About a year ago I created this meme during a discussion on a Facebook group about alien life. The group consensus was that we would never meet an alien race with a humanoid posture or upright bipedal locomotion because it was highly unlikely that this arrangement would evolve independently again. Now I am at best a curmudgeon and at worst an asshole, so I got to thinking about that contention and the more I thought about it the less it struck me as a hard and fast rule.
Evolution is essentially conservative, there is a conservation of form and function in evolution because of the way natural law interacts with living beings. For instance a creature that swims in water on Earth or on a planet 20 light years from Earth is probably going to look roughly the same. Since life seems to favor an aquatic origin as that life emerges from the sea of an alien planet evolution of that terrestrial life may already be based on bilateral symmetry. Of course something like an octopus might be the first creature on land, but at least on our planet the race to the surface favored creatures with hard internal or external structures whose bodies were structurally streamlined. I believe these types would most likely emerge first elsewhere as well.
If my conjecture is correct, that bilateral symmetry is favored by aquatic environments leading to quicker more agile creatures, then that conservation of form will follow onto the land leading to creatures that mimic our own evolution. In the meme above the T-Rex and the Ankylosaurus predate the Terror Bird and the Glyptodon by 60 million years, but the body forms are essentially the same…in fact the T-Rex probably had feathers. What does this mean for future encounters with alien life? First, don’t discount the possibility that creatures with similar capacities to ourselves may have similar body structures. It is very possible that higher intelligence requires a bilateral body plan and whose ancestors went through an arboreal stage of development before developing true upright posture. Second, don’t discount running into a nightmare like a Tyrannosaurus when exploring alien environments.
This is just my opinion.