 For thousands of years, humans have crisscrossed the globe, leaving their homes and seeking new ones for reasons from conquest to famine. These migrations leave traces in our genes, clues to otherwise unknown aspects of human prehistory. Now a paper published online in Nature Genetics on 19 September argues that, in our history, men have traveled more widely than women.Researchers probing our past have often studied two kinds of DNA: the paternally inherited Y chromosome and the maternally inherited mitochondrial DNA (mtDNA). When comparing people in far-flung regions, they found that mtDNA was more similar than were the Y chromosomes. This evidence of more homogenized mtDNA suggested that throughout human history, women have spread their genes farther than men. But some geneticists were skeptical, arguing that the paternally and maternally inherited sequences chosen might not be comparable. To help settle the debate, evolutionary geneticist Michael Hammer, postdoc Jason Wilder, and their colleagues at the University of Arizona, Tucson, analyzed regions of the mtDNA and Y chromosome thought to mutate at roughly the same rate and took both kinds of data from the same 389 men around the world. Then they compared the variability among populations in each kind of DNA. In contrast to previous work, they found approximately the same amount of variation in the mtDNA and Y chromosome among people living on different continents. That indicates roughly the same degree of genetic mixing for paternally and maternally inherited sequences. But Hammer argues that in humans, the breeding ratio is slightly skewed--more women than men reproduce. So to achieve the mixing seen in the Y chromosome, the proportionally smaller group of reproducing males must have traveled farther than their female counterparts. Such male wandering "has been proposed before, but this is the first genetic evidence for it," says Hammer. "They did an excellent job," says evolutionary geneticist Henry Harpending of the University of Utah in Salt Lake City, adding that he thinks the genetic regions Hammer analyzed are more appropriate to the question than those used in previous studies. "I was delighted to read [the paper]." |
