DNA reveals one human family
All non-Africans related to group who left continent up to 80,000 years ago
Modern humans evolved in Africa roughly 200,000 years ago. But how did our species go on to populate the rest of the globe?
The question, one of the biggest in studies of human evolution, has intrigued scientists for decades. In a series of extraordinary genetic analyses published this week, researchers believe they have found an answer.
In the journal Nature, three separate teams of geneticists survey DNA collected from cultures around the globe, many for the first time, and conclude that all non-Africans today trace their ancestry to a single population emerging from Africa between 50,000 and 80,000 years ago.
“I think all three studies are basically saying the same thing,” said Joshua M. Akey of the University of Washington, who wrote a commentary accompanying the new work. “We know there were multiple dispersals out of Africa, but we can trace our ancestry back to a single one.”
The three teams sequenced the genomes of 787 people, obtaining highly detailed scans of each. The genomes were drawn from people in hundreds of indigenous populations: Basques, African pygmies, Mayans, Bedouins, Sherpas and Cree native Americans, to name just a few.
In the 1980s, a group of paleoanthropologists and geneticists began championing a hypothesis that modern humans emerged only once from Africa, roughly 50,000 years ago. Skeletons and tools discovered at archeological sites clearly indicated that modern humans lived after that time in Europe, Asia and Australia.
Early studies of bits of DNA also supported this idea. All non-Africans are closely related to one another, the geneticists found, and they all branch from a family tree rooted in Africa.
Yet there are also clues that at least some modern humans may have departed Africa well before 50,000 years ago, perhaps part of an earlier wave of migration.
In 2011, Eske Willerslev, a renowned geneticist at the University of Copenhagen, and his colleagues came across some puzzling clues to the expansion out of Africa by sequencing the genome of an Aboriginal Australian for the first time.
Willerslev and his colleagues reconstructed the genome from a century-old lock of hair kept in a museum. The DNA held a number of peculiar variants not found in Europeans or Asians, raising knotty questions about the origins of the people who first came to Australia and when they arrived.
Intrigued, Willerslev decided to contact living Aboriginals to see if they would participate in a new genetic study. He joined David W. Lambert, a geneticist at Griffith University in Australia.
In collaboration with scientists at the University of Oxford, the researchers also obtained DNA from people in Papua New Guinea. All told, the team was able to sequence 83 genomes from Aboriginal Australians and 25 from people in Papua New Guinea, all with far greater accuracy than in Willerslev’s 2011study.
Meanwhile, Mait Metspalu of the Estonian Biocentre was leading a team of 98 scientists on another genome-gathering project. They picked out 148 populations to sample, mostly in Europe and Asia, with a few genomes from Africa and Australia. They sequenced 483 genomes at high resolution.
David Reich, a Harvard geneticist, and his colleagues assembled a third database of genomes from all six inhabited continents.
Examining their data separately, all three groups came to the same conclusion: people everywhere descend from a single migration of early humans from Africa. The estimates from the studies point to an exodus somewhere between 80,000 and 50,000 years ago.
The new research also suggests that the splintering of the human tree began earlier than experts had suspected. Reich and his colleagues probed their data for the oldest evidence of human groups genetically separating from one another.
They found that the ancestors of the Khoisan, hunter-gatherers living today in southern Africa, began to split off from other living humans about 200,000 years ago. That finding hints that our ancestors already had evolved behaviours seen in living humans, such as language, by that time.