “I bet they’re tuned to the radio outbursts! That’s something nature never got around to doing on Earth… We have animals with sonar and even electric senses, but nothing ever developed a radio sense. Why bother where there was so much light?
     “But it’s different here. Jupiter is drenched with radio energy. It’s worth while using it—maybe even tapping it. That thing could be a floating power plant!”

     Falcon had a momentary glimpse of great tentacles whipping upward and away. He had just time to note that they were studded with large bladders or sacs, presumably to give them buoyancy, and that they ended in multitudes of thin feelers like the roots of a plant. He half expected a bolt of lightning—but nothing happened.

From A MEETING WITH MEDUSA by Arthur C. Clarke (1971)

     Even as he fell through the roaring heart of the Great Red Spot, with the lightning of its continent-wide thunderstorms detonating around him, he knew why it had persisted for centuries though it was made of gases far less substantial than those that formed the hurricanes of Earth. The thin scream of hydrogen wind faded as he sank into the calmer depths, and a sleet of waxen snowflakes - some already coalescing into barely palpable mountains of hydrocarbon foam - descended from the heights above. It was already warm enough for liquid water to exist, but there were no oceans there; that purely gaseous environment was too tenuous to support them.
     He descended through layer after layer of cloud, until he entered a region of such clarity that even human vision could have scanned an area more than a thousand kilometres across. It was only a minor eddy in the vaster gyre of the Great Red Spot; and it held a secret that men had long guessed, but never proved.
     Skirting the foothills of the drifting foam mountains were myriads of small, sharply-defined clouds, all about the same size and patterned with similar red and brown mottlings. They were small only as compared with the inhuman scale of their surroundings; the very least would have covered a fair-sized city.
     They were clearly alive, for they were moving with slow deliberation along the flanks of the aerial mountains, browsing off their slopes like colossal sheep. And they were calling to each other in the metre band, their radio voices faint but clear against the cracklings and concussions of Jupiter itself.
     Nothing less than living gasbags, they floated in the narrow zone between freezing heights and scorching depths. Narrow, yes — but a domain far larger than all the biosphere of Earth.
     They were not alone. Moving swiftly among them were other creatures so small that they could easily have been overlooked. Some of them bore an almost uncanny resemblance to terrestrial aircraft and were of about the same size. But they too were alive — perhaps predators, perhaps parasites, perhaps even herdsmen.
     A whole new chapter of evolution, as alien as that which he had glimpsed on Europa, was opening before him. There were jet-propelled torpedoes like the squids of the terrestrial oceans, hunting and devouring the huge gasbags. But the balloons were not defenceless; some of them fought backs with electric thunderbolts and with clawed tentacles like kilometre-long chainsaws.
     There were even stranger shapes, exploiting almost every possibility of geometry — bizarre, translucent kites, tetrahedra, spheres, polyhedra, tangles of twisted ribbons.
     The gigantic plankton of the Jovian atmosphere, they were designed to float like gossamer in the uprising currents, until they had lived long enough to reproduce; then they would be swept down into the depths to be carbonized and recycled in a new generation.
     He was searching a world more than a hundred times the area of Earth, and though he saw many wonders, nothing there hinted of intelligence. The radio voices of the great balloons carried only simple messages of warning or of fear. Even the hunters, who might have been expected to develop higher degrees of organization, were like the sharks in Earth’s oceans — mindless automata.
     And for all its breathtaking size and novelty, the biosphere of Jupiter was a fragile world, a place of mists and foam, of delicate silken threads and paper-thin tissues spun from the continual snowfall of petrochemicals formed by lightning in the upper atmosphere. Few of its constructs were more substantial than soap bubbles; its most terrifying predators could be torn to shreds by even the feeblest of terrestrial carnivores.
     Like Europa on a vastly grander scale, Jupiter was an evolutionary cul-de-sac. Consciousness would never emerge here; even if it did, it would be doomed to a stunted existence. A purely aerial culture might develop, but in an environment where fire was impossible, and solids scarcely existed, it could never even reach the Stone Age.

From 2010 ODYSSEY TWO by Arthur C. Clarke (1982)

     Peering through the transparent glassteel of the observation bubble, Grant could see that Jupiter was not merely immense, it was alive.
     They were in orbit around the planet now, and its giant curving bulk loomed so huge that he could see nothing else, nothing but the bands and swirls of clouds that raced fiercely across Jupiter's face. The clouds shifted and flowed before his eyes, spun into eddies the size of Asia, moved and throbbed and pulsed like living creatures. Lightning flashed down there, sudden explosions of light that flickered back and forth across the clouds, like signaling lamps.
     There was life beneath those clouds, Grant knew. Huge balloonlike creatures called Clarke's Medusas that drifted in the hurricane-force winds surging across the planet. Birds that have never seen land, living out their entire lives aloft. Gossamer spider-kites that trapped microscopic spores. Particles of long-chain carbon molecules that form in the clouds and sift downward, toward the global ocean below.

     "The atmosphere/ocean system is like nothing we've seen before," Muzorawa said, his tone at last brightening, losing its guarded edge, taking on some enthusiasm. "For one thing, there's no clear demarkation between the gas phase and the liquid, no sharp boundary where the atmosphere ends and the ocean begins."
     "There's no real surface to the ocean," Grant said, wanting to show the older man that he wasn't totally ignorant.
     "No, not like on Earth. Jupiter's atmosphere gradually thickens, gets denser and denser, until it's not a gas anymore but a liquid. It's … well, it's something else, let me tell you."
     Before Grant could respond, Muzorawa hunched closer in his chair and went on, "It's heated from below, you see. The planet's internal heat is stronger than the solar influx on the tops of the clouds. The pressure gradient is really steep: Jupiter's gravity field is the strongest in the solar system."
     "Two point five four gees," Grant recited.
     "That's merely at the top of the cloud deck," Muzorawa said, waggling one hand in the air. "It gets stronger as you go down into the atmosphere. Do you have any idea of what the pressures are down there?"
     Grant shrugged. "Thousands of times normal atmospheric pressure."
     "Thousands of times the pressure at the bottom of the deepest ocean on Earth," Muzorawa corrected. A smile was growing on his face, the happy, contented smile of a scientist talking about his special field of study.
     "So the pressure squeezes the atmosphere and turns the gases into liquids."
     "Certainly! There's an ocean down there, an ocean ten times bigger than the whole Earth. Liquid water, at least five thousand kilometers deep, perhaps more; we haven't been able to probe that far down yet."

     Leviathan followed an upwelling current through the endless sea, smoothly grazing on the food that spiraled down from the abyss above. Far from the Kin now, away from the others of its own kind, Leviathan reveled in its freedom from the herd and their plodding cycle of feeding, dismemberment, and rejoining. To human senses the boundless ocean would be impenetrably dark, devastatingly hot, crushingly dense. Yet Leviathan moved through the surging deeps with ease, the flagella members of its assemblage stroking steadily as its mouth parts slowly opened and closed, opened and closed, in the ancient rhythm of ingestion.
     To human senses Leviathan would be staggeringly huge, dwarfing all the whales of Earth, larger than whole pods of whales, larger even than a good-size city. Yet in the vast depths of the Jovian sea Leviathan was merely one of many, slightly larger than some, considerably smaller than the eldest of its kind.
     There were dangers in that dark, hot, deep sea. Glide too high on the soaring currents, toward the source of the bountiful food, and the waters grew too thin and cold; Leviathan's members would involuntarily disassemble, shed their cohesion, never to reunite again. Get trapped in a treacherous downsurge and the heat welling up from the abyss below would kill the members before they could break away and scatter.
     Best to cruise here in the abundant world provided by the Symmetry, between the abyss above and the abyss below, where the food drifted down constantly from the cold wilderness on high and the warmth from the depths below made life tolerable.
     Predators swarmed through Leviathan's ocean: swift voracious Darters that struck at Leviathan's kind and devoured their outer members. There were even cases where the predators had penetrated to the core of their prey, rupturing the central organs and forever destroying the poor creature's unity. The Elders had warned Leviathan that the Darters attacked solitary members of the Kin when they had broken away from their group for budding in solitude. Still Leviathan swam on alone, intent on exploring new areas of the measureless sea.
     Leviathan remembered when the abyss above had erupted in giant flares of killing heat. Many of Leviathan's kind had disassembled in the sudden violence of those concussions. Even the everlasting rain of food had been disrupted, and Leviathan had known hunger for the first time in its existence. But the explosions dissipated swiftly and life eventually returned to normal again.

From JUPITER by Ben Bova (2000)

     The data file with what little was known about the H'rulka scrolled down the screen. "Floaters!" Kane said, reading. “The presumption is that they're intelligent gas bags that evolved in the upper atmosphere of a gas giant."
     “Interesting, if true," Wilkerson said, reading. “I'd like to know how they managed to develop a technology capable of building starships without access to metals, fire, smelting, solid raw materials, or solid ground."
     “What is it they're supposed to share with the Tushies, Chom?" Kane asked.
     “It is difficult to express," Noam replied, "as are most Turusch concepts. It appears, however, to be a philosophy based on the concept of depth."
     "Yeah, yeah. They order things higher to lower, instead of the way we do it."
     “It's no different than when we say something is second class," Wilkerson said, "and mean it's not as important or as high—up as first class."
     "It's still bass—ackward," Kane said.
     "The three conscious minds of a Turusch are considered by the Turusch to range from ‘high’ to 'here' to ‘low,' " Noam pointed out, "with ‘high‘ being the most primitive, most basic state of intelligence, and ‘low’ the most advanced and complex. For the Turusch, something called the Abyss represents depth, scope, danger, and tremendous power. We think the Turusch evolved to live on high plateaus or mountaintops on their world, with lower elevations representing sources of wealth or power—maybe a food source—as well as deadly windstorms. Abyssal Whirlwinds, they call them."
     “So, if the H'rulka are Jovian-type floaters," Wilkerson mused, “they might relate to the idea of the Abyss as the depths of a gas giant atmosphere. Hot, stormy, high energy, and definitely dangerous. A point of cognitive contact or understanding between them and the Turusch."
     “Sounds far-fetched to me," Kane said. " Besides, intelligence couldn't develop in a gas giant atmosphere. Absolutely impossible."
     "I've learned in this business to mistrust the phrase ‘absolutely impossible,’ Doctor," Wilkerson said. “Why do you think that?"
     “Because the vertical circulation of atmospheric cells in a gas giant atmosphere would drag any life form in the relatively benign higher levels down into the depths in short order," Kane replied. “They would be destroyed by crushing pressures and searing high temperatures. There'd be no way to preserve culture…or develop it, for that matter. No way to preserve historical records…art…music…learning. And, as you just said, they wouldn‘t get far without being able to smelt metals or build a technology from the ground up. " He smirked.
     "No ground.”
     “But we do have lots of examples of Jovian life," Wilkerson said.
     “None of it intelligent, " Kane replied. “It can’t survive long enough.”

