Why Human-Neanderthal Sex Is Tricky to Prove

Why Human-Neanderthal Sex Is Tricky to ProveA bundle of recent genetic studies have suggested modern humans had sex with Neanderthals thousands of years ago when the two populations roamed the planet alongside each other. However, the bones left behind by the two species don't bear any obvious traces of interbreeding, and a new study of monkeys in Mexico shows why we shouldn't expect them to.