What happened when we expanded out of Africa and into Asia -- where did we go and whom did they meet along the way? The latest evidence suggests we left far earlier than previously thought and interbred with a newly-discovered type of ancient human -- the Denisovans. The existence of these people was only established four years ago, when geneticists extracted DNA from a tiny fragment of finger bone. And because our ancestors mated with them, their genes found a home within our DNA. More than that, they've helped us survive and thrive.