     The H’rulka didn’t name their starships. A name suggested an individual personality, and the concept of the individual was one only barely grasped by H’rulka psychology. The H'rulka were, in fact, colony organisms; a very rough terrestrial analogue would have been the Portuguese Man of War…though the H’rulka were not marine creatures, and each was composed of several hundred types of communal polyps, rather than just four. Even their name for themselves —which came across in a hydrogen atmosphere as a shrill, high-pitched thunder generated by gas bags beneath the primary flotation sac—meant something like “All of Us,” and could refer either to a single colony, in the first person, or to the race as a whole.
     Individual H'rulka colonies took on temporary names, however, as dictated by their responsibilities within the community. Ordered Ascent was the commander of Warship 434, itself until recently a part of a larger vessel, Warship 432. The species didn’t have a government as humans would have understood the term, and even the captain of a starship was more of a principal decision maker than a leader.
     "It looks like home," the aggregate being called Swift Pouncer whispered over the private radio link. H'rulka possessed two entirely separate means of speech, two separate languages—one by means of vibrations in the atmosphere, the other by means of biologically generated radio bursts. Their natural radio transceivers, located just beneath the doughnut-shaped cluster of polyps forming their brains, allowed them to interface directly with their machines.
     “Similar," Ordered Ascent replied. “It appears to be inhabited."
     “We are receiving speech from one of the debris-chunks orbiting the world," Swift Pouncer replied. “It may be a vermin-nest. And…we are receiving speech from numerous sources much closer to the local star."
     Ordered Ascent tuned in to the broadband scanners and saw the other signals.
     Those members of Ordered Ascent capable of rational thought chided themselves. No matter how long they served within the far-flung fleets of the Sh‘daar, it was difficult to remember that vermin-nests frequently occurred, not within the atmospheres of true planets (gas giants), but on the inhospitable solid surfaces of debris (terrestrial planets).
     It was an unsettling thought. For just a moment, Ordered Ascent allowed themselves to pull back from the instrumentation feeds, to find steadiness and reassurance in the sight of the Collective Globe.
     The interior of the H'rulka warship was immense by human standards, but cramped to the point of stark claustrophobia for the species called All of Us. The area that served as the equivalent of the bridge on a human starship was well over two kilometers across, a vast spherical space filled by twelve free-floating H'rulka colonies in a dodecahedral array. Connected by radio to their ship, they used radio commands to direct and maneuver the huge vessel, fire the weapons, and observe their surroundings.
     They lived in the high-pressure atmosphere of gas giants, breathing hydrogen and metabolizing methane, ammonia, and drifting organic tidbits analogous to the plankton of Terran oceans. Until one of the Sh'daar's client species had shown them how to use solid materials to build spacecraft that defied both gravity and hard vacuum, they'd never known the interior of anything, never known what it was like to be enclosed, to be trapped inside. The interior of Warship 434 was large enough—just—to avoid triggering a serious claustrophobic-panic reflex in All of Us aggregates. Sometimes, they needed to see other aggregates adrift in the sky simply in order to feel safe.
     Data streamed down the radio link through Directed Ascent's consciousness. The inhabitants of this system were indeed native to the system debris.
     The All of Us race was unaccustomed to dealing with other sentient species. One of the primary reasons for this was, simply, their size; by almost any standards, the H'rulka were giants.
     An adult H'rulka consisted of a floatation gas bag measuring anywhere from two to three hundred meters across, with brain, locomotion and feeding organs, sensory apparatus and manipulators clustered at the bottom. Most other sentient species with which they'd had direct experience possessed roughly the same size and mass ratio to a H'rulka as an ant compared to a human.
     When the H'rulka thought of other life forms as “vermin," the thought was less insult than it was a statement of fact, at least as they perceived it. Within the complex biosphere of the H'rulka homeworld, there were parasites living on each All of Us colony that were some meters across. H'rulka simply found it difficult to imagine creatures as intelligent that were almost literally beneath their notice in terms of scale.
     “Commence acceleration, " Ordered Ascent directed. "We will move into the region of heavy radio transmission, and destroy targets of opportunity as they present themselves. "
     The H‘rulka warship, more than twenty kilometers across, began falling toward Sol, the inner system, and Earth.

From CENTER OF GRAVITY by Ian Douglas aka William H. Keith, Jr. (2011)

     I swim through clouds of sleeting hydrocarbons. No free oxygen here. In the years before idiot robot probes had reported this, before they tumbled helpless into the heat deck below. Now I float through these low-energy chemical agents wheregthel scientists are sure no animal life can persist. Active creatures require higher-energy reactions. So, too, do the voices from the Orb point out that the wavelengths I have observed in the warbling voices here are immense—hundreds of meters long. Far too large for any animal. So they are natural phenomena, and the Orb bids me to explore, measure. perceive this interesting event.
     In the soft waxen snowfall I navigate, and the sonic ripplings come again. This time the magnetic pulse is large, not a mere fluttering on top of the noise spectrum. I follow it to the southeast, downward. muting my fusion reactor to drop swiftly. In this misty torrent infrared and opticals are blind, but the microwaves bring back granulated pictures I can perceive. Ahead small points flicker and dance. I approach. They are below me but I do not know hon far.
     Corey emits a sharp spike of microwave energy and waits for the rebounding echo. Range is only fort; kilometers; she increases her fall. The steep descent takes him through a froth of white hydrocarbons as though she skis down alpine slopes. The gondola sways and creaks. A jolting bump comes as she falls through a pressure differential. The points below swell into grainy blobs.
     Suddenly the clouds disappear and Corey sees that he has emerged from the face of a vast milky wall. A vortex churns here, swirling the cloud banks in long circular arcs a hundred kilometers in diameter. At the center is a clear crystalline cylinder arcing up into heaven, a floor below of misty red. The infrared opticals swivel left, right, up—and Corey sees the source of the warbling.
     Below float things like ball bearings. They seem motionless, suspended in the beautiful clear ammonia They are small and give off a hot white sheen. An echo burst shows their true dimensions; they are at a range of nine kilometers and appear at least half a kilometer in diameter.
     Immense spheres. A consequence of the vortex? The ribbed cloud banks on all sides churn slowly as Corey sinks. The spheres have not moved. Then she notices a small point: the spheres do not rotate with the majestic cloud barrier around them. They are still. Humming, Corey drops further toward them. As she approaches their design breaks and they move in strangely hyperbolic paths. They form a net. They are maneuvering to Corey’s stimulus. In this vast waxen tunnel they maneuver. They are alive. Like Corey.

It begins to rain curling loops of hydrocarbons. Dollops of paste fall past Corey. They billow whitely around me in long filaments, as though spun from spools.     The gondola yaws as I take it down. We flip through the gnawing winds and fall below the misty hydrocarbon snow. The cyclone vent tunnels deep below me and the restless globes seem almost to float above the distant floor. Thirteen kilometers away the clouds revolve tirelessly. The ammonia cirrus is patchy, translucent, and veins of darker blue form faint tributaries beneath the skin.
     I send the signal Mara asked me to. The spheres below reply; magnetic fields weave and shift. I study them in the optical, the microwave.
     “You were right, Mara. There are long arcs across their surfaces. Regular. Rectangular. Inside each band is a pattern of pentagons. ”
     “That's how they broadcast. They form electrical current distributions over their surfaces. Otherwise a perfect sphere could not radiate anything.”
     “Their surfaces are antennas?"
     “They're linked into the magnetic field, Jupiter's natural field, in that region. So when they ripple currents on their surfaces, the field lines carry the signal away."
     "Thus they speak to each other. And to me.”
     “That’s not all. Jupiter is rich in radio energy. They’re linked into that. They probably feed off it, as well as chewing up those waxy hydrocarbons you see. They eat radio waves, the same way plants consume light. ”
     “They are coming nearer.”
     “Yes. There are six of them now. Average diameter one point three six kilometers. No, one point four one—they are expanding.”
     “Probably have sacs inside. They fill up with gas, just like you, then heat it and rise. ”

     “—I’ve calculated the total oscillater strength from a large number of spheres. It’s really impressive.” Mara paused and Bradley bit his lip in concentration. Vance, sittings beside him, seemed lost in his own private calculations.
     “So you don’t think those spherical creatures communicate locally by rippling the magnetic field?” Bradley said.
     “Well, it’s a possibility. The important point is that the signals from Alpha Libra could be made that way. We know Jupiter gives off huge radio bursts every once in a while. We’ve been listening to those radio thunderclaps for over a century now. The point is, that’s just noise. But suppose some life form could tap that source of energy. The same way a small transistor modulates the output of a large power supply, say. They could impress a signal on it, maybe even direct it toward a particular spot in the sky.”
     “I suppose it’s possible…” Vance began.
     “It wouldn’t take many of those spherical creatures to do it, if they were intelligent. I calculated the total oscillator strength for a bunch of spheres, evenly spaced around the planet. They could harness an immense amount of radio energy and modulate it at will.” Mara spoke quickly, precisely.
     “So there need not be any technology on a Jovian-type planet after all,” Bradley said. “It could simply be use of a natural mechanism.”
     “That’s the idea. Those beings down there, or whatever lives on a gas-giant planet in the Alpha Libra system, don’t know beans about electronics. But they sense electromagnetic forces as a part of the ebb and flow of life. They know only fluxes of things. No chemistry, no physics—but they're so big they don't need to know.”

From IF THE STARS ARE GODS by Gregory Benford & Gordon Eklund (1977)

The Jgd-ll-Jagd are a gas-giant dwelling intelligent species originating on a world on the coreward edge of the imperium. Although technically a minor race, they possessed very advanced technology even before they were first contacted by Vilani explorers in about -4200; in the period since, for obscure reasons, they have never employed jump drives, although their slower-than-light ships have ventured several parsecs from Jagd, and Jgdi colonies are spread across three subsectors. Jgd have very occasionally travelled further afield than this in heavy life support units carried by bulk transporters, and Jgd travellers have even collaborated with humaniti (Traveller term for the human species) in exploration and exploitation problems. The Jgd are the most advanced gas-giant dwellers in the Imperium.


The Jgd have roughly spherical bodies, about 3m in diameter. dotted with clusters of sensory cells, and with three long manipulative tendrils distributed regularly round the "equator". The densest mass of sensory organs, plus a large number of manipulative "feelers" and feeding structures, are sited on the lowest point of the body. The species' internal structures are based on a number of thin-walled compartments, one of which (near the body center) houses the brain (or at least the largest neural nexus), but most of which are empty but for gases secreted by the body chemistry. Control of secretion rates and partially-directed release of the gases (mostly hydrogen) give the Jgd considerable control over their atmospheric buoyancy and direction of flight, but these "living balloons" are still rather susceptible to atmospheric currents; it is generally believed that accidental population redistributions were common in primitive Jgdi society, leading to loosely-bonded communal organization and exceptional homogeneity in Jgdi culture.

In so far as such terms have meaning in this context, the Jgd seem to spring from omnivore/intermittent stock. There is only one sex; genetic interchange is achieved by air-borne spores, and reproduction is achieved by a sophisticated form of binary fission. Senses are based on extreme awareness of atmospheric vibration, plus very limited response to a very wide range of electromagnetic waves. Jgd can communicate limited information over long (20km +) distances, using pitch-modulated ultrasonic "whistling", but the primary form of "speech" involves electrical impulses transmitted by direct physical contact. It is thought that this allows the transfer of very large quantities of information at the semi-subconscious as well as the conscious level, further enhancing the homogeneity of Jgd culture.

The Jgd live extremely long lives; apparently. no condition of "old age" exists, although eventually a fissioning Jgd undergoes division of the parent brain, rather than generating a new "child" cerebrum. Average life of an identifiable Jgd individual, barring accident, is approximately 630 plus standard Imperial years.


The Jgd developed a mechanistic civilization when they learned to manipulate crystalline matter from the Jagd "icebergs"; thus crystallography is as central to their technological history as metallurgy is to mankind's. They developed activities akin to farming rather late, but their social systems are immensely refined, and spring from the need to organize for food-gathering and hunting. The basic social unit is termed the "hunt" by human sociologists, and consists of a cooperative body formed for a specific purpose — not always anything as short-lived as a hunt for food. Many "hunts" are millennia old, but even disregarding natural mortality. the membership is extremely flexible, with individuals leaving and joining quite frequently in most cases. Hunts to some extent resemble human businesses, trusts, or colleges, or Hiver nests, but each hunt actually holds a rather deeper role in Jgdi culture than this implies, in a way that only the Jgd themselves really comprehend. The crew of a short-range spaceship will usually comprise one hunt, while an interstellar craft will have three or four 'active' hunts aboard, plus the social nucleus of several more that become active when the ship establishes a colony or base on a new world. The system is remarkably flexible but robust.

The other key element in Jgdi psychology is an obsession with balanced exchanges, apparently running at least as deep as human curiosity, Aslan land-hunger, or Newt orderliness. A Jgd is literally incapable of "unilateral behavior". For example, the Jgd never initiate exploration for its own sake, but only send ships where there is a very high probability of finding exploitable resources, or of establishing a colony that might eventually send vessels back to Jagd. This obsession, apparently linked to the inherently bilateral nature of Jgdi conversation, has resulted in almost all contact between Jgd and other races taking the form of trade . It also causes the Jgd to operate a peculiar (and slightly brutal-seeming) legal system; theft is always punished by fines, violence by violence, and so on (in short, "an eye for an eye"). It is even hypothesized that the Jgd commenced interstellar travel when and only when they were first contacted by humaniti because only then was a degree of symmetry implied by the activity.

The homogeneity of Jgdi culture is a major factor in Jgd society, but it must not be overstated. Jgd are discrete and independent individuals, with distinct personalities and powerful personal drives; they have an idea of private property; they have personal violence, if not wars. Nonetheless, it is important to note that education — in the sense of a transmittal of data — is extremely easy for them; hence almost any Jgd can employ almost any Jgdi device or technique with at least minimal competence. This does not imply that the race lacks individuals specializing in particular fields of competence, merely that total incompetence in any field is rare.


