Humans started in Africa, spread to Asia around 60,000 years ago, thence to Australia 50,000 years ago, Europe 35,000 years ago and America 15,000 years ago. What have not been so well examined, though, are the tree's African roots. The genetic diversity of Africans probably exceeds that of the rest of the world put together. But the way that diversity evolved is unclear.

[http://www.economist.com/science/PrinterFriendly.cfm?story_id=11088535]