Study in Nature suggests new thesis for human evolution – 05/17/2023 – Science

Scientists have revealed a surprisingly complex origin for our species, rejecting the long-held argument that modern humans arose in one place in Africa at a certain point in time.

By analyzing the genomes of 290 living people, researchers concluded that modern humans descended from at least two populations that coexisted in Africa for 1 million years before merging in several independent events across the continent. The findings were published on Wednesday (17) in the journal Nature.

“There is no single place of birth,” said Eleanor Scerri, an evolutionary archaeologist at the Max Planck Institute for Geoarchaeology in Jena, Germany, who was not involved in the new study. “It really puts a nail in the coffin for this idea.”

Paleoanthropologists and geneticists have found evidence that points to Africa as the place of origin of our species. The oldest fossils that can belong to modern humans, dating back 300,000 years, have been excavated there, as have the oldest stone tools used by our ancestors.

Human DNA also points to Africa. Living Africans have a great deal of genetic diversity compared to other populations. That’s because humans lived and evolved in Africa for thousands of generations before small groups – with comparatively small genetic pools – began to expand to other continents.

Within the vast expanse of Africa, researchers have proposed several places as the birthplaces of our species. The first humanoid fossils from Ethiopia have led some researchers to look to East Africa. But some groups of people living in South Africa appeared to be very distantly related to other Africans, suggesting that humans might have a deep history there.

Brenna Henn, a geneticist at the University of California, Davis, and her colleagues have developed software to run large-scale simulations of human history. The researchers created many scenarios of diverse populations existing in Africa at different time periods and then looked at which ones might produce the DNA diversity found in populations living today.

“One might ask what kinds of models are really plausible for the African continent,” said Henn.

The researchers analyzed the DNA of several African groups, including the Mende, farmers living in Sierra Leone, West Africa; the Gumuz, a group descended from hunter-gatherers from Ethiopia; the Amhara, a group of Ethiopian farmers; and the nama, hunter-gatherers of South Africa.

The researchers compared the DNA of these Africans with the genome of a person from Great Britain. They also analyzed the genome of a 50,000-year-old Neanderthal found in Croatia. Previous research has found that modern humans and Neanderthals shared a common ancestor that lived 600,000 years ago. Neanderthals expanded across Europe and Asia, interbred with modern humans from Africa, and became extinct about 40,000 years ago.

The researchers concluded that 1 million years ago the ancestors of our species already existed in two distinct populations. Henn and his colleagues call them Stem1 and Stem2 (Branch1 and Branch2).

About 600,000 years ago, a small group of humans sprouted from Stem1 and became Neanderthals. But Stem1 endured in Africa for hundreds of thousands of years after that, as did Stem2.

If Stem1 and Stem2 were completely separated from each other, they would have accumulated a large number of distinct mutations in their DNA. Instead, Henn and his colleagues found that they remained only moderately different — as distinct as living Europeans and West Africans are today. The scientists concluded that people moved between Stem1 and Stem2, pairing up to have children and mixing their DNA.

The model does not reveal where the populations of Stem1 and Stem2 lived in Africa. And it’s possible that bands from these two groups moved around a lot over the vast periods of time they existed on the continent. Around 120,000 years ago, the model indicates, African history changed dramatically.

In southern Africa, the peoples of Stem1 and Stem2 merged, generating a new lineage that would give rise to the nama and other living humans in that region. In other parts of Africa, a separate merger of the Stem1 and Stem2 groups has occurred. This merger produced a lineage that would give rise to people living in West Africa and East Africa, as well as people who expanded out of Africa.

It is possible that climatic upheavals forced Stem1 and Stem2 populations into the same regions, causing them to merge into unique groups. Some hunter-gatherer bands may have had to withdraw from the coast when sea levels rose, for example. Some regions of Africa have become arid, potentially sending people in search of new homes.

Even after these mergers 120,000 years ago, people with exclusively Stem1 or exclusively Stem2 ancestry seem to have survived. DNA from the Mende people showed that their ancestors interbred with Stem2 people 25,000 years ago. “This suggests that Stem2 was somewhere in West Africa,” said Henn.

She and her colleagues are now adding more genomes from people in other parts of Africa to see if they affect the models.

It is possible that they will discover other populations that survived in Africa for hundreds of thousands of years, helping to produce our species as we know it today.

Scerri speculated that living in a network of mixed populations across Africa may have allowed modern humans to survive while Neanderthals went extinct. In this arrangement, our ancestors were able to maintain greater genetic diversity, which in turn may have helped them withstand changes in climate or even develop new adaptations.

“This diversity at the root of our species may have been the key to our success,” Scerri said.