Almost All Living People Outside Of Africa Trace Back To A Single Migration More Than 50,000 Years Ago

Australian Aborigines have long been cast as a people apart. Although Australia is halfway around the world from our species’s accepted birthplace in Africa, the continent is nevertheless home to some of the earliest undisputed signs of modern humans outside Africa, and Aborigines have unique languages and cultural adaptations. Some researchers have posited that the ancestors of the Aborigines were the first modern humans to surge out of Africa, spreading swiftly eastward along the coasts of southern Asia thousands of years before a second wave of migrants populated Eurasia.

Not so, according to a trio of genomic studies, the first to analyze many full genomes from Australia and New Guinea. They conclude that, like most other living Eurasians, Aborigines descend from a single group of modern humans who swept out of Africa 50,000 to 60,000 years ago and then spread in different directions. The papers “are really important,” says population geneticist Joshua Akey of the University of Washington, Seattle, offering powerful testimony that “the vast majority of non-Africans [alive today] trace their ancestry back to a single out-of-Africa event.”

Yet the case isn’t closed. One study argues that an earlier wave of modern humans contributed traces to the genomes of living people from Papua New Guinea. And perhaps both sides are right, says archaeologist Michael Petraglia of the Max Planck Institute for the Science of Human History in Jena, Germany, a co-author on that paper who has long argued for an early expansion out of Africa. “We’re converging on a model where later dispersals swamped the earlier ones,” he says.

A decade ago, some researchers proposed the controversial idea that an early wave of modern humans left Africa more than 60,000 years ago via a so-called coastal or southern route. These people would have launched their migration from Ethiopia, crossing the Red Sea at its narrowest point to the Arabian Peninsula, then rapidly pushing east along the south Asian coastline all the way to Australia. Some genetic studies, many on mitochondrial DNA of living people, supported this picture by indicating a relatively early split between Aborigines and other non-Africans. But analysis of whole genomes— the gold standard for population studies— was scanty for many key parts of the world.

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Europeans Returned to Africa 3,000 Years Ago

“DNA sequences in the Khoisan people most closely resemble some found in people who today live in southern Europe.”

Call it humanity’s unexpected U-turn. One of the biggest events in the history of our species is the exodus out of Africa some 65,000 years ago, the start of Homo sapiens’ long march across the world. Now a study of southern African genes shows that, unexpectedly, another migration took western Eurasian DNA back to the very southern tip of the continent 3000 years ago.

According to conventional thinking, the Khoisan tribes of southern Africa, have lived in near-isolation from the rest of humanity for thousands of years. In fact, the study shows that some of their DNA matches most closely people from modern-day southern Europe, including Spain and Italy.

Because Eurasian people also carry traces of Neanderthal DNA, the finding also shows – for the first time – that genetic material from our extinct cousin may be widespread in African populations.

The Khoisan tribes of southern Africa are hunter-gatherers and pastoralists who speak unique click languages. Their extraordinarily diverse gene pool split from everyone else’s before the African exodus.

Ancient lineages

“These are very special, isolated populations, carrying what are probably the most ancient lineages in human populations today,” says David Reich of Harvard University. “For a lot of our genetic studies we had treated them as groups that had split from all other present-day humans before they had split from each other.”

So he and his colleagues were not expecting to find signs of western Eurasian genes in 32 individuals belonging to a variety of Khoisan tribes. “I think we were shocked,” says Reich.

The unexpected snippets of DNA most resembled sequences from southern Europeans, including Sardinians, Italians and people from the Basque region (see “Back to Africa – but from where?”). Dating methods suggested they made their way into the Khoisan DNA sometime between 900 and 1800 years ago – well before known European contact with southern Africa (see map).

Archaeological and linguistic studies of the region can make sense of the discovery. They suggest that a subset of the Khoisan, known as the Khoe-Kwadi speakers, arrived in southern Africa from east Africa around 2200 years ago. Khoe-Kwadi speakers were – and remain – pastoralists who make their living from herding cows and sheep. The suggestion is that they introduced herding to a region that was otherwise dominated by hunter-gatherers.

Khoe-Kwadi tribes

Reich and his team found that the proportion of Eurasian DNA was highest in Khoe-Kwadi tribes, who have up to 14 per cent of western Eurasian ancestry. What is more, when they looked at the east African tribes from which the Khoe-Kwadi descended, they found a much stronger proportion of Eurasian DNA – up to 50 per cent.

That result confirms a 2012 study by Luca Pagani of the Wellcome Trust Sanger Institute in Hinxton, UK, which found non-African genes in people living in Ethiopia. Both the 2012 study and this week’s new results show that the Eurasian genes made their way into east African genomes around 3000 years ago. About a millennium later, the ancestors of the Khoe-Kwadi headed south, carrying a weaker signal of the Eurasian DNA into southern Africa.

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• Ethiopians and Khoisan Share the Deepest Clades of the Human Y-Chromosome Phylogeny
• Paleoanthropologist John Hawks’ recent travels in Ethiopia

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Hitchhiking Virus Confirms Saga of Ancient Human Migration

A study of the full genetic code of a common human virus offers a dramatic confirmation of the “out-of-Africa” pattern of human migration, which had previously been documented by anthropologists and studies of the human genome.

The virus under study, herpes simplex virus type 1 (HSV-1), usually causes nothing more severe than cold sores around the mouth, says Curtis Brandt, a professor of medical microbiology and ophthalmology at UW-Madison. Brandt is senior author of the study, now online in the journal PLOS ONE.

When Brandt and co-authors Aaron Kolb and Cécile Ané compared 31 strains of HSV-1 collected in North America, Europe, Africa and Asia, “the result was fairly stunning,” says Brandt.

“The viral strains sort exactly as you would predict based on sequencing of human genomes. We found that all of the African isolates cluster together, all the virus from the Far East, Korea, Japan, China clustered together, all the viruses in Europe and America, with one exception, clustered together,” he says.

“What we found follows exactly what the anthropologists have told us, and the molecular geneticists who have analyzed the human genome have told us, about where humans originated and how they spread across the planet,” said Curtis Brandt

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