Human migration from Africa populated the world
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
on Wednesday, 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
Indians, to name just a few.
The DNA of indigenous populations is essential to understanding human history,
many geneticists believe. Yet until now scientists have sequenced entire genomes
from very few people outside population centers like Europe and China.
The new data already are altering scientific understanding of what human DNA
looks like, experts said, adding rich variations to our map of the genome.
Each team of researchers tackled different questions about our origins, such as
how people spread across Africa and how others populated Australia. But all
aimed to settle the controversial question of human expansion from Africa.
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 archaeological 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, 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 Israel, for example, researchers found a few distinctively modern human
skeletons that are between 120,000 and 90,000 years old. In Saudi Arabia and
India, sophisticated tools date back as far as 100,000 years.Last October,
Chinese scientists reported finding teeth belonging to Homo sapiens that are at
least 80,000 years old and perhaps as old as 120,000 years.
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.
Dr. 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, Dr. 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, who was already meeting with
Aboriginal communities about participating in this kind of research.
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 Dr. Willerslev’s 2011 study.
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, too, sequenced 483 genomes at high resolution.
David Reich, a geneticist at Harvard Medical School, and his colleagues
assembled a third database of genomes from all six inhabited continents. The
Simons Genome Diversity Project, sponsored by the Simons Foundation and the
National Science Foundation, contains 300 high-quality genomes from 142
populations.
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.
Despite earlier research, the teams led by Dr. Willerslev and Dr. Reich found no
genetic evidence that there was an earlier migration giving rise to people in
Australia and Papua New Guinea.
“The vast majority of their ancestry — if not all of it — is coming from the
same out-of-Africa wave as Europeans and Asians,” said Dr. Willerslev.
But on that question, Dr. Metspalu and his colleagues ended up with a somewhat
different result.
In Papua New Guinea, Dr. Metspalu and his colleagues found, 98 percent of each
person’s DNA can be traced to that single migration from Africa. But the other 2
percent seemed to be much older. Dr. Metspalu concluded that all people in Papua
New Guinea carry a trace of DNA from an earlier wave of Africans who left the
continent as long as 140,000 years ago, and then vanished.
If they did exist, these early human pioneers were able to survive for tens of
thousands of years, said Luca Pagani, a co-author of Dr. Metspalu at the
University of Cambridge and the Estonian Biocentre.
But when the last wave came out of Africa, descendants of the first wave
disappeared.
“They may have not been technologically advanced, living in small groups,” Dr.
Pagani said. “Maybe it was easy for a major later wave that was more successful
to wipe them out.”
-New York Times
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