Cultural norms of exogamy and mobility shape hunter-gatherer genetic evolution
Through a hunting and gathering lifestyle, humans have managed to thrive across all terrestrial ecosystems. A key adaptive feature enabling this ecological success is the ability of hunter-gatherer societies to maintain high levels of genetic diversity despite ecological and demographic shocks, but the mechanisms underlying this resilience are poorly understood. Here we integrate genomic, demographic, mobility and ethnographic data from two Central African hunter-gatherer populations to show that genetic diversity emerges from interacting effects of population size, mobility and cultural norms governing marriage. We first demonstrate direct selection against background homozygosity: even modest increases in runs of homozygosity, in the near absence of close-kin marriage, are associated with reduced reproductive success. Despite regional differences in effective population size, clustering of relatives, sedentism and exogamy rules, overall levels of homozygosity are similarly low in both populations. These shared genetic outcomes are achieved through distinct strategies: in one region, strict exogamy combined with high lifetime mobility limits local relatedness, whereas in the other, more relaxed exogamy norms are offset by increased male mate-search distances that reduce offspring homozygosity. Together, our results show that human populations flexibly adjust mobility and social norms to demographic constraints to preserve genetic diversity and avoid fitness costs, revealing culture as a central component of human adaptation.