What would aliens really be like?

In this post last month, I said that one reason I reject alien-abduction stories is that the aliens they describe -- "greys" and suchlike -- are just unimaginative modifications of the human form, and it's very unlikely that evolution on another planet would produce anything so closely resembling us or any other Earthly species.  Aliens with a space-traveling civilization would need to have sophisticated sense organs and organs of manipulation (like hands or tentacles) in order to develop technology in the first place, but that's all we can assume.

So what would life on another planet be like, if it exists at all?  Not just intelligent life that might visit us, but life in general which we might someday discover?  This is obviously going to be very speculative, since we have only one example of a life-bearing planet to look at and thus no general knowledge about what is and isn't possible.  But it's informed by what I know about evolution, at least.

To begin with, all land vertebrates on Earth are four-limbed (except in a few cases like snakes where evolution has done away with limbs that their ancestors had), but this is just because they all evolved from a common four-limbed ancestor.  There's no reason to think that the same would hold on another world.  It would depend on how many limbs the common ancestor of land life had.  Six limbs, or eight, or even ten or more, might actually work better, especially for very large animals.  In some Earthly vertebrates such as birds and bats, the front pair of limbs has evolved into wings, while in a few species -- notably humans -- the same front pair has become specialized into organs of manipulation rather than locomotion.  With more pairs of limbs to work with, evolution might specialize them for several different functions in the same animal.  In an intelligent species, the first pair or even the first two pairs of limbs might specialize for manipulation as our arms and hands do, leaving the rest as legs and/or wings.

If the common ancestor of land life started off with some legs plus other limbs which were more like tentacles (quite possible, assuming the common ancestor emerged from the ocean originally), the tentacles in later intelligent descendants would likely adapt for use as fine-tuned manipulators more easily than front legs did in our own case.

Similarly, all Earthly land vertebrates have two eyes, but again, this is just because they have a two-eyed common ancestor.  A larger number of eyes distributed around the upper part of an organism would allow for 360-degree vision as well as more redundancy in case an eye were injured.  In Earthly animals, the slow speed of nerve impulses dictates that eyes and other major sense organs must be fairly close to the brain, but if faster nerves evolved on another world, eyes and ears and other sense organs could just as well be distributed all over the body.  In such a case the brain would probably be deep inside the body where it would be better protected -- there would be no reason for evolution to place it in an independently-movable sense-organ cluster at the front.  That is, animals would not have a distinct head separated from the rest of the body by a neck.

On the other hand, bilateral symmetry (both sides of the organism being the same) probably exists in animals everywhere.  There are advantages to being able to travel in a more or less straight line, and without bilateral symmetry it's difficult to do that without a brain capable of sophisticated navigation, which would tend to appear only late in evolution.  A few animals on Earth, such as starfish, lack bilateral symmetry, but I wouldn't expect this to be common.

Science fiction sometimes depicts aliens as "reptilian" or "insect-like", but this is almost as nonsensical as the more common fictional aliens which are essentially human-like but with pointed ears or odd prosthetics on their foreheads.  Reptiles and insects are specifically Earthly categories of life, just as humans are; the same categories wouldn't evolve independently on another planet.  Features like jointed exoskeletons, scaly skin, egg-laying, and so forth might well exist, but there's no reason to think they would be sorted the same way as on Earth.  An animal with a jointed exoskeleton, which bore live young like a mammal, had jaws like a reptile with a venomous bite, and also had manipulative (non-exoskeleton-plated) tentacles like a cephalopod is perfectly possible.  It couldn't evolve on Earth because those traits were long ago sorted into separate categories of animals far too different to interbreed with each other, but there's no reason such traits couldn't all exist within a single lineage on another planet.

One thing I would not expect to see on another planet is a really close equivalent of our male-female distinction.  There's a substantial advantage in having different individuals be able to combine genes to produce offspring, in that it allows beneficial mutations to spread rapidly in a population and also combine with each other, rather than each mutation being confined to a single line of descent.  But I see no advantage in the whole population of a species being divided into two categories, each performing only part of the reproductive process, so that only individuals of opposite categories could combine genes.  It would make more evolutionary sense for every individual to be equipped for all aspects of reproduction, so that any two individuals in the population could "mate" with each other and produce offspring.  Even if evolution did divide the population into two categories with different reproductive roles, it's unlikely that the differences between the two categories would match the distinction between our own male and female genders very closely.

Science fiction has occasionally depicted alien species with more than two genders, but this strikes me as unlikely to be common.  It would make the mating process more complex and thus more likely to fail, while increasing the number of individuals needed for it to succeed.  I suppose a three-gender system might evolve on planets where other oddities of life or the environment minimized these disadvantages, but such cases would be rare.

