http://www.latimes.com/science/sciencenow/la-sci-sn-ancient-ethiopian-dna-eurasia-20151008-story.html
DNA from 4,500-year-old Ethiopian reveals surprise about ancestry of Africans
Mota cave
Kathryn and John Arthur
Mota cave in Ethiopia, where researchers found the body of a
4,500-year-old man whose DNA was still preserved.
(Kathryn and John Arthur)
Karen KaplanContact Reporter
DNA from a man who lived in Ethiopia about 4,500 years ago is
prompting scientists to rethink the history of human migration in
Africa.
Until now, the conventional wisdom had been that the first groups of
modern humans left Africa roughly 70,000 years ago, stopping in the
Middle East en route to Europe, Asia and beyond. Then about 3,000
years ago, a group of farmers from the Middle East and present-day
Turkey came back to the Horn of Africa (probably bringing crops like
wheat, barley and lentils with them).
Population geneticists pieced this story together by comparing the DNA
of distinct groups of people alive today. Since humans emerged in
Africa, DNA from an ancient Africa could provide a valuable genetic
baseline that would make it easier for scientists to track genome
changes over time.
Unfortunately, such DNA has been hard to come by. DNA isn’t built to
last for thousands of years. The samples of ancient DNA that have been
sequenced to date were extracted from bodies in Europe and Asia that
were naturally refrigerated in cooler climates.
That’s what makes the Ethiopian man so special. His body was found
face-down in Mota cave, which is situated in the highlands in the
southern part of the country. The cool, dry conditions in the cave
preserved his DNA, and scientists extracted a sample from the petrous
bone at the base of his skull. The resulting sequence is the first
nuclear genome from an ancient African, according to a report
published Thursday in the journal Science.
Radiocarbon dating revealed that the bone was 4,500 years old. That
meant Mota (as the researchers called him) lived before Eurasians
returned to the African continent.
Consistent with that timeline, Mota did not have any of the genetic
variants for light-colored eyes or skin that evolved in the
populations that left Africa. Nor did he have variants that arose in
Eurasian farmers that allowed them to digest milk as adults.
Mota did have three variants that are known to help modern-day
Ethiopians live in high altitudes. (The present-day town of Mota lies
more than 8,100 feet above sea level.)
When the researchers compared Mota’s genome to those of contemporary
humans, the closest match was with the Ari people of southern
Ethiopia.
With this information, the research team was able to investigate the
mysterious group of Eurasians that came to Africa 3,000 years ago.
They created a model that assumed the Ari genome was a mixture of DNA
from Mota and an unknown population from west Eurasia. Then they
“plugged in” DNA from several candidate populations to see if they
could get a combination that looked like Ari DNA.
Two results stood out from the rest. One was for modern-day
Sardinians, who are known to be the closest living relatives to the
earliest farmers. The other was for members of the so-called LBK
culture in Germany, early farmers who lived about 7,000 years ago.
If the Eurasian settlers who arrived in Africa 3,000 years ago were
indeed descendants of the LBK farmers, then the story of their
migration through Africa needs to be revised, the researchers wrote.
By comparing the LBK genome with DNA from Africans alive today, the
scientists calculated that these ancient farmers may have made up 25%
or more of the population in the Horn of Africa during the migration
years. All of those migrants ultimately pushed farther into Africa
than previously thought, they determined.
African populations from the western and southern tips of the
continent got at least 5% of their DNA from these Eurasian migrants,
according to the study. Some groups from Ethiopia, Somalia, Djibouti
and Eritrea can trace more than 30% of their DNA to these migrants.
“The ability to sequence ancient genomes has revolutionized our
understanding of human evolution,” wrote the research team, which was
led by Marcos Gallego Llorente of the University of Cambridge and
Eppie Ruth Jones of Trinity College Dublin. They said they are eager
to find “even older African genomes” that may make the story more
complete.
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Received on Sat Oct 10 2015 - 12:47:31 EDT