The relatively few samples of Neandertal DNA show up in most people with European or Asian ancestry, as much as 2%, which is more than most of us carry from our great-great-great-great-grands. That means that A) *most* of our ancestors were Neandertal (in fact, the very ones which happen to have survived in the ground) or B) there was no or very very little genetic difference between Neandertal and other archaic humans.
See #18.
Well there had to of been very little difference genetically to interbreed with neanderthals. Look at how close some animal species are and they only create sterile hybrids.