Harmful mutations have accumulated during early human migrations out of Africa.
Credit: Image courtesy of Swiss Institute of Bioinformatics

Modern humans (Homo sapiens) are thought to have first emerged in Africa about 150,000 years ago. 100,000 years later, a few of them left their homeland travelling first to Asia and then further east, crossing the Bering Strait, and colonizing the Americas. Excoffier and his colleagues developed theoretical models predicting that if modern humans migrated as small bands, then the populations that broke off from their original African family should progressively accumulate slightly harmful mutations — a mutation load. Moreover, the mutational load of a population should then represent a way of measuring the distance it has covered since it left Africa. In a nutshell: an individual from Mexico should be carrying more harmful genetic variants than an individual from Africa.