Why do so many stories about life on other planets have the aliens looking like humans? Part of it has to do with the limitations on costumes in alien movies, but I think there’s more to it than that. Humans tend to think of just about everything as looking like us: gods, elves, nymphs, giants, angels. Actually, that last one is weird because the Bible mentions a lot of angels looking like bizarre creatures with multiple heads and pairs of wings, but it’s the human image that’s stuck in popular culture. I’m reminded of the Greek philosopher Xenophanes, who said that if horses, lions, or oxen had hands and the ability to create art, they’d represent the gods in their own forms. It’s more or less our own vanity that makes us assume all thinking beings must look like we do.
As far as aliens go, one of the most famous media to represent most aliens as humanoid is Star Trek. The show started out being pretty low-budget, so it’s not too surprising that they mostly just put pointed ears or enlarged foreheads.
Since we haven’t yet seen any extraterrestrial life forms, we have no idea what they’d actually look like, but it seems rather unlikely that the conditions on other planets would have brought forth anything like what we have here on Earth. From what I understand (I like Trek pretty well, but have by no means seen the majority of the episodes), the original series explained this to a certain extent with Hodgkin’s Law of Parallel Planetary Development, developed by a biologist in the late twenty-first or early twenty-second century. Basically, it states that planets with similar environments will produce remarkably similar life forms. This is used to explain how another planet could have a society almost exactly like the Roman Empire, which of course is a huge stretch. Other planets with Earth-like societies are explained in other ways, like being founded by people from Earth or basing their civilizations on Earth media. In fact, if the Wikipedia description is correct, the episode with the Nazi planet has Spock claiming that the odds of such a similar society developing naturally are practically zero. You know what else the odds are against, Spock? Your parents being from species that evolved on completely different planets.
I guess not only do humans and Vulcans look similar, but they’re genetically close enough to produce offspring. I think the main precedent for this might be folklore in which humans have children with fairies, elves, and even animals. J.R.R. Tolkien popularized the concept of such unions in his work, particularly with Elrond being considered half-elven. The influence on Trek might be reflected in the fact that elves and Vulcans have the same main distinguishing trait, i.e., pointed ears. So I’d say this is really more fantasy than science fiction, but that’s bound to happen on occasion.
I had heard that the Next Generation episode “The Chase” dealt with this issue within the Trek universe, and its conclusion is that the similar-looking beings on planets nowhere near each other was the result of a program initiated by the first known humanoid species.
They didn’t want their image to die out, and instead of making statues, they distributed genetic material that would make life on similar planets evolve in much the same way. As such, most of the space-faring species in the galaxy closely resemble each other. The episode demonstrates this by showing humans, Klingons, Cardassians, and Romulans (whom we already know to be descended from Vulcans) finding this out at the same time. There’s a good moral in there about everyone being basically the same, but scientifically it makes little sense. Darwinian evolution is based on natural selection, which means that species better suited to the environment are the ones that will survive and thrive. Even if there IS some kind of genetic programming that is guaranteed to produce humanoids eventually, what are the chances that the conditions on all these planets will allow them to become the dominant species? Not to mention that it’s an example of intelligent design on a show that’s usually fairly anti-religion. It’s fiction, however, and I think there’s probably an intentional element of humor as well. The episode is played totally straight, but I wouldn’t be too surprised to find they were winking at the audience.