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First published in SFWA Bulletin, Winter 1997
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Aliens should have a home planet. They should not be native to a studio prop room. The difficulty, of course, is in deciding how far to depart from intuitively credible Earthly life when depicting natives of other worlds. Too little, and the aliens might as well be humans with corrugated foreheads. Too much, and they become ridiculous. I would like to suggest that steering a successful course around these shoals depends on following biological laws expressed in all life. I am purposely not saying "all life on Earth" because these laws can confidently be expected to hold true wherever there is life, and even if it is silicon-based life or cyberlife. What is more, because we are biological creatures, we have an intrinsic grasp of these rules, and inventions that flout them seem silly or fake or surreal. In contrast our physical intuition does not extend beyond Euclidean space. Science fiction authors can insist on faster than light drives without breaking their spell, because we all know in our bones that nothing happens when you go faster except that you get there quicker. However, a writer whose protagonists were flying lobsters would never get a hearing.
So what are some of these biological rules we're so familiar with but otherwise unaware of?
The first rule of biology is that life evolves by natural selection. Characteristics that help an organism survive and reproduce get passed on to the next generation. Conversely, any characteristic that increases mortality (either of parents or their offspring) disappears with the individuals it did not help. Except for worlds run by angels or genetic engineers, even aliens will be ruled by natural selection. Though this may seem obvious, the implications are rarely considered. What would be wrong, for instance, in describing an alien species as having three eyes--just to make them a tad more weird?
Natural selection has much to say about such things as third eyes. Eyes are incredibly useful, but they are also incredibly vulnerable. Imagine getting something wrong with one of your eyes and having no hospital to go to. The eye leads directly to the brain. Any accident, damage, or infection to an eye is likely to cause death. Is any creature in its right mind going to want extra organs like this, just waiting around, serving no particular purpose until they can kill him? Of course not. One eye is good; two eyes are even better because they can provide wide angle vision or depth perception; three eyes are deadly. More is not better.
The same is true of any extra organ, including tentacles. It's true also of extra joints, such as mechanically dubious double knees bent at right angles when the (very arthritic) alien stands. Up to a point, organs enhance a creature's ability to survive, but anything beyond that point is just one more thing to have accidents with. That is why we don't see creatures with extra eyes or arms running around, and that is why anything that does have extras feels alien. It does not, however, feel real.
Some people might object that insects have lots of extras. They have extra legs and eyes, to say nothing of a whole array of disgusting and busy mouthparts. Well, no, not really. Insects have exoskeletons, which are heavy, and insects are small, so the world they move through is full of large obstacles. Four legs, to say nothing of two legs, simply does not provide the stability needed for such an unwieldy superstructure. Nor do they have extra eyes. Each individual eye on an insect acts almost like a pixel on a screen. Having eight of those (as some spiders do) provides the bare rudiments of vision. Insects with compound eyes have several hundred quasi-pixels per eye, which are still not very many. Similarly with their mouthparts: insects have no teeth inside their mouths, instead they have parts performing the same slicing and grinding functions outside their mouths. Having six pairs of mouthparts is no more a luxury than having sixteen pairs of teeth. What they do NOT have is both mouthparts and teeth.
Which brings me back to aliens: so long as the conditions of their environments or lifestyles require it, aliens may have more or fewer legs or heads than appears natural at first glance. The point is that to render an alien who is more than a pastiche of leftovers from the special effects studio, it is necessary to think through the whole environment the alien comes from.
As an example, consider the little green men of science fiction. As undersize humanoids with photosynthetic skin and pointy ears, they are just a feeble attempt to make weirdness by slapping a bit of the plant kingdom onto the animal kingdom. But is a photosynthetic animal actually impossible? Coral reefs are huge examples of photosynthetic animals. More to the point perhaps, there are mobile animals--marine relatives of snails--which do not digest the chloroplasts in the algae they eat. Instead they transport them carefully to their skin, and spend lots of time lazing in the shallows, "eating" sunlight like a plant.
The big problem any photosynthetic animal faces is that much surface area is needed to generate enough energy just to survive and reproduce. If movement is to be possible as well, it requires many times as much energy, hence even more surface area. Then it would take even more energy to move all the extra "leaves," which would require even more leaves, and so forth and so on. However, this is not to say one couldn't envision a sun-fed alien.
A real little green man would have skin in multitudinous folds that could be erected and extended to catch sunlight. (While we're updating the little green men, let's update their moniker as well to "little green people," since they're hardly likely to be unisexual.) At night, with the folds flaccid, little green people would look like wilted lettuce, and during the day they wouldn't be able to pass sideways through a cathedral door. Yet, as photosynthetic area goes, this is not very much and the LGPs would spend their days largely as couch potatoes, probably leading an amphibious lifestyle at the sunny neighborhood pond. Water, after all, is essential to almost all biological photosynthetic interactions.
Without the need to forage for food, one would expect the little green people to be very poorly muscled and generally slow. Similarly, there would be reason to expect a rather slow and ponderous intelligence, given to the pursuit of topics without any particular practical application, such as the highest possible math, and poetry, or music. It would, actually, be rather difficult to imagine why such creatures developed intelligence at all, given that they don't need it to find food, but it would be an essential exercise since that would be the backdrop to their whole culture and society.
