The ‘Out of Africa’ Story of Human Migration Is Undergoing Major Revision

Artist's depiction of early humans migrating out of Africa

According to the "Out of Africa" model of human migration that is commonly included in textbooks the world over, some anatomically modern humans from Africa migrated in a single, rapid wave across Europe and Asia around 60,000 years ago. The model often holds that once these people were out of Africa, a brief period of interbreeding with Neanderthals occurred, helping to explain why individuals of European and Asian heritage today retain archaic human DNA.

Technological advances in the fields of genetics and archaeology over the past decade, however, are revising the story. The emerging new model, outlined in the journal Science by Petraglia and his team, shows that there were multiple dispersals of modern humans out of Africa, beginning at least 120,000 years ago.

Petraglia explained that modern human fossils dating to between 120,000–70,000 years ago have been unearthed in the Levant: the region that now includes Israel, Lebanon, western Jordan, the Sinai in Egypt, and part of Syria. Co-author Katerina Douka, also from Max Planck, said that the Levant "forms a biogeographical extension of Africa."

Aside from being a convenient place for many Africans to go, the Levant also had plenty of lakes, rivers, grasslands, and savannahs, depending on the year's climate. "This would have drawn in both animals and the hunter-gatherers that tracked them," explained Petraglia. "When the environments worsened, these regions would have become hyper-arid deserts, thus pushing people on, perhaps."

Once the individuals originating from Africa were in Europe and Asia, they encountered the Neanderthals, Denisovans, and possibly other hominid groups that were already in these areas. Anthropologists use the term "archaic" to describe such hominid groups, but this is not in the colloquial lower-level sense.

"Most researchers studying late human evolution will use the term to refer to older lineages, not directly linked to modern lineages, rather than (meaning) less evolved and adapted, or less clever," Douka said. "Neanderthals and Denisovans share a common ancestor dating to 400,000–450,000 years ago, whereas archaic and modern humans share a common ancestor dating to 520,000–650,000 years ago."

Map of sites and postulated migratory pathways associated with modern humans dispersing across Asia during the Late Pleistocene (about 126,000–5,000 years ago)

Despite such evolutionary differences, the various human groups mated with each other — possibly a lot.

Recent genetic research shows that there were multiple interbreeding events. Melanesians today, as a result, are part Denisovan, and non-Africans carry at least 1-4 percent Neanderthal DNA in their genomes. There is even DNA evidence suggesting that another, as of yet unknown, hominid contributed to the present European and Asian gene pool.

Mating between different groups was not limited to Eurasia.

"Interbreeding among various archaic Homo sapiens populations surely occurred in Africa, particularly given that Africa is considered to be the place of origin for modern humans to have arisen," co-author Christopher Bae of the University of Hawaii at Manoa told Seeker.

"This greater genetic diversity in Africa has long been considered part of the justification to consider Africa as having a longer history for modern humans than Asia," Bae said. "On a related note, though, researchers have tied higher population density in Africa as also another explanation for the higher genetic diversity on the continent."

While questions related to the when and where of humanity's origins remain hotly debated, one matter about our collective genetic makeup is clearer: All humans appear to be hominid hybrids, made up of DNA from different and distinct populations.

Adding to the complexity, Petraglia said that in the past, "there were likely many population turnovers, and probably even extinctions of human lineages that left no DNA trace in current peoples."

New research is also revising the history of Neanderthals, whose expansion across multiple regions was much greater than previously believed.

"Neanderthals were not a 'European' population like people thought until about 10 years ago,” Douka explained, “but their range was far greater, and who knows, maybe much greater than we currently think, too."

Skulls of an anatomically modern human (left) and a Neanderthal (right)

Douka mentioned recent evidence suggesting that Neanderthals once lived in China as well as in North Africa. The latter could help to explain why many Europeans who have their DNA analyzed, such as through popular kits sold online, find they possess North African DNA.

There is also no reason to assume that groups stayed put once they migrated to or from Africa. Early modern humans and other hominids could have frequently traveled back and forth, especially during favorable climatic periods.

While out-of-Africa migrations were definitely happening long before 60,000 years ago, there is evidence that such travels escalated beginning around 55,000 years ago and over the following 10,000 years.

"We see modern humans arriving in Siberia certainly by 45,000 years ago, in Europe around the same time, in southern Asia possibly a bit earlier and they are certainly present in Australia by then," Douka said.

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