Neanderthals went extinct roughly 39,000 years ago, but in some sense these close cousins of our species are not gone. Their legacy lives on in the genomes of most people on Earth, thanks to interbreeding between Homo sapiens and Neanderthals.

While Neanderthals and modern humans lived together in Europe for only a relatively short time, two separate studies found inter-species breeding took place from around 50,000 years ago.

Tens of thousands of years ago, modern humans mated with Neanderthals. But exactly how and when that happened -- and who those groups of humans were -- is less known.