The genetic makeup of hundreds of indigenous peoples has provided new clues to human origins, including the Khoisan, a hunter-gatherer tribe living in southern Africa. ERIC LAFORGUE/GAMMA-RAPHO, VIA GETTY IMAGES
Late Homo sapiens evolved somewhere in Africa about 200,000 years ago. But how did our species spread to the rest of the world?
Did humans pour out of Africa in a great migration, or did they leave the continent wave after wave over tens of thousands of years? This is one of the biggest problems in human evolution, and it has puzzled scientists for decades.
Now they may have found the answer.
On Wednesday, the journal Nature published an unprecedented series of genetic analyses, in which three different research teams concluded that the ancestors of all humans outside Africa can be traced back to a group of people who left Africa between 50,000 and 80,000 years ago.
Aubre Lynch, an elder from Australia's Wongatha indigenous community, participated in a genetic analysis study that may help shed light on ancient human migration. PREBEN HJORT/MAYDAY FILM
"I feel like all three sets of studies are basically saying the same thing," said Joshua M. of the University of Washington. Joshua M. Akey said he wrote an explanatory article for the new study. "We know that Africa has spread people to the outside world many times, but now we can trace our ancestors and attribute them to the same source."
The three teams sequenced the genomes of 787 people, obtaining extremely detailed scans of each person's genome. These genomes come from hundreds of indigenous groups around the world, including Basques, African pygmies, Mayans, Bedouins, Sherpas and Cree Indians, to name a few.
Many geneticists agree that the DNA of older indigenous populations may be more important for understanding human history. Only now, however, have scientists begun to sequence the complete genomes of a small number of people outside the core regions of the population, such as Europe and China. Experts say the new findings will change the scientific view of what human DNA looks like, adding richness to our genome map.
Researchers in each group used several genomes to study different questions about human origins, such as how humans traveled through Africa and how others got to Australia. But their aim is to solve the problem of human departing from Africa and expanding outward.
In the 80s of the 20th century, a group of paleoanthropologists and geneticists began to propose the hypothesis that late Homo sapiens left Africa in one fell swoop about 50,000 years ago. Human bones and tools found at archaeological sites clearly indicate the presence of late Homo sapiens in Europe, Asia and Australia.
Early DNA fragment studies also supported this idea. These studies show that all humans outside Africa are closely related to each other, and that they all belong to a clade of a gene tree rooted in Africa.
However, there is also evidence that at least some late Homo sapiens lived outside Africa 50,000 years ago, perhaps during an earlier wave of migration.
In 2011, Eske Willerslev, a prominent geneticist at the University of Copenhagen, and colleagues found evidence that some modern humans were descendants of this early wave of migration.
Using a century-old strand of hair preserved in a museum, Wellerslöf and his colleagues reconstructed the genome of an indigenous Australian — the first of its kind. DNA shows several special variants that are not found in Europeans and Asians.
He concluded that the ancestors of this Australian Aboriginal were from another group of people from outside Africa who migrated east, arriving in East Asia about 62,000 to 75,000 years ago. Tens of thousands of years later, another group of independent Africans expanded into Asia and Europe.
It was difficult to draw such a significant conclusion from a fragile genetic sequence alone, so Wellerslev decided to contact the surviving Indigenous Australians to see if they would be willing to participate in the new genetic research. David M. Geneticist, Griffith University in Australia, David W. Lambert joins him, and he has met with several indigenous communities to discuss similar research.
Their latest paper also includes DNA research from people in Papua New Guinea, a collaboration with scientists at the University of Oxford. In total, the scientists were able to sequence 83 genomes of Indigenous Australians and 25 genomes of people in Papua New Guinea, both of which were much more accurate than Wellerslev's 2011 study.
Meanwhile, Mait Metspalu of the Estonian Biocentre leads a team of 98 scientists on another genome collection effort. They found 148 ethnic groups as samples, mostly from Europe and Asia, but also some genomes from Africa and Australia. They sequenced 483 genomes at high resolution.
David Reich, a geneticist at Harvard Medical School, and his colleagues collected a third genomic database from five continents. The Simons Genome Diversity Project, sponsored by the Simons Foundation and the National Science Foundation, harvests 300 high-quality genomes from 142 populations.
To study the oldest evidence of genetic isolation in human populations, Reich and his colleagues studied the data. They found that the KhoiSan, a hunter-gatherer tribe that has lived in southern Africa, separated from other humans about 200,000 years ago and has been completely isolated from other populations since 100,000 years ago.
Early research suggested that the fragmentation of human living groups occurred at a more recent time. The new findings suggest that 200,000 years ago, our ancestors developed several behaviors that modern humans have, such as language.
After studying data from the Estonian Biocenter, Matspalu and his colleagues came up with a somewhat different result. They compared large amounts of DNA from different genomes to study how early humans inherited it from a common ancestor.
The DNA of almost all people outside Africa today can be traced back to a population 75,000 years ago — presumably a group of Africans who gradually left Africa and settled elsewhere in the world. This is consistent with the findings of the other two groups of scientists.
But in Papua New Guinea, Matspalu and his colleagues found that things were a little different. 98% of all people's DNA can be traced back to the group of people 75,000 years ago, but another 2% is even older.
Some Papua New Guineans — but none at all in the rest of the study — may have carried DNA from an earlier group of people who left Africa, probably 140,000 years ago and then disappeared.
Researchers believe that the second wave that left Africa — the one that produced the rest of the world's population — occurred about 60,000 years later. The ancestors of the Papua New Guineans crossed with the pioneers of this group of people who migrated east, so their descendants carry unique DNA.
Why on earth did these people leave Africa? Scientists have also found some clues to unravel the mystery.
In a fourth paper in the journal Nature, the researchers describe a computer model of Earth's recent climate and ecological history. It suggests that periodic changes in rainfall patterns opened access to Eurasia for Africans, along which they migrated in search of food