According to a number of Hollywood movies and questionable YouTube videos, extraterrestrials look an awful lot like humans. They have hands, complete with fingers and thumbs, attached to arms that protrude out of a torso with two legs holding it all upright. They also have a head and neck poking out of the torso at the other end. This head is usually taken to have eyes and a mouth. Sometimes they even have ears (albeit pointy ones). By all accounts, this is completely humanoid.
I would like to put aside, for the time being, whether these descriptions of alien beings are accurate; I am not trying to suggest that they are. Because I would like to do something else here: I would like to explore the possibility that these accounts are accurate, and from there see what this would suggest about the evolution of life: why is it that aliens look like humans?
This is an interesting question because it's not so obvious that they should look like humans. After all, most animals don't, even the intelligent ones (like whales, dolphins, elephants, or crows). So why is it that aliens from interstellar space — with whom we share no evolutionary history — look like us? On the face of it, this looks like an extremely improbable coincidence. It's like going to an alien planet and discovering that they speak English. This would be highly coincidental. Because there are so many other ways for them to organize a language that aren't English-like, many of which work perfectly fine, so why English? Similarly: there are many ways that an alien's body can look that aren't humanoid, many of which probably do just fine, so why do all the aliens look humanoid?
Never leave your home without a tinfoil hat. If you do, the aliens will zap your brain cells one-by-one with battery acid in order to cover-up the truth about the fact that they are zapping your brain with battery acid.
There are several answers one could take to this question. One answer is to say that we do not actually know what extraterrestrial life looks like. We, instead, only know what it looks like when we depict them in movies. And in movies it is cheaper to slap some fake pointy ears on an actor, or paint his face green, than to actually go through the effort of imagining a novel body-plan and then playing it out through CGI technology. According to this answer, then, the only aliens are Hollywood aliens, and its no mystery why Hollywood aliens have a humanoid form. But this answer is lame and boring, so we can skip it.
The second answer is that there is some form of convergent evolution going on. It must be the case that the humanoid form evolves independently across interstellar space, because it is the most adaptive body-plan for a life-form that is interested in doing intelligent or quasi-intelligent things (including but not limited to interstellar travel). By this view, intelligence is not just in the head, but in the body, too. So (for example) dolphins and elephants may have smart minds, but they do not have smart bodies.
There is something to this. To see this, consider what an arduous task it would be for dolphins to build a spacecraft: they couldn't do it. And not for lack of brains. The average dolphin could be smarter than Albert Einstein, but that wouldn't change the fact that they can't pick up a damned screwdriver. And how are you going to build a spaceship if you can't even pick up the screwdriver you need to build it? Or the pencil you need to draw out its blueprints and calculate its dimensions? You can't. It's not possible. Smart as they may be, dolphins lack the humanoid form necessary to build interesting technology.
So there is, I would like to suggest, a selection effect: the aliens that build spaceships are not just the aliens that we would consider to be intelligent, but the aliens that have the ability to build spaceships to begin with. These are the aliens that look humanoid. So that is why all aliens look humanoid: it is only the humanoid aliens that can build spaceships, and the rest are condemned to contemplating the mysteries of existence while flipping their flippers in the ocean (or whatever). ∎
