A tuft of tangled human hair is dug out of Greenlands permafrost. From these 4,000-year-old threads of evidence, scientists attempt the impossible to be the first to reconstruct the identity of a Stone Age human being. From the scraps of DNA, an ancient face appears. But as they break into this genetic code, they find far more than they bargained for, and the discovery threatens to turn our long-held concept of how humans populated the earth, on its head.