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. |