People just really like sex.

Neanderthals and humans interbred 100,000 years ago, says study - The Economic Times: "Neanderthals may have been mating with modern humans 100,000 years ago, tens of thousands of years earlier than previously thought, a new study based on several different methods of DNA analysis has found. Scientists provide the first genetic evidence of a scenario in which early modern humans left the African continent and mixed with now-extinct members of the human family prior to the migration "out of Africa" of the ancestors of present-day non-Africans, less than 65,000 years ago."