The first Americans separated in two groups before reconverging

It has previously been thought that the first people to enter the Americas divided into two ancestral branches, one northern and one southern. The northern branch were the direct ancestors to many Native communities now living in the Canadian east. The southern branch is thought to have given rise to all the populations in Central and South America. However, for the first time, a new study has found that the majority – and potentially all – of the Indigenous peoples of the southern continent retain at least some DNA from the northern branch deep in their genetic history.

The study is published in the journal Science, and shows that – while the two branches may have remained separate for thousands of years – they actually came back together either before or during the expansion of people into South America. Researchers analyzed 91 ancient genomes from sites in California and Canada, which show that after the first peoples separated into two populations between 18,000 and 15,000 years ago, the northern branch migrated east to the great lakes region. The researchers believe this population may have been following the retreating glacial edges of the thawing Ice Age. The southern branch is believed to have continued down the Pacific coast, inhabiting islands along the way.

But analysis of Central and South American genomes reveals that the two branches “reconverged” despite being separated for millennia. The study’s authors say that there must have been one or more “admixture” events between the two populations roughly 13,000 years ago. This blending of lineages occurred either in North America before they expanded south, or as people migrated even deeper into the southern continent.

“It was previously thought that South Americans, and indeed most Native Americans, derived from one ancestry related to the Clovis people,” says Dr. Toomas Kivisild, co-senior author of the study from the University of Cambridge. The Clovis culture – named for the 13,000-year-old stone tools found near Clovis, New Mexico – were a prehistoric people once believed to be ancestral to all Native Americans. “We now find that all native populations in North, Central and South America also draw genetic ancestry from a northern branch most closely related to Indigenous peoples of eastern Canada. This cannot be explained by activity in the last few thousand years. It is something altogether more ancient,” Kivisild says.

Interestingly, when present day Central and South American populations were analyzed in this study, the highest proportion of northern branch genetics in South America was found very far south in southern Chile. “It’s certainly an intriguing finding, although currently circumstantial – we don’t have ancient DNA to corroborate how early this northern ancestral branch arrived,” explains Dr. Christiana Scheib, who conducted the work while at the University of Cambridge and is first author of the study. “It could be evidence for a vanguard population from the northern branch deep in the southern continent that became isolated for a long time – preserving a genetic continuity.”

The researchers believe that more should be done to include Indigenous communities in ancient DNA studies in the Americas. Genetic analysis of ancient people may have an adverse effect for linked Indigenous communities, but engagement work can help mitigate harm to the community and give Indigenous peoples a voice in the research.

“The lab-based science should only be a part of the research. We need to work with Indigenous communities in a more holistic way,” says Schieb. “From the analysis of a single tooth, paleogenomics research can now offer information on ancient diet and disease as well as migration. By developing partnerships that incorporate ideas from Native communities, we can potentially generate results that are of direct interest and use to the Indigenous peoples involved.”

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By Connor Ertz, Earth.com Staff Writer