Physiological weirdness can create a sense of strangeness. However, there is a reason all the large, active life we see breathes oxygen. Oxygen is particularly suited to participating in electron transport chains, which is how all biological energy is generated, whether it is sugars made in photosynthesis or ATP made from sugars. The amount of energy generated in an oxygen-based, or aerobic, metabolism is eighteen times greater than the closest runner-up. We actually have carbon-breathers and sulfur-metabolizers right here on Earth. They're all single cells. Insufficient energy is produced to support a multicellular slime mold, to say nothing of an intelligent being capable of creating an alphabet.
It is conceivable that in the absence of atmospheric oxygen, and given enough time, other metabolisms could be dominant, but any plant, that is any photosynthetic organism that does develop aerobic metabolism on such a world will outcompete its neighbors. In a geologically short period of time the world will become oxygenated and the former inhabitants will be relegated to dim, dark corners. (Imagine if those sulfur-breathers had reached all the way to intelligent life when that happened....)
Instead of an anaerobic metabolism, there could be creatures with oxygen-based metabolisms in an anaerobic atmosphere. Aliens could breathe carbon and store oxygen instead of breathing oxygen and storing sugars. They'd need some fanciful chemistry, since a biologically viable form of oxygen storage would have to evolve. Further, since oxygen is so reactive, creatures storing large quantities of oxygen would tend to explode on contact with sparks. One can imagine some rather interesting social taboos developing.
Like anaerobes, photosynthesizers are in metabolic difficulties, but the problem there is in energy yield, not in energy supply since plants have the usual high-yield oxygen-based metabolism. The little green people discussed earlier have enough energy to survive on nothing but sunlight, but how are they to find enough energy to reproduce? Producing sperm or eggs and either gestating or caring for the young takes lots of calories. (Tangentially, any intelligent species is likely to care for its young, because intelligence implies the ability to learn, and that implies a somewhat helpless infancy when the young need to be taught.)
The little green people could solve the calorie problem by supplementing their diet of light with actual food. Since animals provide the most nutrition for the least digestive effort, the normally complacent green people are likely to suddenly show a more sinister side. They won't be snacking on harmless plants. They might develop a digestive surface all along their bellies, for instance, oozing corrosive digestive acids. (That way they wouldn't need to maintain a whole digestive system they rarely use.) They could lie in wait for prey--they wouldn't be very good at running for it--and fall on it when it passed too close, smothering it with their skin folds, and digesting it where it lay. The little green people would be masters of strategy at predicting the movements of prey. Strange umbrella-shaped egg and sperm sacs would sprout around their necks....
Once the young were large enough to start living on sunlight in the neighborhood pond, everyone would go back to higher math.
Some writers interested in biology have presented aliens with strange methods of reproduction, such as more than two sexes, or only one gender that buds off progeny without the benefit of sex at all. Rarely, however, does science fiction top the strangeness of arrangements found here on Earth. Some fungi, for instance, have many different mating strains each of which could be considered a sex. Pairs of mating strains are compatible and produce offspring. The pairs recognize each other, but the only way for anyone else to tell is to wait and see who pairs with whom.
Another method, practiced by plants, is to alternate generations. Adults do not simply produce offspring. Adults, in effect, produce proto-eggs and proto-sperm. These grow to form complete multicellular individuals that sometimes resemble the adult and sometimes, as in the case of moss, are quite different. The proto-egg and proto-sperm individuals then produce the actual eggs and sperm that fuse to form a new adult. This isn't quite the same as having more than two sexes, but it's certainly strange from a mammalian perspective.
Equally strange is the ability of some invertebrates, even some fish, to change sex during their lifetimes. Some reptiles do not inherit their sex. It is determined by environmental factors such as what their mothers ate or the temperature of incubation while they were in the egg. (Imagine visiting an alien culture panicking strangely over the failure of the harvest of some obscure vegetable, only to find that they now produce nothing but males and go extinct.)
There are lizards and pond animals who are almost entirely parthenogenetic, the eggs forming new adults without sexual reproduction. Plain asexual reproduction is common in plants and in simple organisms like sponges or flatworms. In that case, though, it's important to remember that the progeny are formed from one or a few undifferentiated cells. Because of the unitary nature of organic development it is impossible, for instance, to duplicate an arm lengthwise, as shown in drawings in Barlowe's Guide to Extraterrestrials. An arm could bud off from a point at the shoulder joint, and the whole arm would grow at once, just as it does in an embryo. Likewise, an individual could not split lengthwise, like a one-celled Paramecium. An asexually produced individual would grow as a unit from an undifferentiated cell or mass of cells.
Completely unisexual species, by the way, have a problem. The biological purpose of sex is not to have fun, or even merely to reproduce, but to reproduce with variability in the offspring. That variability accelerates the pace of evolution, allowing an intelligent species to evolve in a geologically reasonable period of time. To get an idea of how slowly asexual species evolve, consider bacteria and some of the other one-celled organisms, which are the only truly asexual living things on earth. They have been essentially unchanged in a billion years, or three billion years for some of the bacteria. With such a glacial evolutionary pace, the development of an intelligent, unisexual race would need some explaining. (The simplest might be that they came from a sexual species that was also capable of asexual reproduction, for whom sex became a deadly liability due, perhaps, to a virulent venereal disease.)
