This week' issue of the Proceedings of the US National Academy of Science has an interesting article on the conclusions that can be drawn from our genome and what we known of the Neanderthal and Denisovian genomes that we have got
It seems that we all split from a common ancestor about 25,900 generations ago, and the Neanderthals and Denisovians split only some 300 generations later.
Whatever the genetic innovation might have been, it seems to have helped the creatures who carried it to invade new areas and get isolated from one another.