A slight tangent, but I sometimes wonder how 'humanoid' any potential intelligent alien species must be, if they exist. Some science fiction writers depict alien species as quite morphologically similar to humans – Romulans and Klingons in Star Trek – or radically different – Hutts in Star Wars, Hanars and Krogans in Mass Effect. If you take a quick look at "Star Wars alien species" or "Mass Effect alien species", most of the differences imagined by writers are around the head shape, and the overall distribution of muscle mass around the rest of the body. So most alien species you meet in pop sci fi is 90% identical to humans, with a different head. Human heads in this sense are pretty basic. We don't have a leku like Twi'leks, ridged foreheads like Klingons, long necks and slender frames like Kaminoans, a small ring of horns and skin patterns like Zabraks, or scalp-crests like Asari.

But if we did, would these traits ever go away? Would human evolution eventually "revert" us back to our present form? Because something tells me that we wouldn't, and the selection process would basically say "good enough". In other words, the reason we don't look like Asari or Twi'leks is probably pure luck, and the reason alien species may look the way they do (again, if they exist) is also pure luck. But they probably share some of the general characteristics of humanoids, such as limbs not necessary for motion and with opposable fingers, a head separate from the rest of the body, etc.