What happened when early humans ventured out of Africa and into Asia? Where did they go and whom did they meet along the way? The latest evidence suggests they left far earlier than previously thought and interbred with a newly discovered type of ancient human — the Denisovans, whose existence was established only five years ago when geneticists extracted DNA from a tiny fragment of finger bone. Because our ancestors mated with them, their genes found a home within our DNA. More than that, they’ve helped us survive and thrive.

• Kerry Shale