Well, the amount of Neanderthal DNA in modern human DNA indicates that there was always some overlap in time.<p>I find it easy to imagine a scenario where Neanderthals left their group to join nearby Homo Sapiens groups, as it became obvious that the Homo Sapiens hunting methods developed on the savannah were far more effective than traditional Neanderthal ambush hunting techniques in a post Ice-Age Europe characterised by forest retreat / grassland encroachment.