If I remember rightly the theory on the asteroid was that it allowed mammalian life to flourish, filling the gap left behind by the extinction of the big dinos. Pre-extinction mammals were small, burrow-dwelling creatures, and of course, they were warm blooded, giving them an advantage over cold blooded dinosaurs in the colder climate of the new Earth. Though, of course, there is disagreement on the reptilian nature of the dinosaurs too... Some of them at least were more bird-like... They had feathers... Read this really interesting book on how you could in theory 'grow' a dinosaur from a chicken's egg with a little genetic modification there a while back...

Never heard of the out of Africa part, but it makes sense. Leaving behind the warmer climate and more favorable ecosystem meant that genes for innovation and creativity were selected for, allowing for the devopment of a species of smartasses, as Hankster might say.