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

by David Reich

The test we developed is now called the “Four Population Test,” and it has become a workhorse for comparing populations. The test takes as its input the DNA letters seen at the same position in four genomes: for example, two modern human genomes, the Neanderthal, and a chimpanzee. It examines whether, at positions where there is a mutation distinguishing the two modern human genomes that is also observed in the Neanderthal genome—which must reflect a mutation that occurred prior to the final separation of Neanderthals and modern humans—the Neanderthal matches the second human population at a different rate from the first. If the two modern humans descend from a common ancestral population that separated earlier from the ancestors of Neanderthals, there is no reason why the mutation is more likely to have been passed down one modern human line than another, and thus the rate of matching of each of