Biology constrains the development of intelligence. Once again, it's all the fault of natural selection. Intelligence will increase only if having it helps an animal survive or reproduce, otherwise less intelligent animals will leave just as many offspring. Intelligence, for instance, would be wasted on a plant or a fungus. It lives in or on its food, so it needs no help finding that. It cannot escape from predators in any case, so there is no point in being aware of them. And, since it's fastened to the ground, finding mates is a matter of growing toward them or broadcasting and trapping pollen.
Mobility is a major source of the type of problems intelligence can help to solve, including the need to find food, but a plain earthworm has enough intelligence for that. Survival is a minor problem compared to tough ones like figuring out your neighbors. Interesting levels of intelligence appear only in social animals. All the most intelligent animals on Earth, including whales, dolphins, parrots, and primates (including humans), are social.
Social groupings are advantageous to animals whose food is plentiful, because then there are many eyes to watch for predators instead of just one pair. However, for animals whose food is scarce, such as tigers or some hummingbirds, any help friends can provide is cancelled by their desire to eat the same food. Intelligent alien carnivores would have to come from a very specific, ecologically rich kind of world.
Not all social groupings are likely to lead to intelligence. Whenever food is ridiculously easy to find, as for herbivores, intelligence does not seem to follow. Animals who eat fruit, however, are another matter. Fruit is plentiful within a very limited area when it ripens, so friends are not competition. Because it is found in very specific places, it rewards creatures bright enough to remember where to find it. In other words it's good to be both smart and social if you eat fruit. It is interesting that both parrots and primates are frugivorous. Possibly, from the standpoint of a dolphin, its food sources share these characteristics of being locally plentiful yet requiring intelligence to find.
Scavengers of large animals are also in this position. Many animals can feed since the carcass will spoil anyway, and remembering where to find it the next day means more meals. Based on fossil evidence some anthropologists think that humans may have gotten their start scavenging hippo and elephant carcasses, whose skin was too thick for animals without the ability to use cutting stones. (Consider that to this day we do not usually eat truly fresh red meat. Beef, for instance is hung for many days before being sold.) Somewhere in the story of an alien race there need to be echoes of the prime factors shaping their intelligence and culture.
How realistic is the development of intelligence in that science fiction favorite, the bug-eyed monster? Not impossible, actually. Physics and chemistry constrain the size of cells to about what we see, and a brain with enough neurons to do something interesting has to be quite large. Whether it has to be human-sized is debatable, but it has to be much larger than an ant's. So the bug-eyed insectoid has to approach a human, or at least a dog, in size. But part of the reason we don't see huge insects is that Earth-type exoskeletons become too unwieldy at large sizes. On Earth the conflict between brain size and exoskeleton weight is unresolvable. But if light and strong carbon-fiber exoskeletons had evolved on the bug-eyed monster's world....
Technological ability, as a particularly interesting branch of intelligence, seems to develop only when the animal in question has some way of manipulating its world. For a long time anthropologists assumed that humans' brains grew larger first, and then all that budding intelligence led to greater use of the forelimbs, resulting in dexterous hands and upright walking. Fossils have shown that idea to be backwards. Humans, or proto-humans, had opposable thumbs while their brains were still the size of a chimpanzee's and before they walked for a living. It was the immense evolutionary advantage conferred by the ability to manipulate things that drove humans to free their forelimbs from walking and that led to bigger brains better able to think of things for those hands to do. Thus, any alien no matter how strange, if it has any level of technology at all, needs to have some means of manipulation built into its anatomy. Asimov's gaseous blob extruded pseudopodia. A walking plant might have modified, grasping fruiting bodies. However it does it, an intelligent alien must have hands.
Asimov, Card, Poul Anderson, and others have discussed world-building, especially the ways in which the physical environment constrains biology. One factor which has not received enough attention is evolutionary time. Life takes time to evolve, so it won't appear around a supernova that burns only a few hundred million years (though colonizers might live there). Low energy anaerobes would take tens of billions of years to evolve all the way to civilization (assuming Earth is typical of how long it takes high energy oxygen-users to accomplish the same thing). Only very small stars are long-lived enough for such expanses of time. However, orbits within the zone of liquid water lie very close to small stars, and in that range a planet could become tidally locked and cease rotating. That would result in yet another set of difficult problems for life to adapt to. Finally civilization itself takes time to evolve. A culture that survives despite severe geological or climatic upheaval would need to show some unique adaptations.
In summary, real aliens are the way they are for a reason. I've tried to give an example of odd but (hopefully) believable aliens in the Green People. The physics and chemistry of an alien world constrains its inhabitants' biology. Their biology is the sum of millions of responses to natural selection. Their minds will be based on their biology and ecology. Their cultures will be influenced by their biology as well as their dreams. If the reader gets to glimpse the forces that shaped them, those aliens will feel real.
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