From Infidel753:
In this post last month, I said that one reason I reject alien-abduction stories is that the aliens they describe — “greys” and suchlike — are just unimaginative modifications of the human form, and it’s very unlikely that evolution on another planet would produce anything so closely resembling us or any other Earthly species. Aliens with a space-traveling civilization would need to have sophisticated sense organs and organs of manipulation (like hands or tentacles) in order to develop technology in the first place, but that’s all we can assume.
So what would life on another planet be like, if it exists at all? Not just intelligent life that might visit us, but life in general which we might someday discover? This is obviously going to be very speculative, since we have only one example of a life-bearing planet to look at and thus no general knowledge about what is and isn’t possible. But it’s informed by what I know about evolution, at least.
To begin with, all land vertebrates on Earth are four-limbed (except in a few cases like snakes where evolution has done away with limbs that their ancestors had), but this is just because they all evolved from a common four-limbed ancestor. There’s no reason to think that the same would hold on another world. It would depend on how many limbs the common ancestor of land life had. Six limbs, or eight, or even ten or more, might actually work better, especially for very large animals. In some Earthly vertebrates such as birds and bats, the front pair of limbs has evolved into wings, while in a few species — notably humans — the same front pair has become specialized into organs of manipulation rather than locomotion. With more pairs of limbs to work with, evolution might specialize them for several different functions in the same animal. In an intelligent species, the first pair or even the first two pairs of limbs might specialize for manipulation as our arms and hands do, leaving the rest as legs and/or wings.