Because the out-of-Africa migration was so recent, the genetic differences we see among geographical groups—sometimes called races—are minor. This fact is highly counterintuitive. We see many superficial differences between an African and a Caucasian, such as skin color, facial form, and hair type, and assume that these superficial differences must reflect extensive genetic differences. This assumption made sense when the standard theory about the origin of modern humans was the multiregional one. If European and African populations really had been geographically isolated from each other for as long as 2 million years, then we would expect significant genetic differences among populations.

We expect isolated populations to diverge genetically over time as different mutations occur and are fixed in each population. The longer two populations have been isolated from each other, the more genetic differences between them we expect to see (Chapter 21). Isolation lasting 2 million years implies that the differences are extensive, but isolation of just 60,000 years suggests they are relatively few. Patterns of genetic variation among different human populations support the hypothesis that human populations dispersed as recently as 60,000 years ago. What we see when we look at genetic markers—variable A’s, T’s, G’s, and C’s in human DNA—is that there is indeed very little genetic differentiation by what is sometimes called race.

In short, there’s a disconnect: Different groups may look very different, but, from a genetic perspective, they’re not very different at all. Any two humans may differ from each other by, on average, only 3 million base pairs, and statistical analyses have shown that approximately 85% of that genetic variation occurs within a population (for example, the Yoruba in West Africa); 8% occurs between populations within races (for example, between Yoruba and Kikuyu, another African group); and the remaining 7% occurs between races. The characteristics we use when we assess an individual’s ethnicity, such as skin color, eye type, and hair form, are encoded by genetic variants that lie in that 7%. If Earth were threatened with destruction and only one population—Yoruba, for example—survived, 85% of the total amount of human genetic variation that exists today would still be present in that population.