Charles Q. Choi, Live Science, April 21, 2014

Modern humans may have dispersed in more than one wave of migration out of Africa, and they may have done so earlier than scientists had long thought, researchers now say.

Modern humans first arose between 100,000 and 200,000 years ago in Africa. But when and how the modern human lineage then dispersed out of Africa has long been controversial.

Scientists have suggested the exodus from Africa started between 40,000 and 70,000 years ago. However, stone artifacts dating to at least 100,000 years ago that were recently uncovered in the Arabian Desert suggested that modern humans might have begun their march across the globe earlier than once suspected.

Out of Africa models

To help solve this mystery, Katerina Harvati, a paleoanthropologist at the University of Tübingen in Germany, and her colleagues tested four competing out-of-Africa models. Two models involved a single dispersal–one involved a route northward, up the Nile River valley and then eastward across the northern end of the Arabian Peninsula into Asia; the other involved a “beachcomber” route along the southern coast of the Arabian Peninsula into Asia. Two other models involved multiple dispersals, with both models involving routes along the northern and southern ends of the Arabian Peninsula–one involved connections and gene flow between these routes, and the other did not.

The investigators used these models to predict how much the genes and skull measurements of different groups in Africa, Asia and Australia might have diverged from one another given how separated they were by space and time. Then, the researchers compared these predictions with actual gene and skull data from 10 African, Asian and Australian human populations.

The researchers found that both the genetic and skull data supported a multiple-dispersal model involving several migrations.

“It is really exciting that our results point to the possibility of a multiple-dispersals model of modern humans out of Africa,” Harvati said. “A multiple-dispersals scenario, with earlier modern humans leaving Africa as early as 130,000 before present, can perhaps account for part of the morphological and genetic patterns that we see among modern human populations.”

The first wave of migrations probably followed the southern coast of the Arabian Peninsula as early as 130,000 years ago to Australia and the west Pacific region, while the second wave traveled along the northern route about 50,000 years ago, the researchers said. These waves of migration appear relatively isolated from each other.

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