Everybody is African in origin. Barring a smattering of genes from Neanderthals and other archaic Asian forms, all our ancestors lived in the continent of Africa until 150,000 years ago. Some time after that, say the genes, one group of Africans somehow became so good at exploiting their environment that they (we!) expanded across all of Africa and began to spill out of the continent into Asia and Europe, invading new ecological niches and driving their competitors extinct.

There is plenty of dispute about what gave these...