Another thing I wouldn't expect to see:  wheels.  They work well for machines, but there are two reasons why they wouldn't evolve in animals.  First, there's no plausible way for nerves and blood vessels to get across the bearings.  Second, wheels only work where the terrain to be traversed is completely flat.  Even minor irregularities would require a wheeled animal to have some other type of limbs to get over them, and in practice it's hard to imagine a realistic natural environment where wheels would be practical.  The first problem, though not the second, also applies to the evolution of propellers in flying animals or aquatic ones.

The very concept of a "species" may not be universal.  Earthly bacteria of different species can exchange genetic material -- it's conceivable that on another planet where the organic system for transcribing DNA (or whatever was used for encoding genetic information) was more versatile, animals might be able to exchange genes with a wide range of other animals even if they were not particularly similar.  In such a case, human explorers would find a vast range of individual animals different from and similar to each other to varying degrees and in various ways, but not neatly divisible into separate species as Earthly animals are.

Can we even assume that life on another planet would have something like the distinction between plants and animals that Earthly life has?  Probably.  Sort of.  It seems likely that life anywhere would include a class of forms which extract energy from sunlight, by photosynthesis or something similar, and that these forms would tend to be fixed in place, or not move around much, not being able to produce enough energy to do so -- and that later other forms would evolve which specialized in "eating" the concentrations of stored energy which these "plants" represented, which they could only do if they could move around.  But intermediate forms might also exist, capable of consuming plants but also retaining the ability to photosynthesize as a back-up when richer "foods" were unavailable.  Or mobile forms might evolve the ability to extract nutrients from soil or air rather than from consuming other living things, depending on what their biochemistry, and thus their nutritional needs, was like.

Speaking of plants, even if most planets have them, they probably aren't green.  Black leaves would be more efficient, since they absorb the most light and reflect the least, and from a plant's viewpoint any light which is not absorbed is wasted.  Green leaves work well enough, and that's what evolution on Earth settled upon, but even if the same happened on another planet, the happenstance of mutation might well have produced some other color.  On worlds where the animal/plant distinction was fuzzier, plants might well have more pro-active defenses against animals seeking to eat them, such as eating them in return à la the Venus flytrap, emitting jets of toxic vapor or liquid, etc.

The evolution of high intelligence in plants is probably not common.  Life forms which are fixed in place don't usually face the kinds of challenges for which intelligence would provide a major advantage.  But if there are ecosystems which do pose such challenges, and if the plants in such a case had some kind of nervous system, intelligence could arise.  I don't know whether we would really classify such things as plants, though, even if they were rooted in place and fueled mostly by photosynthesis.

As for aliens capable of building a technological civilization -- the kind of aliens who might visit us someday -- as I said above, we can assume they'd need sophisticated sense organs, and organs of manipulation to enable early tool-making which would eventually lead to advanced technology.  The latter might be hands, tentacles, something like an elephant's trunk, or perhaps even mouthparts adapted for the purpose, as with Larry Niven's fictional "puppeteer" species.  But as I hope I've shown, aliens with these traits would still likely be very different in form from ourselves.

Notice, by the way, that I haven't even raised the issue of different environments driving local evolution in even more alien directions.  Everything I've said here assumes planets with essentially the same kind of environments as Earth.  Stronger or weaker gravity, brighter or dimmer sunlight, higher or lower temperature, soil with different chemical composition, a much denser or thinner atmosphere or one with a different mix of gases, etc. would all produce evolutionary outcomes even more different from Earthly norms.

There may be planets out there with ecosystems as rich and diverse as Earth's.  But nothing we find there is going to look much like any species on Earth.  I know of the argument from convergent evolution -- cases on Earth where animals of different lineages end up looking similar because they evolved in similar environments.  But those animals had a common ancestor which bequeathed them a whole set of common features -- two eyes, two ears, four legs, basically similar skeletons, muscles, etc. -- making it easy for them to evolve into very similar forms because they were fairly similar to start with.  Alien species would have no such common origins or features with Earthly species.

Science-fiction movies favor "aliens" which are really just slightly-modified humans, like the Klingons or the Na'vi, because this allows for facial expressions, gestures, male-female psychological dynamics, etc. which make it easier to relate to them as characters.  Sometimes SF offers "reptilian" or "insect-like" aliens which, at least, give audiences a ready-made set of expectations about their traits, based on what real reptiles and insects are like.  But the likely outcomes of actual evolution on other worlds operate under no such constraints.