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

by David Reich

The problematic mutations in the Neanderthal genome form a sharp contrast with more recent mixtures of divergent modern human populations where there is no evidence for such effects. For example, among African Americans, in studies of about thirty thousand people, we have found no evidence for natural selection against African or European ancestry.43 One explanation for this is that when Neanderthals and modern humans mixed they had been separated for about ten times longer than had West Africans and Europeans, giving that much more time for biological incompatibilities to develop. A second explanation relates to the observation, from studies of many species, that when infertility arises between populations, it is often due to interactions between two genes in different parts of the genome. Since two changes are required to produce such an incompatibility, the rate of infertility increases with the square of population separation time, so a ten-times-larger population separation translates to one hundred times more genetic incompatibility. In light of this the lack of infertility in hybrids of present-day humans may no longer seem so surprising.