The new data led the researchers to a puzzling conundrum: If people had continued to move around at the rate seen during the studied period, the regional differences would have gradually begun to disappear. The genomes of people in Eastern Europe, for instance, would have become indistinguishable from those in western Europe and North Africa and vice versa. However, most of these populations—even today—remain genetically distinct. That may be, in part, because individuals were not always reproducing in the locations where they died, and some may have traveled during their lifetimes but returned home before having children.

Maybe this is evidence of an ongoing slave trade, where slaves were sold outside their regions of origin and had low birth rates.