(AFP) – Dec 22, 2008
PARIS (AFP) — Men significantly outnumbered women in the "out-of-Africa" migration some 60,000 years ago that eventually populated the rest of the world, according to a new study.
Africa is known to be the cradle of human evolution, and recent studies show that the peoples today inhabiting other continents originate from a relatively small band of Homo sapiens sapiens who moved through the Near East, into Europe and beyond some 50,000 and 70,000 years ago.
But until now no one had figured out a way to determine what the sex-ratio of this so-called founding population might have been.
A quartet of researchers led by Alon Keinan at the Harvard Medical School thought that the secret might be locked inside differences in genetic code across distinct geographic regions.
They knew that the percentage of X chromosomes in a given population varies depending on the proportion of men.
The "X" and "Y" chromosomes determine sex -- men have one of each, while women have two X chromosomes. The other 22 chromosome pairings in the human genome are all the same.
It was also known that this ratio affects the rate at which mutations randomly spread through the X chromosome over dozens or hundreds of generations as compared to the mutation rate in other, non-sex, chromosomes.
Keinan and colleagues reasoned that if X-chromosomes changed more quickly than expected, then it almost certainly meant that our common ancestors who wandered out of Africa were predominantly male.
To test their theory, they compared the genetic makeup of Africans first with northern Europeans, and then again with Asians.
"The results point to a period of accelerated drift on chromosome X that largely occurred after the split of West Africa and non-Africans, but before the separation of North Europeans and East Asian," the conclude.
Genetic drift is a term that refers to random mutations in genes, as opposed to changes that occur through natural selection.
Keinan acknowledged that if a small fraction of the women in the migratory exodus from Africa had given birth to all of the children, there might still have been parity in the number of males and females.
But this seemed highly unlikely, he said, adding that his findings were "in line with what anthropologists have taught us about hunter-gatherer populations in which short distance migration is primarily by women and long distance migration primarily by men."
The study was published in Nature Publishing Group's journal Nature Genetics.
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