Jgdi thought is alien to all other races' intelligence; hence communication is a persistent problem. The obvlous difficulty of simply conversing is generally solved by use of powerful human or Jgdi computer translators, but even these tend to struggle with many concepts; nor is pronunciation of synthesized phonemes always easy (the race name is a human corruption of something produced by an early Jgdi machine). In general, relations with humaniti and other races are restricted to trade and informational exchanges.

The Imperium classifies the Jgd as a friendly associate species with autonomous government; actually, no formal pacts exist, although relations are in a state of stable equilibrium. Jgd-inhabited systems will always be "patrolled" by a number of large and powerful vessels; these rarely take much interest in human affairs unless Jgdi interests are threatened. The chief point to note in such systems is that fuel-skimming a Jgd world is extremely unwise; shock waves from the pass will cause severe damage to the beings and their environment, and their response is certain to involve high-energy weapons fire. For this reason, Jgd systems are well-marked with navigational beacons. (Traveller tramp merchant ships routinely skim gas giants for free fuel)

Other races get on with the Jgd even less well than does humaniti (although there are Jgd colonies in the Centaur empire); mankind at least has long experience with the race. and the Jgdi exchange-obsession corresponds effectively to the human tradition of mercantile economics. There are no records of the Jgd hiring alien mercenaries for any but short-term jobs, or of small Jgd groups or individuals settling for long with other races save out of necessity.

The Jgd failure to construct jump drives is a mystery (the task could easily be performed by Jgd technology). One theory is that the race actually refuses to do so because it is impossible to enter into an exchange relationship with hyperspace, making the subject anathema to them. More plausible theories hold that jump travel is dangerous to them. Certainly, the Jgd travel units occasionally loaded onto human jump ships carry extremely heavy insulation.

From CONTACT: THE JGD-LL-JAGD by Phil Masters. Journal of the Traveller's Aid Society #17 (1981)

By the time I arrived in Los Angeles on February 13, 1979 to join the team of Cosmos Artists and work on the production of the visual effects, I had completed many dozens of preliminary concept drawings of the basic design of the HFS cloudscape environment, along with about a hundred conceptual designs for a variety of species of the three principle types which constituted the hypothetical biosphere and its ecological system: Hunters, Floaters and Sinkers — creatures with diverse shapes and anatomies, aerodynamic and buoyancy properties, propulsion mechanisms, feeding modes, social and navigation or migratory behaviors, communication, reproductive and defense and even stealth strategies, and more, that would not only fit them for life within a global atmospheric habitat, depending on conditions at various altitudes and within clouds of various composition, as well as the other dynamic aspects of weather, but also provide the basis for local or close-range interaction with other individuals of the same or different species, either in encountering those of their own kind (for reproductive purposes, or to maintain mutual proximity in gregarious social collections, for example), in encountering potential prey (Floaters grazed on diminutive but abundant Sinkers, while Hunters ambushed Sinkers for their stores of purified hydrogen as well as for nutrition), or encountered potential predators (both Floaters and Hunters could evolve elaborate defense strategies against attack by other Hunters). Carl's early speculations on Jovian atmosphere life in the 1960s had evolved and he eventually worked out the basic hunters, floaters and sinkers ecosystem which he described in a paper with Ed Salpeter: "Particles, Environments, and Possible Ecologies in the Jovian Atmosphere", which appeared in The Astrophysical Journal, Supplement Series in late 1975. Meanwhile I had been exploring the wide range of potential planetary habitats and designing a great many life-forms in the fleshed-out detail necessary for illustration, that could conceivably evolve and flourish within habitable environments, including Jovian-type atmospheres.

Alien Contact

Drake Equation

Back in 1961, there was a scientific conference held in the Green Bank facility about the search for extraterrestrial intelligence. In it, the host Dr. Frank Drake presented his now-famous "Drake Equation". The equation calculates N, which is the number of civilizations in our galaxy that it would be possible to communicate with by radio. After all, this equation was invented for a conference about communicating with aliens by radio.

It is a pity that we have not got a clue about the values of the last four parameters.

This means that the equation is pretty worthless for calculating the actual number of radio-using aliens out there. But it can be useful to study how proposed values for the parameters will affect N.

Note that N is the number of radio-using alien civilizations. Science fiction authors have been using the Drake Equation to calculate the number of alien civilizations, which is not quite the same thing. But close.

Authors can start off with a desired value for N, and work backwards to find values for the other parameters that will give the desired result. Or use their personal best guess for the parameters and see what value of N pops out.

The Drake Equation is:

N = R* × ƒp × ne × ƒl × ƒi × ƒc × L


  • N = the number of civilizations in our galaxy with which radio-communication might be possible
  • R* = the average rate of star formation in our galaxy
  • ƒp = the fraction of those stars that have planets
  • ne = the average number of planets/moons that can potentially support life per star that has planets
  • ƒl = the fraction of planets that could support life that actually develop life at some point
  • ƒi = the fraction of planets with life that actually go on to develop intelligent life (civilizations)
  • ƒc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
  • L = the length of time for which such civilizations release detectable signals into space

Current Estimates

R* the average rate of star formation in our galaxy
NASA and ESA data suggest current rate of star formation is about 7 per year.
ƒp the fraction of those stars that have planets
Microlensing surveys suggest this is pretty close to 1.
ne the average number of planets/moons that can potentially support life per star that has planets
0.4 if you are optimist, 0.1 if you are a pessimist.
0.4 is based the probability a planet is in the star's habitable zone, determined by solar heating. 0.1 is based on the galactic habitable zone, determined by regions of the galaxy with enough heavy elements and lack of near-by deadly supernovae.
Things get more uncertain when you consider that many moons (such as Europa or Titan) might support life. This drastically increases the number of habitable sites in a given solar system.
And proponents of the Rare Earth hypothesis say in order for their hypothesis to be true, it must be so closed to zero that Terra is the only one. Which violates the mediocrity principle and the Copernican principle, as well as being no fun at all for science fiction authors.
ƒl the fraction of planets that could support life that actually develop life at some point
The first SWAG parameter.
1.0 if you are an optimist, 0.13 if you are a pessimist.
1.0 is baased on the fact that life arose on Terra almost immediately after favorable conditions arose. 0.13 is based on an estimate by Charles H. Lineweaver and Tamara M. Davis based on a statistical argument derived from the length of time life took to evolve on Terra.
ƒi the fraction of planets with life that actually go on to develop intelligent life (civilizations)
The second SWAG parameter.
The value of this parameter is controversial, which is a code word for "who the heck knows?" Pretty much every value between 0.0 and 1.0 has been proposed, depending upon the proposer's particular axe to grind.
ƒc the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
The third SWAG parameter.
Also controversial. Some civilizations who have the technology to communicate might be paranoid enough that they keep silent. Yet other civilizations might not have the technology to communicate, but do have technology sufficiently noisy that it can be detected. Again: "who the heck knows?"
L the length of time for which such civilizations release detectable signals into space
The fourth SWAG parameter.
Most controversial of all. At its most innocuous, this could measure how long it takes for a civilization to become paranoid about giving away their position. At its most controversial, this could measure the average lifetime of a technological civilization, which is where the debate turns ugly. Over population, global warming, global thermonuclear war, and other terms for the Four Horsemen of the Apocalypse start being thrown around, and the discussion rapidly goes downhill from there.
More science-fictionally L could measure how long it takes a civilization to be cut short in an unexpected apotheosis by a Vingian Singularity

There have been several suggested modifications to the Drake Equation.

Alien civilizations might colonize other worlds. In a paper called The Great Silence — The Controversy Concerning Extraterrestrial Intelligent Life they derive three equations to calculate the effects of this on N. These equations require calculus so I'm not going to bother writing about them. You can find them in the report.

A given planet might give rise to several alien civilizations. An additional parameter is added for the Reappearance Factor, the average number of times a planet engenders alien civilizations. Like the other parameters this is very hard to estimate. A lot depends upon what kills off a given civilization, specifically how much it spoils the planet for making a new civilization. A little thing like global thermonuclear war and nuclear winter would eradicate a civilization but the planet would totally recover in a few million years. But if the primary star grew so swollen that it vaporized the planet, that would be the end. Another factor is that the first civilization to arise on a planet might use up all the fossil fuels and easily reached ores. The subsequent civilizations are at a disadvantage. They have to jump directly to off-shore oil drilling instead of just shooting a bullet in the ground like Jed Clampett.

An alien civilization, perfectly capable of sending radio messages, just might be paranoid enough that they keep silent. There might be civilization-killers lurking about, no sense attracting their attention. This is called the METI factor, for Messaging to ExtraTerrestrial Intelligence.

The July 2013 issue of Popular Science in an article about the TV show Doctor Who adds the parameter ƒd, which is the fraction of civilizations that can survived an alien attack from space. The "d" is for "Dalek".

Contact Motivation

An alien civilization of similar technological advancement to Terra could contact them first. The standard motives from 1950's SF novels are, according to Solomon Golumb:

  • Help!
  • Buy!
  • Convert!
  • Vacate!
  • Negotiate!
  • Work!
  • Discuss!

Sir Arthur C. Clarke notes that the nasty little short story by Damon Knight adds an eighth motive: Serve!

But don't forget the ever popular Interstellar Trading.

Contact Anti-motivation

There are also anti-motivations. Even if the human race does not want to go all genocidal on a newly discovered alien civilization's posterior, neither do you want to make it easy for them to kill you. As far back as Murray Leinster's classic "First Contact" (1945) the warning is when one of your starships encounters an alien starship, neither can let the other discover the location of their home planet. At least without finding their location as well. If the Terran starship stupidly lets the Blortch starship find the location of Terra, well Terra is at the Blortch's mercy. The Blortch can send their entire star fleet to blow Terra to Em-Cee-Squared, secure in the knowledge that the Terran star fleet has no idea where in the universe to dispatch a retaliation task force.

This only happens when mutually alien ships encounter each other in deep space. Naturally if the Terran exploration ship encounters the Blortch ship while both are orbiting the Blortch homeworld, well the cat is already out of the bag. Then the problem is how does the Terran ship get the vital information back to Terra without leading the Blortch back to your home.

Things can get quite ugly. In Michael McCollum's Antares Passage (1998) all ships have explosive charges on their navigation computers and the astrogators have been brainwashed to commit suicide if they are in danger of being captured by the enemy. In the beforementioned "First Contact", the human and alien ship try to destroy each other in battle, knowing that neither one dare run for home.

If you are really desperate, you will have to trigger the ship's self-destruct mechanism.

“Blasters, sir? What for?”

The skipper grimaced at the empty visiplate.

“Because we don’t know what they’re like and can’t take a chance! I know!” he added bitterly. “We’re going to make contacts and try to find out all we can about them—especially where they come from. I suppose we’ll try to make friends—but we haven’t much chance. We can’t trust them a fraction of an inch. We’ daren’t! They’ve locators. Maybe they’ve tracers better than any we have. Maybe they could trace us all the way home without our knowing it! We can’t risk a nonhuman race knowing where Earth is unless we’re sure of them! And how can we be sure? They could come to trade, of course—or they could swoop down on overdrive with a battle fleet,that could wipe us out before we knew what happened. We wouldn’t know which to expect, or when!”

Tommy’s face was startled.

“It’s all been thrashed out over and over, in theory,” said the skipper. “Nobody’s ever been able to find a sound answer, even on paper. But you know, in all their theorizing, no one considered the crazy, rank impossibility of a deep-space contact, with neither side knowing the other’s home world! But we’ve got to find an answer in fact! What are we going to do about them? Maybe these creatures will be aesthetic marvels, nice and friendly and polite—and, underneath, with the sneaking brutal ferocity of a mugger. Or maybe they’ll be crude and gruff as a farmer—and just as decent underneath. Maybe they’re something in between. But am I going to risk the possible future of the human race on a guess that it’s safe to trust them? God knows it would be worthwhile to make friends with a new civilization! It would be bound to stimulate our own, and maybe we’d gain enormously. But I can’t take chances. The one thing I won’t risk is having them know how to find Earth! Either I know they can’t follow me, or I don’t go home! And they’ll probably feel the same way!

He pressed the sleeve-communicator button again.

“Navigation officers, attention! Every star map on this ship is to be prepared for instant destruction. This includes photographs and diagrams from which our course or starting point could be deduced. I want all astronomical data gathered and arranged to be destroyed in a split second, on order. Make it fast and report when ready!”

