The New York Times
Scientists have uncovered a surprisingly complex origin of our species, rejecting the longheld argument that modern humans originated at a specific point in time in Africa.
By analyzing the genomes of 290 living humans, the researchers concluded that modern humans descended from at least two populations that coexisted in Africa for a million years before merging in several independent events across the continent. The results were published in the journal Nature on Wednesday (17).
“There isn’t a 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’s really a testament to that idea.”
Paleoanthropologists and geneticists have found evidence that suggests our species originated in Africa. The oldest fossils of modern humans have been unearthed there, dating back 300,000 years, as well as the oldest stone tools of our ancestors.
Human DNA also points to Africa. Compared to other populations, living Africans show great genetic diversity. That’s because humans lived and evolved in Africa for thousands of generations before small groups with comparatively small genetic pools began expanding to other continents.
Within the vast expanse of Africa, researchers have suggested several places as birthplaces for our species. The first humanoid fossils from Ethiopia have caused some researchers to look to East Africa. However, some groups of people living in South Africa appeared to be very distantly related to other Africans, suggesting that people there may have a long history.
Brenna Henn, a geneticist at the University of California, Davis, and her colleagues have developed software to run largescale simulations of human history. The researchers created many scenarios of different populations that existed in Africa at different times, and then examined which of these could potentially produce the DNA diversity found in populations alive today.
“One might wonder which 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 huntergatherers in Ethiopia; the Amhara, a group of Ethiopian farmers; and the Nama, huntergatherers 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 50,000yearold Neanderthal found in Croatia. Previous research has found that modern humans and Neanderthals shared a common ancestor that lived 600,000 years ago. Neanderthals spread across Europe and Asia, interbred with modern humans from Africa, and died out about 40,000 years ago.
The researchers concluded that the ancestors of our species already existed in two distinct populations a million years ago. Henn and his colleagues call them Stem1 and Stem2 (Branch1 and Branch2).
About 600,000 years ago, a small group of humans grew out of Tribe 1 and became Neanderthals. But tribe 1 survived in Africa for hundreds of thousands of years thereafter, as did tribe 2.
If Stem1 and Stem2 were completely separated from each other, they would have accumulated a large number of different mutations in their DNA. Instead, Henn and his colleagues found that they remained only moderately different as different as Europeans and West Africans living today are. The scientists concluded that humans moved between tribe 1 and tribe 2, mating to have children and mixing their DNA.
The model does not show where tribe1 and tribe2 populations lived in Africa. And it’s possible that bands from these two groups moved around a lot over the long periods they’ve existed on the continent. The model shows that African history changed dramatically around 120,000 years ago.
In southern Africa, the tribe 1 and tribe 2 peoples merged, creating a new lineage that gave rise to the Nama and other people living in that region. Separate fusion of the Stem1 and Stem2 groups occurred in other parts of Africa. This merger created a lineage that produced people who lived in West Africa and East Africa, as well as people who expanded out of Africa.
It is possible that climatic upheavals forced the Stem1 and Stem2 populations into the same regions, causing them to merge into unique groups. Some huntergatherer groups may have had to retreat from the coast when, for example, sea levels rose. Some regions of Africa have become dry, potentially sending people in search of new homes.
Even after these fusions 120,000 years ago, humans with only Tribe 1 or Tribe 2 ancestry seem to have survived. The DNA of the Mende people showed that their ancestors interbred with the Stamm2 people 25,000 years ago. “This suggests that Stem2 was located somewhere in West Africa,” Henn said.
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 have survived in Africa for hundreds of thousands of years and contributed to the emergence of our species as we know them today.
Scerri theorized that living in a network of mixed populations across Africa may have allowed modern humans to survive while Neanderthals died out. This allowed our ancestors to preserve greater genetic diversity, which in turn may have helped them survive climate change or even develop new adaptations.
“This diversity at the root of our species may have been key to our success,” Scerri said.
Translated by Luiz Roberto M. Gonçalves