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

by David Reich

Instead, it is clear that the Y chromosome was a nonrepresentative part of the genome where certain genetic types were more successful at being passed down to later generations than others. In principle, one possible explanation for this is natural selection, whereby some Y chromosomes gave a biological advantage to those who carried them, such as increased fertility. But the fact that this genetic pattern manifested itself around the same time in multiple places around the world—in a period coinciding with the rise of socially stratified societies—is too striking a pattern to be explained by natural selection at multiple independently occurring advantageous mutations. I think a more plausible explanation is that in this period, it began to be possible for single males to accumulate so much power that they could not only gain access to large numbers of females, but they could also pass on their social prestige to subsequent generations and ensure that their male descendants were similarly successful. This process caused the Y chromosomes these males carried to increase in frequency generation after generation, leaving a genetic scar that speaks volumes about past societies. It is also possible that in this period, individual women began to accumulate more power than they ever had before. Yet because it is biologically impossible for a woman, even a very powerful one, to have an extremely large number of children, the genetic effects of social inequality are much easier to ...