He released the button. He looked suddenly old. The first contact of humanity with an alien race was a situation which had been foreseen in many fashions, but never one quite so hopeless of solution as this. A solitary Earth-ship and a solitary alien, meeting in a nebula which must be remote from the home planet of each. They might wish peace, but the line of conduct which best prepared a treacherous attack was just the seeming of friendliness. Failure to be suspicious might doom the human race—and a peaceful exchange of the fruits of civilization would be the greatest benefit imaginable. Any mistake would be irreparable, but a failure to be on guard would be fatal.

From FIRST CONTACT by Murray Leinster (1945)

“Yes. The region of the galaxy from which you have come is that which we call the desert. It is an area almost entirely devoid of planets. Would you mind telling me which star is your home?”

Cohn stiffened.

“I’m afraid our government would not permit us to disclose any information concerning our race.”

“As you wish. I am sorry you are disturbed. I was curious to know — ” He waved a negligent hand to show that the information was unimportant. We will get it later, he thought, when we decipher their charts.

“There are no charts,” he grumbled, “no maps at all. We will not be able to trace them to their home star.”

The reports were on his desk and he regarded them with a wry smile. There was indeed no way to trace them back. They had no charts, only a regular series of course-check coordinates which were preset on their home planet and which were not decipherable. Even at this stage of their civilization they had already anticipated the consequences of having their ship fall into alien hands.

From ALL THE WAY BACK by Michael Shaara (1952)

(ed note: human ("monster") ship has surprised the alien Ryall planet and Ryall ship the Space Swimmer)

     “I have a message for you from Ossfil of Space Swimmer.”
     “Proceed with the message.”
     “‘The monsters have me surrounded and I am unable to reach the gateway. I am taking evasive action, but will not be able to escape. Request instructions. Ossfil, commanding Space Swimmer.’“
     Varlan muttered a few deep imprecations to the evil star before replying. “Transmit the following: ‘From Varlan of the Scented Waters to Ossfil of Space Swimmer. As a minimum, you will destroy your astrogation computer and trigger the amnesia of your astrogator. After that is done, you may act on your own initiative.’“

     “What of the astronomical data in his computer?”
     “I have given the order that he destroy his computer and trigger his astrogator’s amnesia. Failing that, of course, he will destroy his ship.”

     “It is regrettable, Varlan of the Scented Waters, but I still have considerable astronomical data in my brain, including knowledge of the positions of many of the gateways throughout the hegemony.”
     Varlan “blinked” in horror at Salfador’s revelation.

     “You must have been fitted with an amnesia spell. Give me your trigger code and I will excise the knowledge from your brain,” she said.
     The miserable look on Salfador’s features was all the answer she needed. Even so, he said, “I’m afraid that I was never fitted with such. I had not intended to be an astrogator on a starship, and therefore, had no need.”

     To ask a philosopher to camp in the woods like a barbarian was unthinkable. Even more unthinkable, however, was allowing Salfador to fall into the grasp of the monsters. Finally, she said: “You know what you must do, of course.”
     Salfador signaled his agreement. “I have already done so. There are many poisons in the medical kits. I injected myself with one before coming here. Do not fear. My death will be quite painless.”

(ed note: the humans have captured the alien ship Space Swimmer, and are puzzling over the alien's strange behavior)

     “Naw. Shot him with a dart. He’ll be all right, ‘cept that he’s crazy as a high plateau jumper.”
     “How so?”
     “I found him amidships in one of the equipment rooms. He had this big bar he’d ripped out of some machinery and was using it to beat holy hell out of some access panel. Looked to me like he wanted to get through it and into the machinery beyond.

     “What did you say just now, Corporal?” he asked.
     “I said this damned crazy centaur attacked me, sir...”
     “No, about his trying to smash a machine. What machine?”
     “‘Fraid I don’t recognize this alien machinery too good, sir.”
     “Take me to it.”
     Sayers led the way, followed by Philip Walkirk and Sergeant Barthol. They moved through gloomy corridors until they reached a small compartment almost at the very center of the spherical ship.
     “Yonder machine over there, sir!” Sayer said, playing the beam from his hand lamp over a dented access panel.
     Philip gazed at the panel, blinked, and then emitted a low whistle.
     “This thing important, sir?” Barthol asked.
     “You might say that,” Philip replied. “What Corporal Sayers refers to as ‘yonder machine’ is their astrogation computer. The fact that he was trying to beat it to death may mean that their normal destruct mechanism failed to operate properly.”
     “That good, sir?”
     Philip Walkirk’s sudden laughter startled the two noncoms. “That box, Sergeant, may well contain information vital to the conduct of the war.”
     “What information, sir?”
     “If we’ve been very, very lucky, we may just dredge up a foldspace topology chart for the whole damned Ryall hegemony!”

(ed note: an alien Species called the Makers needs a faster-than-light starship drive. Over a period of several thousand years they send out STL robot space probes with artificial intelligence to seek out other species, and ask them about FTL.

Probe 53935 was passing by the solar system in 2065 CE when it spotted the signature of an FTL starship around Procyon. It didn't have enough reaction mass to head to Procyon, so it decided to visit Terra and bargain for some remass. Ordinarily it would have ignored Terra because they were too technologically primitive to be worthwhile.

Due to some unfortunate squabbles between have and have-not nations, the probe was destroyed by the Pan-African alliance. A part of the probes AI survived. It decided to commit suicide and told the Terran forces to back out of the blast radius.

The United Nations convinced the AI to make a bargain. The UN would mount a STL expedition to Procyon and transport the AI there. It would then help the AI fulfil its prime directive and carry the secret of FTL back to the Maker civilization. In exchange the AI would give the UN some of the Maker's technology.

About 300 years later the colonist at Procyon called "Alphans" find the secret of FTL, and head back to Terra in their first starship. They bring along the AI. They treat the original bargain made with the AI to be a sacred trust, and are committed to bringing the secret of AI back to the Makers.

Unfortunately the government of Terra is apprehensive, and feel zero obligation to honor the bargain.)

      “Williams is concerned that there are a great many species in this galaxy who would regard a starship full of humans the way we would look upon an ownerless cow with a bag of gold strapped to its neck. He fears an attack on Earth should the Alphans lose the secret of the FTL drive to aliens.”
     “Why attack us?”
     “We are potential competitors.”
     “Surely there are security measures we could take to hide the location of our home system, Sergei. Hypnosis, drugs, orders for astrogators to suicide on capture, that sort of thing.”
     Vischenko shook his head. “It might not be that simple, Executive. An FTL starship leaves a radiation wake wherever it goes. Using the proper instruments, this trail can be detected decades later. That was how the probe knew that Procyon was the site of an FTL base. It is also the reason we were so quick to detect the Alphans’ arrival. By the way, we are still tracking their wake. Scientists tell me we’ll be able to watch it all the way back to Procyon, some twelve light-years distant.”
     Duval considered Vischenko’s words for long seconds, and then nodded slowly. “I’m beginning to see your point. We’ll proceed with caution, at least until we know what we’re up against.”

     “Well,” (Henri) Duval (the equivalent of a World President) asked, turning to his one-time mentor, “what do you think?
     Josip Betrain was an old man, even by contemporary standards. He was also a sick man. He suffered from a degenerative nerve disorder that caused his body to twitch continuously. After fifteen decades of life, Betrain had not long to live. That fact gave him an unusually clear view of things, and made him a particularly valued advisor to the Chief Executive.
     “You are trying to decide whether we should participate in this adventure of theirs to search out the probe’s creators?”
     “And you want my advice?”
     “I presume that was a rhetorical question since you know damned well that I do.”
     “My advice to you, Executive, is simple. You are going to end up going out to the stars whether you like it or not. You had best like it.”
     Duval sighed “You know, Josip, I sometimes think you overplay your role as my Oracle of Delphi. Would you care to explain your last remark?”
     “Nothing mysterious about it. If you decide to allow the expedition, you will have to build a fleet of ships of your own to accompany them. It is patently obvious that they will never find the Maker world on their own. The logistics are far too great for their society to handle. And, Henri, if you decide that fulfilling this ‘Promise’ of theirs endangers the Earth, then you will need that star fleet even more.
     “I don’t follow you.”
     “Sure you do. However, when the facts are unpleasant, even a Chief Executive tends to avert his eyes. Therefore, let history record that it was from my lips that the fateful words first fell.” Betrain drew himself up and seemed to gather strength from somewhere within. When he spoke, the usual hoarse whisper was replaced by a reedy monotone.
     “If Professor Williams’ scenario is correct, Executive, it is your solemn duty to prevent the Alphans from giving the FTL secret to any alien species whatsoever! That includes the beings that built the probe. Once the secret is out, it is out!  Obviously, the only way to stop the spread of a dangerous technology is at the source. To do that, you will have to take control of Procyon VII itself. You may well be forced into becoming that which you have long despised — an imperialist aggressor who launches an unprovoked attack against people who have done nothing to deserve it.”
     “I take it then, Josip, that you agree with Professor Williams’ assessment of the danger?”
     “I do not!” the old man growled. There was a deep rattle in his chest and he was overcome with a wracking cough. After the spasm passed, he lifted himself upright, stared at Duval with rheumy eyes, and continued. “Study your history, man! Every forward step the race has ever taken was opposed by someone afraid of what we would find. So far, the ‘naysayers’ have been 100% wrong. As a result, history does not speak kindly of leaders who lose their nerve at critical moments.
     “But my opinion doesn’t count. Neither does yours. We cannot lose sight of the possibility that the Colin Williams’s of this world might be right this time. Maybe the galaxy is full of alien monsters just waiting for a shipload of hayseed humans to blunder into their clutches.”

From PROCYON'S PROMISE by Michael McCollum (1985)

The Fermi Paradox

Sooner or later one has to confront the Fermi Paradox. A good overview of the problem is David Brin's Xenology: The Science of Asking Who's Out There and The 'Great Silence': the Controversy Concerning Extraterrestrial Intelligent Life. For more detail, try Where Is Everybody?: Fifty Solutions to the Fermi Paradox and the Problem of Extraterrestrial Life by Stephen Webb.

The Fermi Paradox points out that:

  • There is a high probability of large numbers of alien civilizations
  • But we don't see any

So by the observational evidence, there are no alien civilizations. The trouble is that means our civilization shouldn't be here either, yet we are.

The nasty conclusion is that our civilization is here, so far. But our civilization is fated for death, and the probability is death sooner rather than later. This is called The Great Filter, and it is a rather disturbing thought.

And the problem is not just that we see no alien civilizations. It is the fact that humans exist at all. Terra should by rights be an alien colony, with the aliens using dinosaurs as beasts of burden.

Using slower-than-light starships it would be possible to colonize the entire galaxy in 5 million to 50 million years. By one alien civilization. Naturally the time goes down the higher the number of civilizations are colonizing.

So during the current life-span of our galaxy, it would have been possible for it to be totally colonized 250 to 2500 times. At a minimum.

The Fermi Paradox asks why isn't Terra an alien colony right now?

Granted an alien civilization might not be interested in colonization. There might be thousands of civilizations all content on their home planets, with nary a thought of colonization at all. But remember it only takes one. For anti-colonization bias to be a solution to the Fermi paradox, every single freaking civilization would have to share it with no exceptions at all. If there is even one then the galaxy is colonized in the blink of a galactic eye.

Again, the problem with no alien civilizations existing is that it implies our civilization should not exist either. A galaxy with 400 billion stars and 13.8 billion years of time to play with, it should have produced either millions of civilizations or zero civilizations. But not just one civilization. Violates the mediocrity principle and the Copernican principle, that does. Every single time people have theorized that Terra has a central specially favoried position in the universe, it has turned out to be ludicrously wrong.

Which means our civilization exists so far, but it is due to become extinct quite soon.

This means it is going to be real bad news if we discover any alien life forms at all in our solar system, even bacteria. It will imply that life is common in the universe, life on Terra is not special, The Great Filter must have wiped out all the other civilizations, and we are next.

Naturally there are quite a few solutions proposed. Stephen Webb's book has fifty of them. Some examine the Drake Equation's parameters with an eye towards finding unexpected constraints on the values.

The Wikipedia article has a broad outline of various classifications the solutions fall into. Refer to that article for details.

