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

by David Reich

The problems faced by modern humans with Neanderthal ancestry went beyond reduced fertility, as it turns out that Neanderthal ancestry is not just reduced on the X chromosome and around genes important in male reproduction, but is also reduced around the great majority of genes (there is far more Neanderthal ancestry in “junk” parts of the genome with few biological functions). The clearest evidence for this came from a study in 2016, in which we published a genome-wide ancient DNA dataset from more than fifty Eurasians spread over the last forty-five thousand years.41 We showed that Neanderthal ancestry decreased continually from 3 to 6 percent in most of the samples we analyzed from earlier times to its present-day value of around 2 percent at later times and that this was driven by widespread natural selection against Neanderthal DNA.