Humans Migrated Out of Africa, Then Some Went Back, Study Says

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Other models have suggested that humans left Africa in multiple waves of migration via northern and southern routes.

The single "out of Africa" dispersal is believed to have given rise to all modern non-African populations.

However, scientists have been puzzled by two genetic populations found only in northern and eastern Africa, whose ancestors appear to have been Asian.

(Get the basics on human population genetics.)

In the new study, scientists sequenced the mitochondrial DNA, which is passed from mother to daughter, from 81 individuals in both of these genetic groups.

They found that the two populations must have arisen in southwestern Asia and returned to Africa about 40,000 to 45,000 years ago.

The groups did not, however, follow the same southern coastal route back that was used in the single dispersal out of Africa.

Instead, the study suggests, they arrived from the Middle East, the same area from which another genetic group—one typical among Europeans—was at the same time moving toward Europe.

"It's a finding that … supports the view that the first [Late Stone Age] cultures in North Africa and Europe had a common homeland in the Levant," Torroni said.

Vincent Macaulay, the lead author on one of the two single-dispersion studies published in Science last year, agrees with the findings.

"These results make perfect sense and wrap up some loose ends," said Macaulay, a genetic statistician at the University of Glasgow in Scotland.

Changing Climate

The authors of the new study believe that before reaching the Levant, migrating humans may have paused at the Persian Gulf for some time because of a hostile climate.

Environmental evidence suggests that migrating north from southwestern Asia would have been impossible earlier than 50,000 years ago because of a vast desert that extended from northern Africa to central Asia.

"When weather conditions improved, the desert was fragmented and reduced in size," said Anna Olivieri, a geneticist in Torroni's lab and a co-author of the study.

"The human groups living in the coastal regions of southwestern Asia were able to move inland."

"Some of them colonized first the Levant and from there all surrounding regions including Europe and North Africa," she said.

"Consider also that the Sahara desert in North Africa was reducing its size. Thus, that region became interesting from a human colonization perspective."

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