  • Few, if any, other civilizations currently exist
    • No other civilizations have arisen (see also Rare Earth Hypothesis)
    • It is the nature of intelligent life to destroy itself
    • It is the nature of intelligent life to destroy others (Berserker Hypothesis)
    • Life is periodically destroyed by naturally occurring events
    • Human beings were created alone
    • Inflation hypothesis and the youngness argument (multiple universes with synchronous gauge probability distribution)
  • They do exist, but we see no evidence
    • Communication is improbable due to problems of scale
    • Intelligent civilizations are too far apart in space or time
    • It is too expensive to spread physically throughout the galaxy
    • Human beings have not been searching long enough
    • Communication is improbable for technical reasons
    • Humans are not listening properly
    • Aliens aren't monitoring Earth because Earth is not superhabitable
    • Civilizations broadcast detectable radio signals only for a brief period of time
    • They tend to experience a technological singularity
    • They are too busy online
    • They are too alien
    • They are non-technological
    • The evidence is being suppressed (the Conspiracy Theory)
    • They choose not to interact with us
    • They don't agree among themselves (no talking with Terrans until Galactic UN is in agreement)
    • Earth is deliberately not contacted (Zoo Hypothesis)
    • Earth is purposely isolated (Planetarium Hypothesis)
    • It is dangerous to communicate
    • The Fermi paradox itself is what prevents communication (implies that communcation is lethal)
    • They are here unobserved

Dr. Geoffrey A. Landis has a possible solution based on Percolation Theory. A more depressing solution is in Toolmaker Koan by John McLoughlin. It argues that any intelligent species that invents tools starts a process of accelerated progress that inevitably leads to extinction by warfare over dwindling resources.

A more nasty solution is in the classic The Killing Star by Charles Pelligrino and George Zebrowski, Run To The Stars by Michael Rohan, and Antares Dawn by Michael McCollum (see below). It boils down to a variant on the Berserker Hypothesis.

In A Fire Upon The Deep, Vernor Vinge postulates a solution based upon Terra being located in the less desirable geographic region of the galaxy.

Fermi's Nightmare

When we’re considering what kind of tropes to use in a science-fictional setting, we need to be aware of an observation most commonly called the Fermi Paradox. It goes something like this:

  • The galaxy is very large (at least a hundred billion stars) and very old (billions of years).
  • Stars with planets appear to be very common, and it seems reasonable to assume that many of those planets provide conditions suitable for life.
  • Given enough time, there seems to be a significant probability that any planet supporting life will eventually give rise to an intelligent species capable of tool use and a high-technology civilization.
  • If a high-technology civilization becomes capable of interstellar travel, even using very slow methods, it should be able to colonize the entire galaxy within a few million years. If easier or faster interstellar travel turns out to be possible, that process could take considerably less time.
  • Therefore, we should see evidence of previous visits to and colonization of our own solar system. Possibly a lot of such evidence.
  • We don’t. Where is everybody?

An astute reader will notice that there’s all manner of hand-waving in that argument. When Enrico Fermi walked through it back in 1950, we didn’t know very much about the galaxy around us. Most of the probabilities and quantities implicit in the argument were unclear. Today we have more evidence for a few items – we know that most stars probably have planets, for example, because we’ve detected thousands of them in recent years. Still, at a lot of points we’re arguing from a sample size of one – our own situation – and that’s always dangerous.

It’s entirely possible that Fermi’s observation isn’t a paradox at all. Perhaps life is much rarer than we assume. Or perhaps complex life is vanishingly rare – the universe may be crammed full of bacteria, with the appearance of big tool-using animals like us as an aberration. Or perhaps high-technology civilizations almost never figure out the trick of interstellar travel, either because they don’t survive long enough, or because interstellar travel is even harder than we think. We need more data.

On the other hand, when we want to design a space-operatic setting, we have to implicitly assign values to several of those quantities. So it behooves us to assign values that make sense together, and don’t run us straight into Fermi’s Paradox at warp speed.

I’d like to suggest the following rule of thumb:

If a given science-fiction setting has multiple interstellar civilizations, and the typical civilization undergoes territorial expansion at a rate of 1% per year, then no civilization should be expected to survive longer than 1,000 years. For every factor of ten by which the growth rate is reduced, the allowable lifespan for interstellar civilizations will increase by a factor of ten.

The reasoning here is fairly straightforward.

Starting with a single fully occupied star system, a civilization which grows at 1% per year doubles its territory in not quite 70 years. Now it fully occupies two star systems (or, more likely, it has that one home system and small colonies in several other star systems). In another 70 years, it fully occupies four systems. In another 70 years, it fully occupies eight. The power of compound-interest expansion: in about 2,500 years that civilization has fully occupied one hundred billion star systems, and at that point the Milky Way is full to bursting.

If there are multiple interstellar cultures around, and that kind of growth is typical for them, then we have a problem. In the past billion years, Earth should have been overrun many times over. The Fermi Paradox is in full force, unless something comes along to eat civilizations for dinner long before they reach that point. That could be a recurring natural disaster, or an intelligent super-cultural force that cuts young civilizations short. Or maybe civilizations tend to stop their territorial expansion, turn to other concerns, and then die out. It’s your setting, your choice.

For the purpose of this rule of thumb, I stipulate that the lifespan limit is only 1,000 years. This is a nice round number, and it permits us to assume the presence of many interstellar civilizations at any given time, all of them following the same dynamics of growth and decay.

If we want star-faring cultures to live longer, then we have to adjust the other parameter in the model, their typical rate of growth. Given how the math works, if we divide the growth rate by ten, the allowable lifespan in turn grows almost exactly by a factor of ten. So if we want our interstellar civilizations to last on the order of ten thousand years, we need to assume a growth rate of 0.1% per year. A hundred thousand years, 0.01% per year.

Notice what this says about interstellar cultures, assuming that we aren’t living in a “Rare Earth” universe in which there just aren’t any intelligent beings other than ourselves. The Fermi Paradox seems to suggest that longevity requires very slow growth. The growth rates required to permit the existence of million-year-old civilizations are so low that they’re just about indistinguishable from a steady state.

Perhaps this shouldn’t surprise us. After all, on our little planet and for most of human history, our own population growth rates were very low. Only the Industrial Revolution, and subsequent improvements in agricultural technology, sanitation, and medicine, permitted us to undergo a period of rapid expansion. Human population growth peaked at a little over 2% in the early 1960s, is currently back down to a little over 1%, and may not be sustainable at even that pace for very long. The galaxy as a whole is a much bigger field of endeavor . . . but given even a little time, compound interest has a way of overwhelming such differences in scale.

Now, notice one other implication: none of this should be a surprise to any culture that manages to figure out the trick of interstellar travel. By the time such a culture has been out among the stars for a while, it should have a good estimate for every parameter in the relevant mathematics. Which means that if our characters live in a fast-growing interstellar civilization, or they know of other such cultures, they should be very worried.

Why? Well, let’s look at a specific science-fictional universe that I’ve been playing with for some time: the one in the popular Mass Effect series of video games.

In Mass Effect, humanity emerges out into the galaxy in the mid-22nd century, to find a number of other interstellar civilizations already well-established. The oldest of these civilizations date back to about three thousand years ago. We learn of dozens or even hundreds of colony worlds settled in that time, some of them with populations in the billions, which would suggest a typical territorial growth rate that’s modest but still significant – say, about 0.2% or so per year. We also learn that there have been plenty of former interstellar cultures, all of them now extinct.

In the course of the first Mass Effect game, the protagonist discovers the existence of the Reapers, a force of godlike sentient machines that periodically sweep the Milky Way and exterminate all advanced civilizations. The story details the effort to delay the return of the Reapers, and then to defeat them and win the survival of galactic civilization once they do return.

It’s all very well-done space opera, with plenty of attention given to a plausible setting and plot. But there’s one detail that should set off alarms for us: the protagonist has a great deal of difficulty persuading anyone in authority that the Reapers even exist, until it’s far too late.

From one perspective, of course, this is fine. The hero forced to act on her own, because those in authority don’t take a threat seriously, is a perfectly useful trope to apply. Yet after we’ve given the Fermi Paradox some thought, we have to ask how the rulers of any galactic civilization could remain fully ignorant of the implications.

Notice what our rule of thumb suggests for this universe. Interstellar cultures growing at about 0.2% per year tell us that the maximum lifespan for any civilization is somewhere around five thousand years. Most of that time has already passed. Just about everyone who’s paying attention should probably be looking around with a great deal of apprehension right now.

We know that interstellar travel is easy, and that civilizations can grow with significant speed. We know that there have been other interstellar cultures before our own. Why wasn’t the galaxy already full when we arrived? Why are all those other cultures extinct?

What made them extinct? What might be waiting to make us extinct before we manage to fill up the galaxy with our own colonies? Shouldn’t we be trying to find out?

If you’re a potential author, maybe your fictional galaxy won’t have anything in it like the Reapers. But there has to be something to keep the galaxy from becoming over-crowded, many times over during its long history. You need to take a moment and consider what that might be.

From Fermi's Nightmare by Sharrukin (2015)

Exterminomachy and Consequences

RocketCat sez

This is really nasty, but far too plausible. There are too many human beings who would find the paranoid logic to be perfectly reasonable.

It's sort of like a self-fulfilling Berserker Hypothesis.


Little is known of the culture, former civilization, and even biology of the skrandar species. Extreme xenophobes, they had little interaction with the species of the Worlds even post-contact. The destruction of their homeworld along with the rest of Skranpen (Charred Waste)’s1 inner system in the self-induced nova of their sun (on detecting the relativistic approach of the Serene Fleet) has left little archaeological evidence available for study. Even the name of the Skranpen system, like that of the species, is phonemically generated and institute-assigned. What little is known of the skrandar is based on abstractions from damaged and disabled examples of the skrandar berserker probes and the two identified replication sites captured in the Exterminomachy.

What has been extracted from these sources (see declassified reports tagged PYRETIC PHAGE) suggests that the skrandar were in the grip of a peculiar type of madness at the end. It is believed among crypto-archaeologists that the skrandar had a preexisting cultural obsession with the Precursor Paradox: namely, why, when we see evidence of elder races and Precursor civilizations aplenty, and both life and intelligence appear to be relatively common within the Starfall Arc, has the galaxy not been colonized and/or hegemonized long since by ancient civilizations?

(Indeed, given the relative isolation of the Skranpen system, this paradox must have weighed even more heavily on the minds of the skrandar than on those species which originated in more populous galactic neighborhoods.)

The leading hypothesis, therefore, is that xenognosis came as a severe trauma to the skrandar; upon seeing the impossible, in the light of a presumed filter preventing starfaring civilizations from existing, they collectively went mad. If, they reasoned, there was – must be – some reason for the destruction of starfaring civilizations, then they themselves could only escape that fate by becoming that reason. And so they turned as a species to the manufacture of berserker probes designed to cull all other sapient, starfaring life.

It is easy for us today, looking back on the Exterminomachy, to attribute the tragedy of the skrandar solely to some inherent flaw in the species. But consider this: the skrandar were isolated, by their own choice. They had the opportunity, therefore, to go mad quietly, unknown to the rest of the civilized galaxy, hearing no voices but their own unreason.

For this reason, among others, the Exploratory Service at this time maintains its pro-contact, pro-intervention, pro-socialization policy towards emerging species. Whatever the short-term cultural impact of xenognosis might be, in the longer term, they very much endorse the view that an ounce of prevention today is better than a gigaton of cure tomorrow.

1. While identified here as a system of the Charred Waste constellation, the Skranpen system is not connected to the stargate plexus; it is, however, located centrally in the constellation in real space.







(ed note: OMRD stands for "Office of Military Research and Development")

Radio Silence


That is the expected number of intelligent civilizations in our galaxy, according to Drake’s famous equation. For the last 78 years, we had been broadcasting everything about us – our radio, our television, our history, our greatest discoveries – to the rest of the galaxy. We had been shouting our existence at the top of our lungs to the rest of the universe, wondering if we were alone. Thirty-six million civilizations, yet in almost a century of listening, we hadn’t heard a thing. We were alone.

That was, until about five minutes ago.

The transmission came on every transcendental multiple of hydrogen’s frequency that we were listening to. Transcendental harmonics – things like hydrogen’s frequency times pi – don’t appear in nature, so I knew it had to be artificial. The signal pulsed on and off very quickly with incredibly uniform amplitudes; my initial reaction was that this was some sort of binary transmission. I measured 1679 pulses in the one minute that the transmission was active. After that, the silence resumed.

The numbers didn’t make any sense at first. They just seemed to be a random jumble of noise. But the pulses were so perfectly uniform, and on a frequency that was always so silent; they had to come from an artificial source. I looked over the transmission again, and my heart skipped a beat. 1679 – that was the exact length of the Arecibo message sent out 40 years ago. I excitedly started arranging the bits in the original 73 x 23 rectangle. I didn’t get more than halfway through before my hopes were confirmed. This was the exact same message. The numbers in binary, from 1 to 10. The atomic numbers of the elements that make up life. The formulas for our DNA nucleotides. Someone had been listening to us, and wanted us to know they were there.

