Population geneticists expected to find dramatic differences as they got a look at the full genomes — about 25,000 genes — of people of widely varying ethnic and geographic backgrounds. Specifically, they expected to find that many ethnic groups would have derived alleles that their members shared but that were uncommon or nonexistent in other groups. Each regional, ethnic group or latitude was thought to have a genomic “signature” — the record of its recent evolution through natural selection.

But as analyses of genomes from dozens of distinct populations have rolled in — French, Bantu, Palestinian, Yakut, Japanese — that’s not what scientists have found. Dramatic genome variation among populations turns out to be extremely rare.

Instead, it is “random genetic drift” that appears to be more important in sculpting our genes. Drift describes the chance loss of genetic variation that occurred not only in the out-of-Africa migration, but through all of human history as famine, climate change or war caused populations to crash and then recover.