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Ancient African DNA Provides New Look at Patterns of Population Formation

NEW YORK – A team led by researchers at Harvard University and Rice University has analyzed genome-wide ancient DNA data that provides new insight into patterns of movement among people in sub-Saharan Africa during the Pleistocene epoch.

The study, which was published Wednesday in Nature, used DNA data from 34 individuals from ancient African populations. It found genetic evidence for increasing regionalization roughly 20,000 to 80,000 years ago, toward the end of the Pleistocene epoch. According to the researchers, led by senior authors David Reich at Harvard and Mary Prendergast at Rice, this genetic information supports hypotheses "that have emerged from archaeological analysis but remain contested."

The authors noted that archaeological studies have shown that by around 50,000 years ago, "technological innovations and symbolic behaviors" like tools and ornaments were widespread throughout sub-Saharan Africa. Around 20,000 years ago, these items and behaviors were both "nearly ubiquitous" and "regionally diverse," which some have argued indicates that people were by that time living in larger, more structured groups, "with variations in population size and connectivity driving differences in material culture across space and time."

The researchers set out to test this notion by analyzing DNA from a series of ancient individuals. Their dataset consisted of 13 individuals for whom genetic information had already been generated; 15 for whom there was already genetic data but which the researchers supplemented with new, higher quality analyses; and six new individuals buried in today's Malawi, Tanzania, and Zambia. This latter group included samples dated as far back as 18,000 years ago, which the authors said doubled the age of ancient sub-Saharan African DNA data previously available.

Using the available genetic data, the researchers conducted a series of analyses, including principal component analysis and allele-sharing tests, to determine the degrees of relatedness of the individuals to one another and to several current African populations.

They found that the strongest predictor of genetic similarity was geographical proximity, even when considering individuals across "a wide range of time periods, ecological contexts, and archaeological associations." This, the authors wrote, suggests that "long-range migrations were rare" toward the end of the Pleistocene, a notion that they said was backed up by their findings of "excess genetic relatedness at subregional scales but not at longer-distance scales." They added that this indicates that earlier patterns of more extensive "mobility and social interactions" did not "remain consistent" throughout the Late Pleistocene.

"Our genetic results support a scenario in which human mobility and longer-range gene flow occurred with the development and elaboration of long-distance networks approximately [80,000 to 20,000 years ago], contributing to the formation of a population structure that persisted over tens of thousands of years during a period when people were living locally," the authors wrote.