Then it came to me – this original message was transmitted only 40 years ago. This means that life must be at most 20 lightyears away. A civilization within talking distance? This would revolutionize every field I have ever worked in – astrophysics, astrobiology, astro-

The signal is beeping again.

This time, it is slow. Deliberate, even. It lasts just under five minutes, with a new bit coming in once per second. Though the computers are of course recording it, I start writing them down. 0. 1. 0. 1. 0. 1. 0. 0... I knew immediately this wasn’t the same message as before. My mind races through the possibilities of what this could be. The transmission ends, having transmitted 248 bits. Surely this is too small for a meaningful message. What great message to another civilization can you possibly send with only 248 bits of information? On a computer, the only files that small would be limited to…


Was it possible? Were they really sending a message to us in our own language? Come to think of it, it’s not that out of the question – we had been transmitting pretty much every language on earth for the last 70 years… I begin to decipher with the first encoding scheme I could think of – ASCII. 0. 1. 0. 1. 0. 1. 0. 0. That’s B... 0. 1. 1 0. 0. 1. 0. 1. E…

As I finish piecing together the message, my stomach sinks like an anchor. The words before me answer everything.


From Radio Silence credited to bencbartlett ()

The Dark Forest Rule

In 2006 author Liu Cixin wrote a novel named 三体 (The Three-Body Problem) which won the Chinese Science Fiction Galaxy Award in 2006 and the 2015 Hugo Award for Best Novel. It proposed a solution to the Fermi Paradox which was plausible enough to get analyzed in a paper published in the Journal of the British Interplanetary Society (The Dark Forest Rule: One Solution to the Fermi Paradox).

It is very similar to the scenario set out in Pelligrio & Zebrowski's The Killing Star.

The Dark Forest Rule has two basic hypotheses:

  1. Survival is the primary requirement of civilization
    (civilizations that don't care if they live or die won't last long)

  2. Civilization grows and expands continuously, whereas the total cosmic materials remain constant
    (all warfare boils down to two monkeys and one banana)

and two basic concepts:

  1. Suspicion Chain: poor communication between different civilizations in the universe results in civilizations distrusting each other
    (you can't trust that mysterious tribe who lives over the hills, I'll bet you they have horns growing out of their heads and eat human flesh)

  2. Technology Explosion: technologies in civilizations may achieve explosive breakthroughs and development at any time, which are beyond the accurate estimation of any distant civilization with its own technological level
    (you never know when some other nation will unexpectedly invent the Ultimate Weapon while your back is turned)

The result of combining these hypotheses and concepts:

  1. Civilizations in the universe are competing for resources
    (there ain't enough to share)

  2. Civilization cannot trust other civilizations
    (because they have horns growing out of their heads and eat human flesh)

  3. Civilizations cannot be confident about the advancement of their technology
    (at any moment those horned flesh eaters who want our galactic resources might invent a Nicoll-Dyson Beam and kill us all)

In other words It's The Law Of The Jungle. "Every man for himself," "anything goes," "need of the sole outweights the need of the many," "survival of the strongest," "survival of the fittest," "kill or be killed," "dog eat dog" and "eat or be eaten." Basically the "state of nature" as proposed by Thomas Hobbes.

The Three-Body Problem

“Universe is a dark forest. Every civilization is a hunter with gun in hand and he sneaks in the forest. He must be careful enough as there are other hunters in the forest. If he discovered other lives, he can only do one thing: shoot it. In this forest, other lives are hell and constant threats. Any life that will expose his existence will be killed soon. This is the picture of universe civilizations”

From The Three-Body Problem by Liu Cixin (2006)

The hypothesis "survival is the primary requirement of civilization" is not saying that there are no alien civilzations that are moral-advocating or selfless. The hypothesis is saying that such civilizations will be slaughtered by survivalist civilizations. Much like how the Roman civilization was defeated by the Goths and the Song Dynasty was defeated by Mongolian cavalry. In other words "nice guys finish last."

The hypothesis "civilization grows and expands continuously, whereas the total cosmic materials remain constant" does not mean that civilizations will fight because they are greedy for all the resources. It means civilization will fight because they do not know how many resources will ensure survival, the only safe assumption is "all of them." In a broader sense all activities of all living civilizations increase entropy (by the second law of thermodynamics). Therefore other civilizations must be killed to slow down the heat death of the universe, thus prolonging the time for this civilization to live.

The limit of the speed of light means that two-way communication between civilizations can take years to centuries. This helps create the suspicion chain. But even with rapid communication you will instantly find yourself in the middle of The Prisoner's Dilemma which is the suspicion chain raised to the second power.

In addition, even limited communication with another civilization might inadvertently trigger in them a technological explosion, and suddenly you will find that you've brought a knife to a gun fight.

Given all that, the only safe strategy is to instantly try and kill any alien civilizations you come across. Especially since chances are they will have followed the same train of logic, and will instantly try to kill you once they discover you exist.

If you kill them, the worst thing that might happen is you'll discover they were a non-expanding moral-advocation race. Which is a shame, but even so they were creating entropy.

If you leave them alone, you are rolling the dice and risking the extinction of your entire species.

Of course when you try to kill them, you'll have to be covert about it. Just in case they turn out to be more powerful than you are. You'll have to try to avoid letting them discover that your species even exists in the first place. Keeping in mind that if they are incredibly more advanced than you are, they will have found you first and you are doomed. But there isn't much you can do about that. Given the "Apes or Angels" scenario, chances are there will be a huge technological inequality between the two civilizations. Meeting between civilizations of the same tech level will be rare.

The big draw-back to instantly killing a newly discovered alien civilization is that another ultra-highly advanced alien civilization will notice your attack (that is, a civilization vastly more advanced than you are). Then they will probably obliterate you. If you are worried about that, the best strategy is to try and hide your entire civilization, and avoid contact with anyone.

Unless the ultra-highly advanced alien civilization does not obliterate you, for fear that a third ultra-ultra-ultra-highly advanced alien civilization will notice the attack and obliterate them.

So, the answer to the Fermi Paradox is either:

  1. All the other civilizations have been killed except for a couple of bloodthirsty ones

  2. All the other civilizations are doing their best to hide, either to avoid attracting the attention a killer civilization or because they are a killer civilzation which thinks that killing is too much of a risk

The Killing Star

From The Killing Star by Charles Pellegrino and George Zebrowski (you really should read this book):

The great silence (i.e. absence of SETI signals from alien civilizations) is perhaps the strongest indicator of all that high relativistic velocities are attainable and that everybody out there knows it.

The sobering truth is that relativistic civilizations are a potential nightmare to anyone living within range of them. The problem is that objects traveling at an appreciable fraction of light speed are never where you see them when you see them (i.e., light-speed lag). Relativistic rockets, if their owners turn out to be less than benevolent, are both totally unstoppable and totally destructive. A starship weighing in at 1,500 tons (approximately the weight of a fully fueled space shuttle sitting on the launchpad) impacting an earthlike planet at "only" 30 percent of lightspeed will release 1.5 million megatons of energy -- an explosive force equivalent to 150 times today's global nuclear arsenal... (ed note: this means the freaking thing has about nine hundred mega-Ricks of damage!)

I'm not going to talk about ideas. I'm going to talk about reality. It will probably not be good for us ever to build and fire up an antimatter engine. According to Powell, given the proper detecting devices, a Valkyrie engine burn could be seen out to a radius of several light-years and may draw us into a game we'd rather not play, a game in which, if we appear to be even the vaguest threat to another civilization and if the resources are available to eliminate us, then it is logical to do so.

The game plan is, in its simplest terms, the relativistic inverse to the golden rule: "Do unto the other fellow as he would do unto you and do it first."...

When we put our heads together and tried to list everything we could say with certainty about other civilizations, without having actually met them, all that we knew boiled down to three simple laws of alien behavior:


    If an alien species has to choose between them and us, they won't choose us. It is difficult to imagine a contrary case; species don't survive by being self-sacrificing.


    No species makes it to the top by being passive. The species in charge of any given planet will be highly intelligent, alert, aggressive, and ruthless when necessary.



Your thinking still seems a bit narrow. Consider several broadening ideas:

  1. Sure, relativistic bombs are powerful because the antagonist has already invested huge energies in them that can be released quickly, and they're hard to hit. But they are costly investments and necessarily reduce other activities the species could explore. For example:
  2. Dispersal of the species into many small, hard-to-see targets, such as asteroids, buried civilizations, cometary nuclei, various space habitats. These are hard to wipe out.
  3. But wait -- while relativistic bombs are readily visible to us in foresight, they hardly represent the end point in foreseeable technology. What will humans of, say, two centuries hence think of as the "obvious" lethal effect? Five centuries? A hundred? Personally I'd pick some rampaging self-reproducing thingy (mechanical or organic), then sneak it into all the biospheres I wanted to destroy. My point here is that no particular physical effect -- with its pluses, minuses, and trade-offs -- is likely to dominate the thinking of the galaxy.
  4. So what might really aged civilizations do? Disperse, of course, and also not attack new arrivals in the galaxy, for fear that they might not get them all. Why? Because revenge is probably selected for in surviving species, and anybody truly looking out for long-term interests will not want to leave a youthful species with a grudge, sneaking around behind its back...

I agree with most parts of points 2, 3, and 4. As for point 1, it is cheaper than you think. You mention self-replicating machines in point 3, and while it is true that relativistic rockets require planetary power supplies, it is also true that we can power the whole Earth with a field of solar cells adding up to barely more than 200-by-200 kilometers, drawn out into a narrow band around the Moon's equator. Self-replicating robots could accomplish this task with only the cost of developing the first twenty or thirty machines. And once we're powering the Earth practically free of charge, why not let the robots keep building panels on the Lunar far side? Add a few self-replicating linear accelerator-building factories, and plug the accelerators into the panels, and you could produce enough anti-hydrogen to launch a starship every year. But why stop at the Moon? Have you looked at Mercury lately? ...

Dr. Wells has obviously bought into the view of a friendly galaxy. This view is based upon the argument that unless we humans conquer our self-destructive warlike tendencies, we will wipe out our species and no longer be a threat to extrasolar civilizations. All well and good up to this point.

But then these optimists make the jump: If we are wise enough to survive and not wipe ourselves out, we will be peaceful -- so peaceful that we will not wipe anybody else out, and as we are below on Earth, so other people will be above.

This is a non sequitur, because there is no guarantee that one follows the other, and for a very important reason: "They" are not part of our species.

Before we proceed any further, try the following thought experiment: watch the films Platoon and Aliens together and ask yourself if the plot lines don't quickly blur and become indistinguishable. You'll recall that in Vietnam, American troops were taught to regard the enemy as "Charlie" or "Gook," dehumanizing words that made "them" easier to kill. In like manner, the British, Spanish, and French conquests of the discovery period were made easier by declaring dark- or red- or yellow-skinned people as something less than human, as a godless, faceless "them," as literally another species.

Presumably there is some sort of inhibition against killing another member of our own species, because we have to work to overcome it...

But the rules do not apply to other species. Both humans and wolves lack inhibitions against killing chickens.

Humans kill other species all the time, even those with which we share the common bond of high intelligence. As you read this, hundreds of dolphins are being killed by tuna fishermen and drift netters. The killing goes on and on, and dolphins are not even a threat to us.

As near as we can tell, there is no inhibition against killing another species simply because it displays a high intelligence. So, as much as we love him, Carl Sagan's theory that if a species makes it to the top and does not blow itself apart, then it will be nice to other intelligent species is probably wrong. Once you admit interstellar species will not necessarily be nice to one another simply by virtue of having survived, then you open up this whole nightmare of relativistic civilizations exterminating one another.

It's an entirely new situation, emerging from the physical possibilities that will face any species that can overcome the natural interstellar quarantine of its solar system. The choices seem unforgiving, and the mind struggles to imagine circumstances under which an interstellar species might make contact without triggering the realization that it can't afford to be proven wrong in its fears.

Got that? We can't afford to wait to be proven wrong.

They won't come to get our resources or our knowledge or our women or even because they're just mean and want power over us. They'll come to destroy us to insure their survival, even if we're no apparent threat, because species death is just too much to risk, however remote the risk...

The most humbling feature of the relativistic bomb is that even if you happen to see it coming, its exact motion and position can never be determined; and given a technology even a hundred orders of magnitude above our own, you cannot hope to intercept one of these weapons. It often happens, in these discussions, that an expression from the old west arises: "God made some men bigger and stronger than others, but Mr. Colt made all men equal." Variations on Mr. Colt's weapon are still popular today, even in a society that possesses hydrogen bombs. Similarly, no matter how advanced civilizations grow, the relativistic bomb is not likely to go away...

