tangentially related, i don't think finding humanoid species elsewhere in the galaxy is really that far fetched.
the most simple multicelled organism is essentially a tube that takes food from one end and poops from the other, just like OPs image. (FRONT AND BACK ASYMMETRY)
when organisms get large enough, the force of gravity starts affecting them, giving their bodies a top and a bottom. (TOP AND BOTTOM ASYMMETRY)
there is no pressure to favor the left and right side of an organism, unlike the front and back and top and bottom, and there for you get bilateral symmetry (LEFT AND RIGHT SYMMETRY)
Now, life as we know it would almost certainly have started in an aqueous environment. if there is land, eventually that life will spread to it.
land requires different types of locomotion than water. legs are clearly a good design choice, slithering is more niche, and flight isn't going to happen right out of the sea. how many legs? for small animals like insects, legs are cheap, and they can sport dozens of em. but for large animals, legs are much more expensive. 3 legs provides stability, but not when moving, so 4 legs is what probably most often will occur. more than 4 legs is probably mostly energetically prohibitive for large animals.
animals that are able to move with 2 of 4 legs will develop increased dexterity with the legs not being used for movement. this allows these animals increased control over manipulating their environments. highly evolved animals may lose the ability to ambulate with the modified limbs, which then become arms.
finally, animals that routinely manipulate their environment become more intelligent, and animals with the most ability to manipulate their environment will become the most intelligent.
maybe alien life is more strange than i can imagine, but if i saw a humanoid on another planet, it wouldn't surprise me as much as it probably should
