Human Evolution: Tale of the Y

Nothing against fossils, but when it comes to tracing the story of human evolution they’re taking a back seat lately to everything from DNA to lice, and even the DNA of lice. A few years ago scientists compared the DNA of body lice (which are misnamed: they live in clothing, not the human body) to that of head lice, from which they evolved, and concluded that the younger lineage split off from the older no more than 114,000 years ago, as I described in a cover story last year. Since body lice probably arose when a new habitat did, and since that habitat was clothing, that’s when our ancestors first needed a haberdasher. The Y chromosome has been an even greater source of clues to human evolution, showing among other things that the most recent common ancestor of all men alive today lived 89,000 years ago in Africa, and that the first modern humans walked out of Africa about 66,000 years ago and became the ancestors of everyone outside that natal continent.

The Y chromosome is at it again. Scientists reported this week that an analysis of Y chromosomes in a dozen African populations sheds light on one of the more controversial questions in human prehistory: did innovations such as animal herding spread because their inventors did, migrating to new places and teaching the natives new tricks, or because the idea spread on its own, as neighboring tribes noticed the new trick and adopted it, and then neighbors of those guys did the same, on and on until the idea had spread like wildlfire?

According to a paper in the online version of Proceedings of the National Academy of Sciences, pastoralism—cattle- and sheep-herding—arrived in southern Africa 2,000 years ago on a wave of human migration from eastern Africa, not by the spread of ideas to neighbors near and far.

“There's a tradition in archaeology of saying people don’t move very much; they just transfer ideas,” said genetic anthropologist Joanna Mountain of Stanford University, senior author of the paper with geneticist Peter Underhill. But in this case, at least, the people themselves moved.

Scientists knew about two prehistoric migrations of Bantu-speaking people from eastern Africa, where pastoralism first arose, to southern Africa: 30,000 years ago and again 1,500 years ago. But anthropological evidence showed that the first sheep and cattle herds existed in southern Africa 2,000 years ago. That suggested that the idea jumped from group to group (“hey, look what those guys are doing”) without the people themselves actually trekking south.

The Stanford scientists analyzed genetic variation on the Y chromosome, which is passed almost intact from father to son. The only change through the generations occurs through rare mutations. By counting and comparing mutations, geneticists can trace ancestries of living men, in this case 13 populations in Tanzania and in the Namibia-Angola-Botswana border region of southern Africa. In this case, it revealed a novel mutation in some men in both places, which implies that those men had a common ancestor. The novel mutation arose in eastern Africa about 10,000 years ago and was carried by migration to southern Africa about 2,000 years ago not by Bantu-speakers, in whom the mutation is absent, but in speakers of what’s called the Nilotic language. These unsuspected ancestors first brought herds of animals to southern Africa before the Bantu migration.

Why did they migrate south? Underhill suspects that a shift in rainfall 10,000 years ago caused some people to stay in rainy areas and grow crops, while others moved to dry regions and lived the nomadic life of herders, he and colleagues proposed in the June issue of the journal Antiquity.