Who We Are and How We Got Here: Ancient DNA and the New Science of the Human Past

by David Reich

genetic data are truly ancient. Take for example the founding of the ancestral population of non-Africans. Any genetic analysis of non-Africans reveals evidence of a population bottleneck dating to some time before fifty thousand years ago—that is, a small number of individuals giving rise to many descendants today. In 2009, I worked with Alon Keinan, a postdoctoral scientist in my laboratory, to compare genetic variation on the X chromosome, the larger of our two sex chromosomes, to the rest of the genome. To our surprise, we found much less genetic variation in non-Africans on chromosome X than would be expected from the level of variation in the rest of the genome, assuming that males and females participated equally in the founding of the ancestral population of non-Africans. The pattern was too extreme to be explained by a simple scenario of more men than women participating in the founding of the ancestral population of non-Africans. But we discovered that one scenario that could explain the pattern is that after the initial budding off of the ancestral population of non-Africans, there was genetic input into this population from males of other groups. Since males carry one copy of chromosome X for every two copies of other chromosomes, a process of repeated waves of male immigration would decrease X-chromosome diversity (meaning that there would be less genetic variation in the population) compared to the rest of the genome, producing the pattern we observed.