We ask that you try just one more thought experiment. Imagine yourself taking a stroll through Manhattan, somewhere north of 68th street, deep inside Central Park, late at night. It would be nice to meet someone friendly, but you know that the park is dangerous at night. That's when the monsters come out. There's always a strong undercurrent of drug dealings, muggings, and occasional homicides.

It is not easy to distinguish the good guys from the bad guys. They dress alike, and the weapons are concealed. The only difference is intent, and you can't read minds.

Stay in the dark long enough and you may hear an occasional distance shriek or blunder across a body.

How do you survive the night? The last thing you want to do is shout, "I'm here!" The next to last thing you want to do is reply to someone who shouts, "I'm a friend!"

What you would like to do is find a policeman, or get out of the park. But you don't want to make noise or move towards a light where you might be spotted, and it is difficult to find either a policeman or your way out without making yourself known. Your safest option is to hunker down and wait for daylight, then safely walk out.

There are, of course, a few obvious differences between Central Park and the universe.

There is no policeman.

There is no way out.

And the night never ends.

From The Killing Star by Charles Pellegrino and George Zebrowski
A Minor Flaw

Attacking with relativistic rockets may be a good idea if there are only two technological species, but if there are two then it seems to me that it is likely there will be more. Using a relativistic rocket to destroy a planet will reveal your position AND indicate that you are hostile to any possible third race that is out there.

To extend the Central Park analogy, the muzzle flash when you fire off your gun reveals your position and identifies that you are hostile to anyone else out there.

Bill Seney
Another Minor Flaw

After The Killing Star I found a flaw in Pellegrino's logic, called him, explained it to him, and he conceded the point.

Here it is: OK, you've detected radio signals from X light years away and, following the logic, prepare to send a planet-killer at the source. Only ... will the civilization still be killable by a single fractional-C strike when you get there? If it isn't, you now have definitely pissed-off neighbors that want you dead. And civilizations advance.

By the time you've registered the signal and can send a planet-killer back, optimistic assumptions about the speed of your NAFAL (not as fast as light) drives (say, 0.2c) suggest it's been a minimum of 6 * X years since the target civilization's radio output became detectable (which will be some time after the discovery of radio).

Everything hinges on your estimate of the interval between commencement of large-scale radio emissions and self-sufficient offworld colonies; call that N. The radius in lightyears beyond which you don't dare try for the kill is N / 6 . For a reasonable conservative assumption about N — say, 300 years, that comes to 50 lightyears. Not a large distance.

Go ahead and play with the assumptions — speed of NAFAL drives, the radio-to-colonies interval. It's pretty hard to come up with a plausible scenario in which launching the civ-killer looks like a good idea.

Eric Raymond (2015)

Run To The Stars

From Run To The Stars by Michael Scott Rohan (1982). The heroes have discovered the Dreadful Secret that the BC world government is hiding: explorers have discovered the first known alien species, and BC is sending a huge missile to kill all the aliens.

"Alien," muttered Ryly, and coughed rackingly, unpleasant in the confined space. "The Colony - people, that was different, but - Bellamy, hey, hold on. Think a minute. So what you say's true - couldn't the BC still be right? I mean, these're aliens, man! Better we'd never contacted them, but now they've found us - hell, we can't trust them! We can't be sure! It's the human race at stake."

"Ye're sayin' that genocide - worse than that, even - that ye like the idea?" demanded Kirsty.

"Hell, no, think I'm Stalin or somethin'? Like I said - better we'd laid low, shut up, kept to ourselves, safe, Earth and the Colony both. But these things, we can't afford to take a risk with them! Better the missile cleans the mistake off the slate, things quiet down an' we're safe again. I don't like it, I hate it - but then I'm not so wild about some of the things you feel you were justified in doin' either..."

..."Ryly, you're no fool, but you're bloody well talking like one. That missile can be tracked, man! With the mass it'll have by the time it connects it'll leave a wake of gravitational disturbance - on interstellar radiation, for a start - pointing right back this way. That's why it's a one-shot weapon - no second chances! Safe? What's safe? As if we could somehow hide away from the rest of the universe. Not as long as we use any kind of broadcast communication, we can't Think of it! Just round here, in our own little neighborhood, three planets inhabited, two with intelligent life, two with roughly the same kind of life! There must be millions of inhabited worlds out there, whatever the experts spout. Some like us, some not. Sooner or later one of them's bound to track back our communications overspill and find us. What then? Under the bed?"

"If that missile hits the target," said Kristy venomously, "we'll have tae hide. Shrink back into our own wee system, never make a noise, never stir outside it. What if any other race ever found out what we'd done? Then we'd never be safe. They'd never trust us. Not for an instant. There's bound to be some of them who think like you, Ryly. We'd be giving them grand evidence, wouldn't we? They'd wipe us out like plague germs and feel good about it!"

My own imagination was striking sparks off Kirsty's and kindling an evil flame. "Unless..." I began, and actually had trouble shaping the thought. "Unless we got them first. At once, on first contact. A pre-emptive strike, before they could possibly have a chance to find out about us. Hellfire, isn't that a glorious future history for us! A race of paranoid killers, skulking in our own backwater system when we might have had the stars! Clamping down on exploration, communications, anything that might lead someone else to us and make us stain our hands again with the same old crime... Carrying that weight down the generations. What would that make of us?"

"Predators," breathed Kirsty, "Carrion-eaters - no, worse, ghouls, vampires, killing just tae carry on our own worthless shadow-lives."

From Run To The Stars by Michael Scott Rohan (1982)


Daniel Krouse brought to my attention some important new ideas on this matter:

Peter Watts wrote a book, "Blindsight" that covers a first contact scenario from a new and interesting angle ... I wanted to share with you an excerpt that I feel would serve as a good example on your Aliens page. It has a lot in it actually, as the whole of it tackles first contact from an evolutionary and game theory POV and raises some good points (such as the possibility that even our TV signals could be considered a hostile action). But my favorite bit and the part I include here is where he expands on Powell and Pellegrino's 3 assumptions with a 4th one: Technology implies belligerence.

Daniel Krouse

Once there were three tribes. The Optimists, whose patron saints were Drake and Sagan, believed in a universe crawling with gentle intelligence — spiritual brethren vaster and more enlightened than we, a great galactic siblinghood into whose ranks we would someday ascend. Surely, said the Optimists, space travel implies enlightenment, for it requires the control of great destructive energies. Any race which can't rise above its own brutal instincts will wipe itself out long before it learns to bridge the interstellar gulf.

Across from the Optimists sat the Pessimists, who genuflected before graven images of Saint Fermi and a host of lesser lightweights. The Pessimists envisioned a lonely universe full of dead rocks and prokaryotic slime. The odds are just too low, they insisted. Too many rogues, too much radiation, too much eccentricity in too many orbits. It is a surpassing miracle that even one Earth exists; to hope for many is to abandon reason and embrace religious mania. After all, the universe is fourteen billion years old: if the galaxy were alive with intelligence, wouldn't it be here by now?

Equidistant to the other two tribes sat the Historians. They didn't have too many thoughts on the probable prevalence of intelligent, spacefaring extraterrestrials — but if there are any, they said, they're not just going to be smart. They're going to be mean.

It might seem almost too obvious a conclusion. What is Human history, if not an on going succession of greater technologies grinding lesser ones beneath their boots? But the subject wasn't merely Human history, or the unfair advantage that tools gave to any given side; the oppressed snatch up advanced weaponry as readily as the oppressor, given half a chance. No, the real issue was how those tools got there in the first place. The real issue was what tools are for.

To the Historians, tools existed for only one reason: to force the universe into unnatural shapes. They treated nature as an enemy, they were by definition a rebellion against the way things were. Technology is a stunted thing in benign environments, it never thrived in any culture gripped by belief in natural harmony. Why invent fusion reactors if your climate is comfortable, if your food is abundant? Why build fortresses if you have no enemies? Why force change upon a world which poses no threat?

Human civilization had a lot of branches, not so long ago. Even into the twenty-first century, a few isolated tribes had barely developed stone tools. Some settled down with agriculture. Others weren't content until they had ended nature itself, still others until they'd built cities in space. We all rested eventually, though. Each new technology trampled lesser ones, climbed to some complacent asymptote, and stopped — until my own mother packed herself away like a larva in honeycomb, softened by machinery, robbed of incentive by her own contentment. (ed note: Read the book for that bit to make sense)

But history never said that everyone had to stop where we did. It only suggested that those who had stopped no longer struggled for existence. There could be other, more hellish worlds where the best Human technology would crumble, where the environment was still the enemy, where the only survivors were those who fought back with sharper tools and stronger empires. The threats contained in those environments would not be simple ones. Harsh weather and natural disasters either kill you or they don't, and once conquered — or adapted to — they lose their relevance. No, the only environmental factors that continued to matter were those that fought back, that countered new strategies with newer ones, that forced their enemies to scale ever-greater heights just to stay alive.

Ultimately, the only enemy that mattered was an intelligent one.

And if the best toys do end up in the hands of those who've never forgotten that life itself is an act of war against intelligent opponents, what does that say about a race whose machines travel between the stars? The argument was straightforward enough. It might even have been enough to carry the Historians to victory — if such debates were ever settled on the basic of logic, and if a bored population hadn't already awarded the game to Fermi on points. But the Historian paradigm was just too ugly, too Darwinian, for most people, and besides, no one really cared any more. Not even the Cassidy Survey's late-breaking discoveries changed much. So what if some dirtball at Ursae Majoris Eridani had an oxygen atmosphere? It was forty-three light years away, and it wasn't talking; and if you wanted flying chandeliers and alien messiahs, you could build them to order in Heaven. (ed note: Again, read the book to understand Heaven) If you wanted testosterone and target practice you could choose an afterlife chock-full of nasty alien monsters with really bad aim. If the mere thought of an alien intelligence threatened your worldview, you could explore a virtual galaxy of empty real estate, ripe and waiting for any God-fearing earthly pilgrims who chanced by. It was all there, just the other side of a fifteen-minute splice job and a cervical socket. Why endure the cramped and smelly confines of real-life space travel to go visit pond scum on Europa?

And so, inevitably, a fourth Tribe arose, a Heavenly host that triumphed over all: the Tribe that Just Didn't Give A Sh*t. They didn't know what to do when the Fireflies showed up. So they sent us, and — in belated honor of the Historian mantra — they sent along a warrior, just in case. It was doubtful in the extreme that any child of Earth would be a match for a race with interstellar technology, should they prove unfriendly. Still, I could tell that Bates' presence was a comfort, to the Human members of the crew at least. If you have to go up unarmed against an angry T-rex with a four-digit IQ, it can't hurt to have a trained combat specialist at your side.

At the very least, she might be able to fashion a pointy stick from the branch of some convenient tree.

From Blindsight by Peter Watts (2006)

We Know You Are Out There


We made a mistake. That is the simple, undeniable truth of the matter, however painful it might be. The flaw was not in our Observatories, for those machines were as perfect as we could make, and they showed us only the unfiltered light of truth, The flaw was not in the Predictor, for it is a device of pure, infalliable logic, turning raw data into meaningful information without the taint of emotion or bias. No, the flaw was within us, the Orchestrators of this disaster, the sentients who thought themselves beyond such failings. We are responsible.

It began a short while ago, as these things are measured, less than 66 Deeli ago, though I suspect our systems of measure will mean very little by the time anyone receives this transmission. We detected faint radio signals from a blossoming intelligence 214 Deelis outward from the Galactic Core, as photons travel. At first, crude and unstructured, these leaking broadcasts quickly grew in complexity and strength, as did the messages they carried. Through our Observatories we watched a race of strife and violence, populated by a barbaric race of short-lived, fast-breeding vermin. They were brutal and uncultured things which stabbed and shot and burned each other with no regard for life or purpose. Even their concepts of Art spoke of conflict and pain. They divided themselves according to some bizarre cultural patterns and set their every industry to cause of death.

They terrified us, but we were older and wiser and so very far away, so we did no fret. Then we watched them split the atom and breech the heavens within the breadth of one of their single, short generations, and we began to worry. When they began actively transmitting messages and greetings into space, we felt fear and horror. Their transmissions promised peace and camaraderie to any who were listening, but we had watched them for too long to buy into such transparent deceptions. They knew we were out here, and they were coming for us.

