Science Forums 2

[pgrmdave]

I think that how they look would be largely determined by their environment. Consider what would have evolved differently if earth had a different mass, and had twice, or half the gravity that it has. If there was much less gravity, then it would be less necessary to have as much muscle to move, and more mass could be dedicated to other activities - thought, metabolism, disease fighting. In a lower oxygen environment, things would live longer but, assuming they still need oxygen, they would need to develop a way to either efficient use or gather the oxygen. In a higher oxygen environment, things would die quicker, but may not need such specialized organs as lungs and gills to obtain oxygen, instead absorbing it directly into the body.