It has long been posited by paleoanthropologists that modern humans originated in Africa within the last 200,000 years from a single group of ancestors. Modern humans continued to evolve in Africa, the theory suggests, and then spread to the Middle East by 100,000 years ago (and possibly as early as 160,000 years ago) and ultimately reached Europe.
But the latest study challenges these conclusions and proposes new theories.
By analyzing the genomes of 290 living people, researchers concluded that modern humans descended not from one, but from at least two populations that coexisted in Africa for a million years before merging in several independent events across the continent. The findings were published on 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 puts a nail in the coffin of that idea.”
Paleoanthropologists and geneticists have ample evidence that points to Africa as the origin of our species: the oldest stone tools used by our ancestors, and human DNA, but exactly where, and was it a single source?
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 existing in Africa over different periods of time and then observed which ones could produce the diversity of DNA found in people alive today.
The researchers analyzed DNA from a range of African groups, including the Mende, farmers who live in Sierra Leone in West Africa; 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.
They compared these Africans’ DNA with the genome of a person from Britain. They also looked at the genome of a 50,000-year-old Neanderthal found in Croatia. Previous research had 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 coming out of Africa, and then became extinct about 40,000 years ago.
The researchers concluded that as far back as a million years ago, the ancestors of our species existed in not one, but two distinct populations. Dr. Henn and her colleagues call them Stem1 and Stem2.
About 600,000 years ago, a small group of humans budded off from Stem1 and went on to become the Neanderthals. But Stem1 endured in Africa for hundreds of thousands of years after that, as did Stem2.
If Stem1 and Stem2 had been entirely separate from each other, they would have accumulated a large number of distinct mutations in their DNA. Instead, Dr. Henn and her colleagues found that they had remained only moderately different — about as distinct as living Europeans and West Africans are today. The scientists concluded that people had moved between Stem1 and Stem2, pairing off to have children and mixing their DNA.
About 120,000 years ago, the model indicates, African history changed dramatically. For undetermined reasons, people from Stem1 and Stem2 merged in southern Africa, giving rise to a new lineage.
It’s possible they will discover even other populations that endured in Africa for hundreds of thousands of years, ultimately helping produce our species as we know it today; to use an analogy, one that emerged not from a single trunk, but from multiple branches.
Dr. Scerri speculated that living in a network of mingling populations across Africa might have allowed modern humans to survive while Neanderthals became extinct. In that arrangement, our ancestors could hold onto more genetic diversity, which in turn might have helped them survive through shifts in the climate, or even evolve new adaptations.
“This diversity at the root of our species may have been ultimately the key to our success,” Dr. Scerri said.
