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We are all Africans: DNA analysis points to a single major exodus

If you go back far enough, the ancestors of all people traced to Africa. That’s clear. But there has been considerable room for debate about exactly when and how often modern humans made their way out of Africa to take up residence in distant places worldwide. Nor is it clear what developments or other factors could have led our human ancestors for a journey so dangerous and uncertain (or trips) in the first place.

By analyzing 787 complete human genomes newly sequenced representing more than 280 diverse populations and little studied, three new studies two of which received funding from the NIH-now help fill some of those missing pages of our history evolutionary. Evidence suggests that genomics the first human inhabitants of Eurasia came from Africa and began to diverge genetically at least 50,000 years ago. While new studies differ somewhat in their conclusions, the results also support the idea that our modern human ancestors out of Africa dispersed mainly in a single migration event. If a previous trip and not ultimately occurs, he left few traces in the genomes of people alive today.

In one of three studies published in Nature [1] NIH dealer David Reich of the School of Medicine at Harvard, Boston, and colleagues report the genome sequences 300 people from 142 different populations around the world. DNA samples, part Simons Genome Diversity Project, were carefully chosen to cover most of the variation in human genetics, language and culture. In a stroke of good timing, David spoke at the NIH last week, so I had the opportunity to give a consultation on these results.

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Reanalysis of this diverse set of DNA data suggests that the ancestors of all humans today have slowly begun to split into different populations as long 200,000 years ago. By 100,000 years ago, the ancestors of some modern human populations, including several populations of hunter-gatherers in Africa, and showed substantial differences at the genetic level. That’s long before archaeological evidence shows signs of modern human behavior, including the use of more complex tools and ornaments such as necklaces.

As people outside Africa, researchers estimate that the oldest genetic differences date back about 50,000 years. That is consistent with previous evidence that modern humans made their way from Africa to Eurasia at that time.

The three studies weigh on the long debate among experts on Africa if the ancestors of the Aborigines in Australia and New Guinea Papuans could have gone around 120,000 or 130,000 years ago, long before the event that led to the first people Eurasia. Reich and colleagues report no evidence in the sequence data for a dispersion of modern humans such principles.

This is the general conclusion, also a second report Nature led by Eske Willerslev of the University of Copenhagen in Denmark [2]. Willersley and colleagues generated high quality genomes for Aboriginal Papuans 83 and 25 individual covering most of the Australian continent and the highlands of New Guinea. It is estimated that the Aborigines and the Papuans have been separated for the past 37,000 years, long before physical division of their ancient continent known as Sahul shared. Researchers also the Australian Aborigines and New Guinea Papuans Eurasians diverged around 58,000 years ago, concluded the consistent with the hypothesis of a single trip outside Africa.

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In the third Nature study [3] Luca Pagani and Mait Metspalu Estonian Biocentro in Tartu and colleagues analyzed 483 human genomes (including 379 sequences obtained recently genome). Collectively, the 125 genomes represented populations around the world.

propose a slightly different model, including echoes of a previous migration potential. While his analyzes suggest that the genomes of modern Papuans primarily derived from a single migration of humans out of Africa, report traces of DNA representing at least 2 percent of the genome of Papua reflecting perhaps before and otherwise extinct outside the African migration. These results lead to suggest that modern humans living outside Africa over 75,000 years ago may have contributed to some non-African descent living today.

As to what might have led our ancestors to make such an uncertain journey all those years ago, of course we will never have the full story. But Reich and his colleagues say they see no evidence that one or a few large genetic mutations have no change in human traits and behavior from the moment of that fateful departure. Instead, they argue that it is more likely that cultural and environmental changes exert a selective influence on mating and survival, and therefore were more likely to be the forces ultimate driving stimulating the transformations made Homo sapiens what we are today.

Courtesy NIH, USA

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