db 88 88 88888888888 888b 88 ad88888ba d88b 88 88 88 8888b 88 d8" "8b d8'`8b 88 88 88 88 `8b 88 Y8, d8' `8b 88 88 88aaaaa 88 `8b 88 `Y8aaaaa, d8YaaaaY8b 88 88 88""""" 88 `8b 88 `"""""8b, d8""""""""8b 88 88 88 88 `8b 88 `8b d8' `8b 88 88 88 88 `8888 Y8a a8P d8' `8b 88888888888 88 88888888888 88 `888 "Y88888P" Sci-fi shows like STAR TREK are dominated by aliens that are at essentially humans with superficial makeup, and even productions with larger budgets, like STAR WARS, are predominantly inhabited by humanoid aliens. Obviously the real reasons for this are budget and familiarity, but could it also be argued that intelligent life might convergently evolve similar humanoid forms? That is what I will try to justify here. I have no particular background to draw on; so the following is more or less uninformed musing based on first principles. As a starting point, I will assume that evolutionary pressure guided by the survival of the fittest is what decides which shape will be taken by life forms. This implies that intelligent life must evolve from non-intelligent life. I will also assume that evolution will tend to produce the simplest solution to problems, including minimising the amount of genetic information that must be maintained. Life on Earth is fundamentally carbon-based; our genetic information is encoded via nucleic acids, and our proteins are chains of amino acids. I don't know my chemistry well enough to say whether life based on another fundamental element would be possible (silicon is sometimes mentioned), or how that would affect the macroscopic structure of life. Life can exist without oxygen (it still does, in the form of anaerobic bacteria), but oxygen seems like a rather efficient way of releasing energy. It may not be the only element that can perform oxygenation, but it may be the most prevalent one in the universe. If life is based on carbon and water, then it is likely to prefer roughly the same temperature range as us. Much lower, and water will freeze. Much higher, and it will boil. Similarly for pressure. This argues for a planet similar to Earth, at a similar distance to its Sun, which will probably imply it has a similar amount of gravity, give or take an order of magnitude. This will affect the maximum size of any life that evolves - while it may be smaller or larger than humans, it is unlikely to be enormously so. Although life is most likely to arise in water, because it acts as a solvent and thus makes a rich variety of chemical compounds easily available, it is plausible that intelligent tool-making life will benefit from dry land. This is simply because setting things on fire is such an effective and simple way of accessing more energy than can be provided by an organism's own body. The evolution of intelligence is inherently about harnessing ever increasing amounts of the energy in the environment. Now we're at carbon- and oxygen-based tool-making life on dry land. This still leaves a lot of design freedom for what it might look like. First, in order to gain the maximum amount of body parts relative to the amount of genetic information that has to be maintained, well-optimised life exploits symmetry. Most terrestrial animal life is laterally symmetric (your left and right sides look the same), but radial symmetry is also found to some extent (e.g. octopuses), and of course many plants appear to be radially symmetric as well. I have no very strong argument for why intelligent life would prefer lateral symmetry, but I wonder if having a "front" where energy-costly sensors (e.g. eyes) can be focused is efficient if you have enough intelligence to ensure they're always pointed in the most profitable direction. Obviously, radially symmetric intelligent life will look *very* different from humans. Beyond lateral symmetry, the humanoid body plan has a few more identifying characteristics: two legs, two arms, and a head on top. The purpose of arms is to be able to handle tools, which is definitely crucial for intelligent life. From an evolutionary perspective, it makes sense for arms to be an adaptation of what might once have been another pair of legs. While you need two legs for efficient locomotion, one arm might have been sufficient, but lateral symmetry essentially gives you two of each limb for free, so there's no great cost here. Two eyes are significantly better than a single one, because you gain depth perception. But why have two complex eyes rather than a multitude of simpler but specialised eyes? Perhaps it is simpler to have only two instances of the supporting neural and muscular machinery. It is my understanding that most of the life on earth with more than two eyes doesn't quite have the ability to focus them the way two-eyed organisms do. Any organism will want an orifice for ingesting food and an orifice for expelling waste, and putting the waste orifice on the other end of the body means you don't have to worry about the waste containing unhealthy things. You'll definitely want to put various sensor organs very close to the food ingestion orifice, to determine whether whatever you're considering eating is going to be good for you. The humanoid face does this pretty well: a nose just above the mouth to maximise olfactory impressions, eyes just above that, and the tongue as a final line of detection. These sensors provide a lot of information, and moving information is expensive is slow, so you want to minimise the distance to the decision-making centre (the brain), which is therefore right next to them. This is also why the ears are located here. It fundamentally makes sense for the most complex sense organs to be very close to the brain and thus each other. To provide 3D hearing, and due to the lateral body layout, the two ears should be located on each side of the head. So now we have a head, two arms, and two legs. We can discuss the fine details: a neck is useful so you can move the sensor cluster (the head) around without having to move the entire body. Something finger-like on the arms is definitely practical in order to allow tool manipulation, but I have no argument for why it should be much like the human hand. Two fingers might be sufficient, but you probably need either that or something with similar functionality, in order to be able to grip tools. The human foot is pretty bad and mostly a relic of the fact that, from an evolutionary perspective, we are new to bipedalism. You definitely don't need that many bones or does; something like an ostrich's foot is much more well-designed. But in any case, STAR TREK aliens tend to wear shoes, so who knows what they look like down there.