A Single Migration From Africa Populated the World, Studies Find

Sep 24, 2016

By Carl Zimmer

Modern humans evolved in Africa roughly 200,000 years ago. But how did our species go on to populate the rest of the globe?

The question, one of the biggest in studies of human evolution, has intrigued scientists for decades. In a series of extraordinary genetic analyses published on Wednesday, researchers believe they have found an answer.

In the journal Nature, three separate teams of geneticists survey DNA collected from cultures around the globe, many for the first time, and conclude that all non-Africans today trace their ancestry to a single population emerging from Africa between 50,000 and 80,000 years ago.

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5 comments on “A Single Migration From Africa Populated the World, Studies Find

  • While there seems to be one predominant exodus from Africa, there are hints of others.


    Hints of an early exodus of modern humans from Africa may have been detected in living humans.
    People outside Africa overwhelmingly trace their descent to a group that left the continent 60,000 years ago.

    Now, analysis of nearly 500 human genomes appears to have turned up the weak signal of an earlier migration.

    But the results suggest this early wave of Homo sapiens all but vanished, so it does not drastically alter prevailing theories of our origins.

    Writing in Nature, Luca Pagani, Mait Metspalu and colleagues describe hints of this pioneer group in their analysis of DNA in people from the Oceanian nation of Papua New Guinea.

    After evolving in Africa 200,000 years ago, modern humans are thought to have crossed through Egypt into the Arabian Peninsula some 60,000 years ago.

    Until now, genetic evidence has shown that today’s non-Africans could trace their origins to this fateful dispersal.

    Yet we had known for some time that groups of modern humans made forays outside their “homeland” before 60,000 years ago.

    Fossilised remains found at the Qafzeh and Es Skhul caves in Israel had been dated to between 120,000 and 90,000 years ago.
    Then in 2015, scientists working in Daoxian, south China, reported the discovery of modern human teeth dating to at least 80,000 years ago.
    An additional piece of evidence recently came from traces of Homo sapiens DNA in a female Neanderthal from Siberia’s Altai mountains. The analysis suggested that modern humans and Neanderthals had begun mixing around 100,000 years ago – presumably outside Africa.

    In order to reconcile this evidence with the genetic data from living populations, the prevailing view advanced by scientists was of a wave of pioneer settlement that ended in extinction.

    But the latest results suggest some descendents of these trailblazers survived long enough to get swept up in the later, ultimately more successful migration that led to the settling of Oceania.

    “The first instance when we thought we were seeing something was when we used a technique called MSMC, which allows you to look at split times of populations,” said co-author Dr Mait Metspalu, director of the Estonian Biocentre in Tartu, told BBC News.

    His colleague and first author Dr Luca Pagani, also from the Estonian Biocentre, added: “All the other Eurasians we had were very homogenous in their split times from Africans.

    “This suggests most Eurasians diverged from Africans in a single event… about 75,000 years ago, while the [Papua New Guinea] split was more ancient – about 90,000 years ago. So we thought there must be something going on.”

    It was already known that Papua New Guineans, along with other populations from Oceania and Asia, derive a few per cent of their ancestry from Denisovans, an enigmatic sister group to the Neanderthals.

    The researchers tried to remove this component, but were left with a third chunk of the genome which was different from the Denisovan segment and the overwhelming majority which represents the main out of Africa migration 60,000 years ago.

    “This third component had intermediate properties which we concluded must have originated as an independent expansion out of Africa about 120,000 years ago,” Dr Pagani told BBC News.

    “We believe this makes up at least 2% of the genome of modern [Papua New Guineans].”

    In a separate paper in the same edition of Nature, Prof David Reich and Swapan Mallick, both from Harvard Medical School, along with colleagues analysed 300 genomes from 142 different populations around the world.

    They found evidence of early splits between populations within Africa, along with a single dispersal that gave rise to non-African populations.

    “I am a bit concerned that poorly modelled features of the methods used by Pagani and colleagues may have contributed to a false-positive signal of early dispersal ancestry in them. However, an alternative possibility is that the truth is around 2%, and this might just be consistent with all three studies.”

    Commenting on the Reich Lab study, Dr Metspalu told BBC News: “They do not detect an early Out of Africa, but they also do not reject it as long as it is just a few per cent in modern humans.”

    Dr Pagani added: “All three papers all reach the same conclusions. That in Eurasians and also [Papua New Guineans] – the majority of their genomes come from the same major migration.”

    Prof Chris Stringer, from London’s Natural History Museum, who was not involved with the genomic studies, commented: “The papers led by Mallick and by Malaspinas favour a single exit from Africa less than 80,000 years ago giving rise to all extant non-Africans, while that led by Pagani favours an additional and earlier exit more than 100,000 years ago, traces of which they claim can still be found in Australasians.

    “Unfortunately, the signs of past interbreeding with a Denisovan-like archaic population which are found at a level of about 4% in extant Australasians, according to the Malaspinas paper, complicate interpretations, as well as the possibility that there may have been yet other ancient interbreedings which are so far poorly understood.”

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  • I am amazed that no one seems to have picked up on the correlation between the 60 – 80k year estimate for the start of the main migration and the Toba event of +/- 72k years ago which came perilously close to wiping out the entire human race. Whether contemporous with the initial effect, or in the aftermath of 1000 years of drought thereafter and the lingering changes to the ecology of home ranges, would have been more than enough to prompt a desperate search for more productive territory. I’m sure when technology allows, it will be found that other hominid populations of the time also suffered the same catastrophic culling.

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  • There is also this population raising questions about earlier migrations!


    Since the initial find, bones and teeth representing as many as 12 H. floresiensis individuals have been recovered at Liang Bua—the only site where H. floresiensis has been found so far. The bulk of the finds related to H. floresiensis date between 100,000 and 60,000 years ago, with stone tools made by this species dating between 190,00 and 50,000 years ago.

    Stone tools found on the island of Flores show that early humans arrived there at least 1 million years ago, but it’s not known how early humans got there as the nearest island is 9 km (6 mi) away across treacherous seas. Paleoanthropologists found many stone tools associated with H. floresiensis, and these tools are broadly similar to those found earlier on Flores and throughout the human evolutionary career (i.e., Lower Paleolithic tools in Asia or Oldowan tools in Africa). There is also evidence that H. floresiensis selectively hunted Stegodon (an extinct type of elephant) as hundreds of Stegodon bone fragments are found within H. floresiensis occupation layers and some of these Stegodon bones show butchery marks.

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