A photo provided by the Max Planck Institute shows Nobel Prize-winning Swedish geneticist Svante Pääbo with a reconstructed Neanderthal skull. As a result of interbreeding between Neanderthals and humans tens of thousands of years ago, most people alive today carry a bit of Neanderthal DNA in their genome — and that residual DNA, in turn, is giving scientists a detailed look at the ancient sexual encounters that put it there. (Frank Vinken/Max Planck Institute via The New York Times) By CARL...