Study offers new twist on how the first humans evolved

Scientists have revealed a surprisingly complex origin of our species, rejecting the long-held argument that modern humans evolved from one place in Africa over a period of time.

By analyzing the genomes of 290 living humans, researchers concluded that modern humans are descended from at least two populations that coexisted in Africa for a million years before merging into several independent events across the continent. The findings were published Wednesday in Nature.

“There is no single birthplace,” 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 hits a nail in the coffin of that idea.”

Paleoanthropologists and geneticists have found evidence pointing to Africa as the origin of our species. The oldest fossils possibly belonging to modern humans, dating back as much as 300,000 years, have been excavated there. So were the oldest stone tools used by our ancestors.

Human DNA also points to Africa. Living Africans have an enormous amount of genetic diversity compared to other people. That’s because humans lived and evolved in Africa for thousands of generations before small groups — with relatively small gene pools — began to expand to other continents.

Within the vast expanse of Africa, researchers have suggested several places as the birthplace of our species. Early hominin fossils in Ethiopia led some researchers to look to East Africa. But some living groups of people in South Africa seemed to be very distantly related to other Africans, suggesting that people there might have deep histories instead.

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

“We could ask what types of models are really plausible for the African continent,” said Dr. Henn.

The researchers analyzed DNA from several African groups, including the Mende, farmers who live in Western Africa’s Sierra Leone; the Gumuz, a group descended from hunter-gatherers in Ethiopia; the Amhara, a group of Ethiopian farmers; and the Nama, a group of hunter-gatherers in South Africa.

The researchers compared the DNA of these Africans with the genome of a person from Great Britain. They also looked at the genome of a 50,000-year-old Neanderthal found in Croatia. Previous research had shown 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 who came from Africa, and became extinct about 40,000 years ago.

The researchers concluded that the ancestors of our species existed in two different populations as far back as a million years ago. Dr. Henn and her colleagues call them Stem1 and Stem2.

About 600,000 years ago, a small group of humans sprouted from Stem1 and became the Neanderthals. But Stem1 persisted in Africa for hundreds of thousands of years afterwards, just like Stem2.

If Stem1 and Stem2 had been completely separated from each other, they would have accumulated a large number of different mutations in their DNA. Instead, Dr. Henn and her colleagues that they had remained only moderately different – about as different as living Europeans and West Africans are today. The scientists concluded that humans had moved between Stem1 and Stem2, mating to have children and mixing up their DNA.

The model does not reveal where the Stem1 and Stem2 people lived in Africa. And it is possible that bands of these two groups moved around a lot during the vast time they existed on the continent. About 120,000 years ago, the model indicates, African history changed dramatically.

In southern Africa, people from Stem1 and Stem2 merged, creating a new lineage that would lead to the Nama and other living people of that region. Elsewhere in Africa, a separate merger of Stem1 and Stem2 groups took place. That fusion produced a lineage that would give rise to living humans in West Africa and East Africa, as well as the humans that expanded from Africa.

It is possible that climate upheavals forced Stem1 and Stem2 humans into the same regions, merging them into single groups. For example, some hunter-gatherer groups have had to retreat from the coast as sea levels have risen. Some regions of Africa became arid, possibly leading people to look for new homes.

Even after these mergers 120,000 years ago, people with only Stem1 or only Stem2 ancestry seem to have survived. The DNA of the Mende people showed that their ancestors interbred with Stem2 people just 25,000 years ago. “It suggests to me that Stem2 was somewhere in West Africa,” said Dr. 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’s possible they might discover other populations that have existed in Africa for hundreds of thousands of years, ultimately helping to produce our species as we know it today.

Dr. Scerri speculated that living in a network of intermixed populations across Africa would have enabled modern humans to survive as Neanderthals went extinct. In that setup, our ancestors could have maintained more genetic diversity, which in turn would have helped them weather shifts in climate, or even evolve new adaptations.

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