This may sound nuts, however I personally do not believe civilization is a result of evolution and natural selection. It is caused by something non-natural, as is our capacity to build civilizations. I agree with Mole here (almost) in that natural selection is not teleological, it does not tend towards consciousness. I think Mole was saying that to mean that there is no reason to suspect any species would replace us, but the logical conclusion we should draw from Mole's statement is that since consciousness is not a result of evolution, it must be the result of something else. I don't know what, though, there are varying theories and models concerning the non-natural theory of consciousness. If we see consciousness as a non-natural experience in itself, it must have been caused by something non-natural. The problem with this theory, and the reason it sounds nuts, is because don't know what this cause is, and thus have to fill in the blanks with our imagination.
If a birdrace were to replace us, which I don't see as impossible, I don't think they would need oppossable thumbs necessarily, they can do other things which their morphology allows them to, but they would have to acquire the level of sapience we currently have (and don't always use very well btw).