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Study Uncovers Dynamics at Play During Hunter-Gatherer Transition to Agriculture

NEW YORK – A team from the US, France, and Ethiopia has retraced population and genetic features found in a present-day population in Southwest Ethiopia that is transitioning from a hunter-gatherer to agricultural lifestyle.

"Our results illustrate that although foragers respond to encroaching agriculture and pastoralism with multiple strategies, including cultural adoption of agropastoralism, gene flow, and economic specialization, they often face population decline," co-senior author Brenna Henn, an anthropology researcher at the University of California at Davis Genome Center and its Center for Population Biology, and her colleagues wrote in Current Biology on Wednesday.

Working with anthropologists and local experts, researchers at UC Davis, Stony Brook University, Washington State University, and elsewhere analyzed array-based genotyping profiles for 83 Chabu individuals and more than 190 additional individuals from the four nearby populations in Southwest Ethiopia — the Bench, Sheko, Majang, and Shekkacho populations.

"[S]outhwest Ethiopia is home to several of the world's remaining hunter-gatherer groups, including the Chabu people, who are currently transitioning away from their traditional mode of subsistence," the authors explained.

By comparing patterns in these populations to one another, and to nearly a dozen other populations in Eastern, Central, and Western Africa, they got a glimpse at population sizes over time, genetic relationships in the region, and factors involved in the ongoing hunter-gatherer-to-agriculture transition taking place in the Chabu population.

"Many ancient DNA studies are concerned with the displacement of hunter-gatherers or population replacement during the Neolithic revolution," Henn explained in an email, "but mechanisms are circumstantial because they have only a few ancient genomes to work with at a site or limited evidence of cultural affiliation."

From the genotyping data, together with language, marriage, and other insights, the team saw signs that there have been numerous responses to the arrival of pastoralist or agricultural groups in the Chabu hunter-gatherer individuals — ranging from agropastoralist cultural adoption or economic specialization to gene flow.

When the investigators considered five possible strategies for dealing with the arrival of such agricultural groups, for example, "we find evidence for pretty much all of these strategies," Henn said, noting that "[w]e need to now think more carefully about how ancient DNA can test more sophisticated scenarios, such as a transition to agriculture in response to population decline."

Along with distinct ancestry going back at least 4,500 years in Southwest Ethiopia, the team found that the Chabu population appeared to have undergone significant population declines over roughly the last 1,400 years or so. Those population shifts were not evident in other populations in the region, which appear to have increased modestly or significantly during the same time frame.

The investigators presented their findings and got feedback from the Chabu community in 2019, Henn explained, noting that she hopes to continue incorporating insights from community stakeholders and local experts for genetic studies in the future.

"Continued ethnographic and genetic work in collaboration with the Chabu and other marginalized groups is likely to provide valuable insights into the interactions between farmers and [hunter-gatherers], the drivers of major cultural transitions over long periods of co-existence, and the reasons behind the divergence of demographic histories in genetically and culturally similar groups," the authors concluded.

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