A new paper published in the journal Cell shows that inbreeding has declined over the last 12,000 years, as demonstrated with 411 ancient DNA samples from West Eurasia (defined as Europe) and Central Eurasia (defined as Southwest Asia, the Caucasus and Central Asia).
Samples from modern groups like the Balochi, the Bedouin, or the Sindhi from Pakistan have the highest proportions of individuals with FROH > 0.0391 (50%, 41.3%, and 33.3% respectively).
Some concluding comments of the paper bear repeating in their entirety so as not to lose the nuance of these important caveats to its findings:
Three points further deserve mention regarding mating patterns in human societies.
One is the seeming contrast between the high levels of drift-driven autozygosity (panmictic inbreeding) we report for ancient hunter-gatherer societies and ethnographic studies showing low levels of inbreeding among modern-day hunter-gatherers. For instance, a comparison of inbreeding patterns in a worldwide sample of contemporary hunter-gatherers with Amazonian horticulturalists reported lower inbreeding in hunter-gatherer groups. Hill and colleagues also report low levels of relatedness within modern-day hunter-gatherer bands. However, the mentioned ethnographic findings rely on genealogies and report the prevalence of inbreeding by consanguinity, not inbreeding by drift. In fact, we also find consanguinity to be rare among early Holocene Eurasian hunter-gatherers relative to agriculturalists, consistent with widespread exogamy in modern-day hunter-gatherers. This raises the possibility that reciprocal exogamy and consanguinity avoidance traditions may have been predominant among human foragers since prehistory (but possibly not in archaic hominins).
Second, our results lend support, albeit with limited data, to the hypothesis that extreme consanguinity may have become more common with farming. This result parallels higher within-group marriages among modern-day horticulturalists than foragers. It is also consistent with singular reports on ancient agriculturalist genomes, such as evidence for consanguinity identified in an early Neolithic farmer from Iran, a first-degree incest case from Neolithic Ireland, as well as a recent report on close-kin unions in the central Andes after 1000 CE.In our analysis, among the seven individuals with the highest level of inbreeding (with FROH > 0.125), all four hunter-gatherers appear autozygous by drift, while all three agriculturalists appear autozygous by consanguinity. This appears unlikely to happen by chance (Fisher’s exact test, two-sided p = 0.029). These results are consistent with the view that consanguineous traditions could have thrived in class-based agricultural societies with private property more readily than in more egalitarian hunter-gatherer groups.Finally, we report higher consanguinity in Central versus West Eurasia in contemporary societies, in parallel with earlier work. This is consistent with widespread first- or second-cousin marriage practices in agricultural societies in Middle Eastern and North African countries and in South Asia, including Muslim and Jewish groups, as documented by ethnographic or genomic studies. We note that cousin marriages were also common among royal dynasties and upper classes of Europe until the 20th century, and many prominent European scientists of that period are known to have married their first cousins, including Charles Darwin and Albert Einstein. These traditions are thought to have arisen through various social factors, including the inheritance of property in class societies.Interestingly, we do not observe the relatively high rates of consanguineous marriage observed in modern-day Central Eurasia in any of the past societies we studied, in Antiquity or earlier. We naturally prefer to remain cautious, especially given the limited sample size of our advanced complex agriculturalist samples from West and Central Eurasia (n = 9 and n = 30, respectively). Nevertheless, it appears possible that present-day cultural patterns may have emerged relatively late in time.