Genetic study suggests new model for human evolution

A modern genomic study puts forward an alternative model to the “tree of life” picture for how modern humans evolved.

While it is widely accepted that modern humans, Homo sapiens, diverged from other human species in Africa before spreading around the world, when and how the split between modern humans and other hominoids, such as Neanderthals, continues to be an area of uncertainty.

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The classic theory holds that between 100,000 and 300,000 years ago, one ancestral population of humans diverged from others in the Homo genus, leading to the modern human lineage.

Another theory suggests that this central ancestral population did not evolve in isolation, but was the result of mixing between modern humans and Neanderthal-like hominins, hundreds of thousands of years ago.

Decades of studies into human genomic variation point to the classic “tree-like” model of recent population divergence being accurate. But the fossil evidence suggests otherwise.

“At different times, people who embraced the classic model of a single origin for Homo sapiens suggested that humans first emerged in either East or Southern Africa,” says Brenna Henn, a population geneticist in the Department of Anthropology in the Genome Center at the University of California, Davis. “But it has been difficult to reconcile these theories with the limited fossil and archaeological records of human occupation from sites as far afield as Morocco, Ethiopia, and South Africa which show that Homo sapiens were to be found living across the continent as far back as at least 300,000 years ago.”

Henn’s team took a different approach. Rather than looking at fossils, they examined modern genetic data.

The researchers used contemporary genomic material of 290 individuals from four geographically and genetically diverse African groups to produce the first systematic test of the competing models of modern human evolution.

Tracing the similarities and differences encoded in the genomes of the groups over the past million years, the scientists were able to gain insight into the genetic interconnections across the continent.

The groups were the Nama (Khoe-San from South Africa); the Mende (from Sierra Leone); the Gumuz (recent descendants of a hunter-gatherer group from Ethiopia); and the Amhara and Oromo (agriculturalists from eastern Africa).

Some Eurasian genetic material was also included to negate the traces of colonial incursions and mixing in Africa.

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The team then used an algorithm to test hundreds of possible scenarios. “Those with gene flow back and forth between populations in various parts of the continent over the course of hundreds of thousands of years provided a much better explanation of the genetic variation we see today,” explains Simon Gravel, Associate Professor at Canada’s McGill University, and co-senior author on the paper.

To account for their findings, the researchers put forward a “weakly structured stem” model which has gene flow between ancestral Homo populations over hundreds of thousands of years.

The research is published in Nature.