Humans today are mosaics, our genomes rich tapestries of interwoven ancestries. With every fossil discovered, with every DNA analysis performed, the story gets more complex: We, the sole survivors of the genus Homo, harbor genetic fragments from other closely related but long-extinct lineages. Modern humans are the products of a sprawling history of shifts and dispersals, separations and reunions—a history characterized by far more diversity, movement and mixture than seemed imaginable a mere decade ago.
But it’s one thing to say that Neanderthals interbred with the ancestors of modern Europeans, or that the recently discovered Denisovans interbred with some older mystery group, or that they all interbred with each other. It’s another to provide concrete details about when and where those couplings occurred. “We’ve got this picture where these events are happening all over the place,” said Aylwyn Scally, an evolutionary geneticist at the University of Cambridge. “But it’s very hard for us to pin down any particular single event and say, yeah, we’re really confident that that one happened—unless we have ancient DNA.”
The events that do get pinned down therefore tend to be relatively recent, starting with the migration of modern humans out of Africa 60,000 years ago, during which they interacted with hominin relatives (like the Neanderthals and Denisovans) they met along the way. Evidence of interbreeding during any migrations before then, or during events that transpired earlier within Africa, has been elusive.