Everyone says that sapient alien species will naturally tend to take a humanoid form, but their reasoning is, to be perfectly honest, really shit
The general form of the argument is such:-
Animals must be bilateral and cephalized to be able to be sapient
Sapient life needs to be terrestrial for them to progress technologically
Four legged creatures are ideal for terrestrial locomotion
Sapient life needs prehensile members, and the most plausible way that can happen is through repurposing the forelegs
Following this argument, according to proponents of humanoid aliens, results in humanoid aliens being a plausible thing to arise
But, there are a tonne of holes in this logic
First, the start of the argument; or rather, where it should start. While this argument is so often started at the point of cephalization, this point relies on the unspoken assumption that alien fauna will proceed up to this point exactly as telluric fauna has:- It is assumed that the earliest ancestor of a sapient alien must be a globular thing that crawls on a substrate of its food and has a single mouth and gut, and thus it would adapt as such life did on Earth. It's true, that IF it was so much like the prebilaterian it would likely evolve similarly, but we have no reason to assume it would be in the first place. There are plenty of other ways that proto-animals could exist, and thus ways it could differ from our bilaterians
Consider these:- If rocky/inorganic substrates dominated, animals would have to move in many directions to reach food, leading to a radial body form being more likely; While if the ancestor of the complex animalia didn't live on a solid substrate at all then it would have to adapt for swimming rather than creeping, likely taking on a radial or biradial form due to the lesser effects of verticality
The most dramatic variations involve the possibility that animal life evolves proper structure before adapting for mobility. In this case, the primitive animals could have multiple copies of its feeding structure, which would almost certainly evolve into multiple guts. Sometimes it is argued that this would evolve back into a singular gut, but among real lifeforms with multiple guts or similar, most if not all of these guts are retained amongst different forms. Perhaps it could be different in more active forms, but also perhaps it isn't. Also, a star-shaped or forked form could be imagined to result in multiple cephalized regions (i.e. multiple heads); as cephalization occurs because the front point is usually the first to hit new stimuli, if many points are in the front cephalizing all of them might be most useful. All of this is really speculative, so take it with a grain of salt
The assumption of terrestriality is pretty dubious. While obviously there's quite a lot a creature can do on land that it cannot underwater, none of that is really essential to sapience or civilization. Though, of course, there are a lot of terrestrial animals, so this is more of just a side note
While terrestrial life existing is essentially a given if there's land, the idea that terrestrial life will necessarily be tetramelic is, well, obviously wrong. Outside of gnathostomes, the only examples of four-limbed creatures anywhere are some butterflies and such bugs, and jellyfish. Every other non-gnathostome, and several gnathostomes too, have six or more limbs, or none at all (and if we set aside hexapods then it's usually ten or more limbs). Given this, it's quite likely that many alien worlds wouldn't have tetramelic creatures at all, let alone such creatures being the dominant classes from which sapience arises
Also suspect is the idea that sapient life specifically needs limb-based hands. Of course they do need some sort of prehensile member, but why is the only option a pair of limbs that are exclusively for manipulation? If we look at real animals with prehensile parts, then we see all sorts of parts, from proboscides and tentacles, to grasping hindfeet, to even penes specially adapted for grabbing
In fact, there seem to be a trend indicating that aliens wouldn't have hands like ours; specifically, it is that almost all prehensile members that evolve specifically for manipulation are either facial extremities or genital structures. Prehensile tails, feet, and hands are almost exclusively found in arboreal creatures (or descendents thereof) as an adaptation for climbing; while clearly they can adapt for manipulation, it seems clear that proboscides and such are a much more likely candidate for alien prehensors
Even if we ignore all that, the argument has only really gotten us to assuming sapient aliens will be bipedal tetramelic creatures. While this is often where humanoid alien proponents drop the argument, it does not necessarily mean humanoid. There are plenty of nonhumanoid bipeds on Earth; in fact, most real bipeds aren't even upright at all
Perhaps you might think this is just happenstance, due to Earthly reptiles having a thick tail that can work as a counterweight, but this is not the case. Firstly, because many bipeds without such a tail (such as kangaroos and most birds) have remained as pronogrades, and secondly, pretty much all animals with a small number of limbs evolved from an ancestor with many more; usually, most of the reduced/lost limbs are behind the full limbs, thus resulting in a large abdomen/opisthosoma which would serve as a perfectly good counterweight (perhaps even better than a tail). Thus, most bipedal aliens would likely be able to have the same counterweighted form as Earthly dinosaurs and other bipedal reptiles
All in all, the argument for humanoid aliens breaks down at literally every single juncture, and that isn't even getting into the issue of so many humanoid aliens being essentially Earth tetrapods
TLDR:- Humanoid aliens don't make sense
