Humans should go extinct to give rise to more evolutionary adaptive species. When the asteroid wiped out the dinosaurs only the most resourceful and capable animals survived, which meant the rise of mammals. If a similar event were to occur (and I'm thinking WW3 with a nuclear apocalypse scenario), I doubt most humans would survive. The ones who do however, will breed to become resilient super-humans with super strength and perhaps telepathy and the ability to fold space such that it negates the Earth's gravity and allows them to levitate. These super-humans (who I presume will be in various bunkers) will interact with the horribly mutated animals that lay protective eggs sheltering their young from radiation, and also be immune from radiation themselves (because they've already mutated, they can't mutate any more... duh). However, since as we all know that radiation tends to enlarge animals, if you've got giant rats and giant kangaroos running around those first few humans to come out of the bunker will undoubtedly die a horrible, horrible death. This means that those human survivors will be further refined until eventually they look like those little grey aliens I'm sure you've all read about. Given all the time that's elapsed, these grey men will have devised a machine to traverse through time - a time machine if you will - probably in the shape of a circular disc since it's aerodynamic and tempodynamic. They will use this time disc to come back to the present time right now to study us and our way of life prior to the great atomic annihilation.

Anyway I think that's the most likely scenario. I'm pretty sure it'll happen within my lifetime.