Scientists have revealed a surprisingly complex origin for our species, rejecting the long-held argument that modern humans originated from a single place in Africa during a single time period.
By analyzing the genomes of 290 surviving individuals, the researchers concluded that modern humans are descended from at least two groups that co-existed in Africa for a million years before merging in several independent events across the continent. The results were published on Wednesday nature.
“There is no single place of birth,” said Eleanor Skari, 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 found evidence that points to Africa as the origin of our species. The oldest excavations that may belong to modern humans, dating back up to 300,000 years ago, were discovered there. So were the oldest stone tools that our ancestors used.
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 genetic pools–began to expand to other continents.
Within the vast expanse of Africa, researchers have suggested various places as the birthplace of our species. Early hominid fossils in Ethiopia have prompted some researchers to look to East Africa. But some living groups of people in southern Africa appear to be very distantly related to other Africans, suggesting that humans may have a deep history there 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 for different populations present in Africa over different time periods, and then observed which ones could produce the DNA diversity found in people alive today.
“We can ask what kinds of models are really plausible for the African continent,” said Dr. Henn.
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; 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 to the genome of a British person. They also studied the genome of a 50,000-year-old Neanderthal found in Croatia. Previous research has found that modern humans and Neanderthals share a common ancestor that lived 600,000 years ago. Neanderthals spread throughout Europe and Asia, intermarried with modern humans coming from Africa, and then became extinct about 40 thousand years ago.
The researchers concluded that a million years ago, our ancestors lived in two different groups. Dr.. Hinn and her colleagues call them Stem1 and Stem2.
About 600,000 years ago, a small group of humans exited Stem1 and went on to become Neanderthals. But Stem1 suffered in Africa for hundreds of thousands of years after that, as did Stem2.
If Stem1 and Stem2 had been completely separated 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 remained somewhat different — and just as different from living Europeans and West Africans today. Scientists have concluded that people have switched between Stem1 and Stem2, setting out to have children and shuffling their DNA.
The model does not reveal where the Stem1 and Stem2 peoples lived in Africa. It is possible that groups of these two groups moved a lot over the vast periods of time that they were present on the continent. The model indicates that African history changed dramatically around 120,000 years ago.
In South Africa, people from Stem1 and Stem2 merged, creating a new lineage that would give rise to the Nama and other living humans in that region. Elsewhere in Africa, separate fusions of the Stem1 and Stem2 groups have occurred. This fusion 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 likely that climatic perturbations drove Stem1 and Stem2 individuals into the same regions, leading them to coalesce into single groups. Some groups of hunter-gatherers may have had to withdraw from the coast as sea levels rose, for example. Some regions of Africa have become barren, which can send people in search of new homes.
Even after these mergers 120,000 years ago, people of either Stem1-only or Stem2-only ancestry seem to have survived. The DNA of the Mende people showed that their ancestors interbred with Stem2 individuals only 25,000 years ago. “This suggests to me that Stem2 was somewhere around 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 influence the models.
They will likely discover other populations that lived in Africa for hundreds of thousands of years, eventually helping to produce our species as we know it today.
Dr. Sukkari speculated that living in a network of mixed populations across Africa might have allowed modern humans to survive while Neanderthals died out. In this arrangement, our ancestors could hold on to more genetic diversity, which in turn might have helped them withstand shifts in climate, or even develop new adaptations.
“This diversity in the roots of our species may ultimately be the key to our success,” said Dr. Skerry.