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I mean, star trek has lots of intelligent life that isn't human at all. The default is human like, becuase it's easier to relate to that and becuase it's easier to make shows like that.
Yeah, using humanoid aliens makes it easier to relate to them for obvious reasons (and is a lot cheaper for the costume and effects departments), but it's pretty likely that alien life would be vastly different from humans. Just look at the huge variety of animal life on Earth alone.
As far as I understand it, in terms of physiology, aliens could and likely are different-looking than us humanoids. But in terms of biology, our best guesses are that aliens would also be carbon-based and drink water-- not because we're arrogant enough to assume that all life must be like us, but we have no evidence in any direction to prove or disprove that. We have to start somewhere, otherwise we'd be spreading way too thin. That's why we limit our search to Earth-like planets in the Goldilocks zone that could be capable of holding water-- again, not because we know aliens need water, but because if we don't start with that assumption, we'd get absolutely nowhere.
There are some theories that intelligent alien life might be more humanlike than you would expect considering the variety of conditions in the universe. Chemistry works differently at extreme temperatures in either direction, so even if you skip over the idea of carbon-based life you still hit some limits with what sorts of complex chemistry can happen with extremely heavy or extremely light elements.
If you decide life can still exist in those conditions and start seeking increasingly exotic forms of life that we can’t even comprehend there are some other limits to consider. Aliens that are too large or too small are much less likely to ever become a space-faring species simply due to the physical constraints of building a machine that can escape orbit, even when considering very non-earth-like planets. Certainly traveling through space isn’t required for intelligence, but if we’re ever going to encounter life it’s much more likely that we'd encounter a space-faring species.
Then when you look at the shape of humans, we are symmetrical which has the benefit of allowing binocular vision that gives us a better grasp of 3d space, we have limbs that allow us to move around our environment and manipulate objects to build tools. Certainly there are other ways to accomplish these things, but these traits are simple and shared amongst a lot of life on earth because they are good solutions to a broad variety of environmental challenges.
Basically just consider the things that an intelligent space-faring species would need to be capable of and work backwards to the type of physical characteristics they would need to have and then the type of environment that would allow those traits to evolve. Alternatives are certainly possible but we definitely don’t know what else to look for.
I'm not well versed in alien evolutionary theory at all, but wouldn't differences in gravity, water, temperature, air makeup etc cause a significantly different evolutionary path? Earth is a Goldilocks planet for human life, have we considered that other planets may be a Goldilocks for something else?
I am leaps and bounds from an expert, but as I understand it, it's something that's been thought of, certainly. But it's very very difficult to search for something when the paramaters for it are very broad.
On the flip side, we have a decent idea of what it takes to sustain life as we know it. It's far, far easier for us to search for that.
And Earth is a place where pretty much everything stemmed from the same evolutionary trunk. We are far, far more closely related to octopi or centipedes than we would be to anything that evolved outside our biosphere. In fact it's pretty likely that we may not even recognize alien life as being alive, at least in the sense we are used to. A silicon based or argon based life form would exist and operate in a completely alien manner (heh) to the carbon base that we're used to seeing.
Moral of the story being, the chance of humanoid aliens existing is a miniscule fraction of the chance of alien life existing at all. Much more likely that we'd scan right past an alien lifeform and quite possibly not even notice it was there, especially if we're busy scanning for carbon-based Earth style life forms. Gotta keep your eyes and your options open, because the end result of a completely different evolutionary timeline will inevitably produce a completely different style of creature.
That being said, if earth isn't some incredible fluke, and similar planets exist bearing life, that life will most likely resemble earth life because it will be adapted to similar niches and environmental conditions.
A puddle sits in a ditch and thinks "Wow! This ditch is shaped exactly like me. It must have been made for me"
Other ditches are observably a similar shape, so other puddles must be similar is not a leap in logic.
Makeup is also much cheaper than really good puppets. People have a really rare body shape in nature, so it seems unlikely that aliens would have identical two legs to arms hairless upright configuration.