The Orchestrators consulted the Predictor, and the output was dire. They would multiply and grow and flood out of their home system like some uncountable tide of Devourer worms, consuming all that lay in their path. It might that 68 Deelis, but they would destroy us if left unchecked. With aching carapaces, we decided to act, and sealed our fate.

The Gift of Mercy was 84 strides long with a mouth 2/4 that in diameter, filled with many 44 weights of machinery, fuel, and ballast. It would push itself up to 2/8th of light speed with its onboard fuel, and then begin to consume interstellar Primary Element 2/2 to feed its unlimited acceleration. It would be traveling at nearly light speed when it hit. They would never see it coming. Its launch was a day of mourning, celebration, and reflection. The horror of the act we had committed weighed heavily upon us all; the necessity of our crime did little to comfort us.

The Gift had barely cleared the outer cometary halo when the mistake was realized, but it was too late. The Gift could not be caught, could not be recalled or diverted from its path. The architects and work crews, horrified at the awful power of the thing upon which they labored, had quietly self-terminated in droves, walking unshielded into radiation zones, neglecting proper null pressure, safety or simply ceasing their nutrient consumption until their metabolic functions stopped. The appalling cost in lives had forced the Orchestrators to streamline the Gift's design and construction. There had been no time for the design or implementation of anything beyond the simple, massive engines and the stablizing systems. We could only watch in shame and horror as the light of genocide faded in infrared against the distant void.

They grew, and they changed, in a handful of lifetimes. They abolished war, abandoned their violent tendencies and turned themselves to the grand purpose of life and Art. We watched them remake first themselves, and then their world. Their frail, soft bodies gave way to gleaming metals and plastics, they unified their people through an omnipotent communications grid and produced Art of such power and emotion, the likes of which the Galaxy has never seen before. Or again, because of us.

They converted their home world into a paradise (by their standards) and many 106s of them poured out into the surrounding system with a rapidity and vigor that we could only envy. With bodies built to survive every environment from the day-lit surface of their innermost world, to the atmosphere of their largest gas giant and the cold void in between, they set out to sculpt their system into something beautiful. At first we thought them to be simple miners, stripping the rocky planets and moons for vital resources, but then we began to see the purpose to their construction, the artworks carved into every surface, and traced across the system in glittering lights and dancing fusion trails. And still, our terrible Gift approached.

They had less than 22 Deeli to see it, following so closely on the tail if its own light. In that time, oh so brief even by their fleeting lives, more than 1010 sentients prepared for death. Lovers exchanged last words, separated by worlds and the tyranny of light speed. Their planet-side engineers worked frantically to build sufficient transmission to upload countless masses with the necessary neural modification, while those above dumped lifetimes of music and literature from their databanks to make room for passengers, Those lacking the required hardware of the time to acquire it consigned themselves to death, lashed out in fear and pain, or simply went about their lives as best they could under the circumstances.

The Gift arrived suddenly, the light of its impact visible in our skies, shining bright and cruel even to the unaugmented ocular receptor. We watched and we wept for our victims, dead so many Deelis before the light of their doom had even reached us. Many 64s of those who had been directly or even tangentially involved in the creation of the Gift sealed their spiracles was past as a final penance for the small roles they had played in this atrocity. The light dimmed, the dust cleared, and our Observatories refocused upon the place where their shining blue world had once hung in the void, and found only dust and the pale gleam of an orphaned moon, wrapped in a thin, burning wisp of atmosphere that had once belonged to its parent.

Radiation and relativistic shrapnel had wiped out much of the inner system, and continent-sized chunks of molten rock carried screaming ghosts outward at interstellar escape velocities, damned to wander the great void for an eternity. The damage was apocalyptic, but not complete. From the shadows of the outer worlds, tiny points of light emerged, thousands of fusion trails of single ships and world ships and everything in between, many 106s of survivors in flesh and steel and memory banks, ready to rebuild. For a few moments we felt relief, even joy, and we were filled with the hope that their culture and Art would survive the terrible blow we had dealt them. Then came the message, tightly focused at our star, transmitted simultaneously by hundreds of their ships.

"We know you are out there, and we are coming for you."


From We Know You Are Out There by Panini's Cupcake (2008)

Why I Don't Worry


Last time I explained why I don't lie awake nights worrying that contact with extraterrestrial civilizations will lead to humanity getting conquered by invading alien armies. It's just too hard to be worth the effort.

But what if the aliens don't want to conquer the Earth? What if they just want to wipe us out?

It's actually easier to destroy rival civilizations across interstellar distances than it is to conquer them. Conquest, after all, involves fairly large armies, supplies — and above all, deceleration when the starships reach the target system. Relativisitic warheads don't need any reaction mass or energy to slow down (though they might need a motor for terminal guidance). A few bricks hitting the Earth at 99.9 percent of lightspeed would do as much damage as the asteroid which killed the dinosaurs. (And those bricks could be intelligently targeted to maximize the harm they do.)

At least one conference, featuring the likes of Isaac Asimov and Jill Tarter (I can't find a link to reference it), proposed that launching a salvo of relativistic projectiles would be the optimum strategy for any species which so much as detects another advanced civilization nearby. Get them before they get us!

As with interstellar conquest, I don't buy it. There are sound logical reasons why first strikes across interstellar distances are very bad ideas.

Hi There! Launching an interstellar death-barrage is not something you can hide. The energy output of a relativistic rocket is very bright. Any civilization with immense space-based telescopes (like the kind we're planning to use for detecting extrasolar planets) can spot them at arbitrarily large distances. If the projectiles are launched using some kind of ground-based laser or maser system, the launching beam is like a beacon shining in the direction of the target.

This is important for two reasons.

First, the target world gets some warning that the strike is on its way. That means they can they could launch a counter-strike during the flight time of the projectiles.

(How do they know it's a salvo of projectiles rather than a fleet of friendly starships? The color and brightness of the exhaust reveals the energy output involved; the Doppler shift reveals the acceleration profile and thus the mass of the payload. If someone's launching small objects at very high velocities, it's not a friendly gesture. About the only way to fool the target is to go all-in and launch a very big projectile, at a velocity which might match the mission profile for a starship. But when the "starship" fails to start decelerating, the target system still gets a clue something is amiss, and a slower vehicle could be intercepted.)

Second, other civilizations can also see this happening — civilizations the would-be genocidal lunatics don't know about. And even if you're kind of on the fence about the wisdom of firing off pre-emptive genocidal relativistic kill-vehicle attacks on other civilizations, watching someone else do it would overcome a lot of objections, and move the demonstrated genocidal lunatics to the top of everyone else's hit list. So … don't do it.

Time Lag, Again. I think we all agree that attacking a superior civilization is a bad idea, right? Some piddly Kardashev Class I outfit decides to take pot-shots at a big-time Kardashev II crew, they're gonna get messed up but good. Stands to reason.

This means, of course, that if you are a big-time Kardashev II civilization, you really don't have to worry much about the Kardashev I peons bothering you. They may be primitive, but they're not stupid. They know you can mess them up if they start something.

So nobody's going to be shooting at superior civilizations, and nobody's going to be shooting at inferior civilizations, either. What does that leave? Well, you can target civilizations at about your level of technology, just in case they have similar ideas about you.

See the problem yet? Time lag! Suppose you launch your attack at a peer civilization 50 light-years away. Even if the missiles are going nearly the speed of light, they're still going to spend half a century getting there. During which time the target civilization has half a century of technological progress. So if your aggressors are Stalin's Russia, launching the missiles in 1950, the weapons arrive in Clinton's America in 2000, equipped with interceptors which can easily handle them.

This is especially important because of course you can't know just how much progress a distant civilization will make during the transit time. Maybe you're advanced enough to catch them off-guard and wipe them out … but maybe you aren't, and now you've got an implacable enemy.

Of course, the odds of even detecting a peer or near-peer civilization are remote. Given the immense age of the universe and the long time-scale of life on Earth, it's highly improbable that any aliens we detect would be close to us in technology. It's far more likely that we'll pick up indications of Godlike Kardashev II civilizations, or send out probes which observe species just figuring out how to make tools, than beings with similar capabilities to our own.

The gods are safe from us, and we're safe from the primitives. So nobody has to get pre-emptively violent.

Deterrence. Once you're capable of building weapons which can strike across interstellar distances, you're also capable of hiding weapons across interstellar distances — tucking away a little counter-strike force in nearby star systems, or deep in the Kuiper Belt. There's no way for an attacker to know in advance if you've got some entertaining surprises in store — and the consequences for that attacker are likely to be dire if you do.

This means that the rational assumption, for anyone interested in keeping their species and civilization intact, is to assume all possible adversaries have just such a counterforce in being. So don't attack them.

If I can think of these things, so can alien strategic planners. And really, it's difficult to imagine any being with the ability to construct interstellar vehicles thinking it's a good idea to launch unprovoked attacks on newly-discovered civilizations. There are simply too many unknowns.

So that's why I don't worry about E.T. trying to wipe us out. Back to whatever you were doing.


For stories about primitive and super-advanced extraterrestrials, buy my ebook Outlaws and Aliens!

The Prisoner's Dilemma

The problem of whether to commit genocide upon an alien race or not is vaguely related to the famous "prisoner's dilemma".

The problem is that the Prisoner's Dilemma makes it all too likely that Paranoia beats reason. For those unfamiliar with it... here's the Space version.

Race A and B both have roughly comparable technology, but don't understand each other. Each race has 2 options: Launch missiles or Ignore each other.

If Both races open fire, both races are devastated but not destroyed.

If one race opens fire and the other ignores it, they're utterly exterminated.

If both races ignore each other, they live in peace and are fine.

The problem is, neither can really communicate with each other. And although the cooperative choice of ignoring each other is best, the risks of them firing first while you ignore them are too great. Thus, this scenario via game theory, will always result in missiles being exchanged.

Laura 'Nephtys' Reynolds
Race B IgnoresRace B Attacks
Race A IgnoresBoth live constant fearRace A exterminated
Race B lives free of fear
Race A AttacksRace A lives free of fear
Race B exterminated
Both are devastated but not destroyed

As the Wikipedia article shows, the dilemma comes when you assume that each race is trying to maximize it's survival.

Say you are Race A. If Race B ignores you, your best outcome is to attack. Then you do not have to live in fear, spend resources on building defenses, and so on. If race B attacks, your best outcome is still to attack, since the alternative is extermination.

And since Race B will make the same determination, both races will attack and be devastated but not destroyed.

An outside observer will note that if the two races are taken as a group, the best outcome of the group is for both races to cooperate. If either attacks, the outcome for the group will be worse. And if both attack, both races receive a worse outcome than if they had both ignored each other.

So if both races selfishly look out for themselves, both will attack and the result is devastation. If both races altruistically think about the group, both will ignore and both will live. And if one race is selfish while the other is altruistic, yet again it will be proven that nice guys finish last.

And it actually doesn't matter if they can communicate with each other or not, a given race cannot be sure if the other is being truthful. If the two races can communicate, they run into the "cooperation paradox". Each race must convince the other that they will take the altruistic option despite the fact that the race could do better for themselves by taking the selfish option.

Cooperatewin some-win somelose all-win all
Defectwin all-lose alllose some-lose some
CooperateD, DC, B
DefectB, CA, A

Of course the prisoner's dilemma is a very artificial set-up, in real life the results would not be quite so clean-cut. To the right are two formulations of the prisoner's dilemma matrix.

In the Detailed matrix, A, B, C, and D are various outcomes, and the relative value of the outcomes are B > D > A > C. If those relative values are true, the prisoner's dilemma is present. In the first example, B = alive and free from fear, D = alive but in constant fear, A = alive but devastated and C = exterminated.

The prisoner's dilemma does have some vague similarities to the old cold war doctrine of Mutual Assured Destruction, though they are actually not very closely related. The prisoner's dilemma also does not work in those cases where what is bad for one player is equally bad for the other. An example is the game of "chicken" as seen in the 1955 film Rebel Without A Cause, where the drivers of both cars race to a deadly cliff and the first one to "chicken out" loses. But game theorists are working on a new approach called "Drama Theory" (warning: commercial website. No endorsement implied.)

"Gently, Sandy," First Lieutenant Cargill interjected. "Dr. Horvath, I take it you've never been involved in military intelligence? No, of course not. But you see, in intelligence work we have to go by capabilities, not by intentions. If a potential enemy can do something to you, you have to prepare for it, without regard to what you think he wants to do."

From The Mote In God's Eye by Larry Niven and Jerry Pournelle (1975)

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