How We Got Here: DNA Points to a Single Migration From Africa
Modern humans evolved somewhere in Africa roughly 200,000 years ago. But how did our species go on to populate the rest of the globe?
Did humans flood out of Africa in a single diaspora, or did we trickle from the continent in waves spread out over tens of thousands of years? The question, one of the biggest in human evolution, has plagued scientists for decades.
Now they may have found an answer.
In a series of unprecedented genetic analyses published on Wednesday in the journal Nature, three separate teams of researchers conclude that all non-Africans today trace their ancestry to a single population emerging from Africa between 50,000 and 80,000 years ago.
“I think all three studies are basically saying the same thing,” said Joshua M. Akey of the University of Washington, who wrote a commentary accompanying the new work. “We know there were multiple dispersals out of Africa, but we can trace our ancestry back to a single one.”
“我觉得三组研究基本上说的都是同一件事，”华盛顿大学的约书亚·M·阿基(Joshua M. Akey)说，他为这个新研究撰写了一篇解释性文章。“我们知道，非洲有多次向外界传播人口，但现在我们可以追溯自己的祖先，把他们归结到同一个来源。”
The three teams sequenced the genomes of 787 people, obtaining highly detailed scans of each. The genomes were drawn from people in hundreds of indigenous populations around the world — Basques, African pygmies, Mayans, Bedouins, Sherpas and Cree Indians, to name just a few.
The DNA of older indigenous populations may be essential to understanding human history, many geneticists believe. Yet until now scientists have sequenced few whole genomes from people outside population centers like Europe and China. The new findings already are altering scientific understanding of what human DNA looks like, experts said, adding a rich diversity of variation to our map of the genome.
Each team of researchers used sets of genomes to tackle different questions about our origins, such as how people spread across Africa and how others populated Australia. But all aimed to settle the question of human expansion from Africa.
In the 1980s, a group of paleoanthropologists and geneticists began championing a hypothesis that modern humans emerged only once from Africa, roughly 50,000 years ago. Skeletons and tools discovered at archaeological sites clearly indicated the existence of modern humans in Europe, Asia and Australia.
Early studies of bits of DNA also supported this scenario. All non-Africans are closely related to one another, the studies found, and they all branch from a genetic tree rooted in Africa.
Yet there are also clues that at least some modern humans lived outside Africa well before 50,000 years ago, perhaps part of an earlier wave of migration.
In 2011 Eske Willerslev, a renowned geneticist at the University of Copenhagen, and his colleagues reported evidence that some living people descended from this early wave.
Willerslev and his colleagues reconstructed the genome of an aboriginal Australian from a century-old lock of hair kept in a museum — the first reconstruction of its kind. The DNA held a number of peculiar variants not found in Europeans or Asians.
He concluded that the ancestors of Aboriginals spit off from other non-Africans and moved eastward, eventually arriving in East Asia 62,000-75,000 years ago. Tens of thousands of years later, a separate population of Africans spread into Europe and Asia.
It was big conclusion to draw from a single fragile genome, so Willerslev decided to contact living Aboriginals to see if they would participate in a new genetic study. He joined David W. Lambert, a geneticist at Griffith University in Australia, who was already meeting with aboriginal communities about beginning such a study.
仅凭一份脆弱的基因序列很难得出这样重大的结论，所以韦勒斯勒夫决定联络尚健在的澳大利亚土著人，看他们是否愿意参与新的基因研究。澳大利亚格里菲斯大学的遗传学家大卫·W·兰伯特(David W. Lambert)也加入进来，他已经和若干土著人社区会面，商讨进行类似研究。
Their new paper also includes DNA from people in Papua New Guinea, thanks to a collaboration with scientists at the University of Oxford. All told, the scientists were able to sequence 83 genomes from aboriginal Australians and 25 from people in Papua New Guinea, all with far greater accuracy than in Willerslev’s 2011 study.
Meanwhile, Mait Metspalu of the Estonian Biocentre was leading a team of 98 scientists on another genome-gathering project. They picked out 148 populations to sample, mostly in Europe and Asia, with a few genomes from Africa and Australia. They sequenced 483 genomes at high resolution.
与此同时，爱沙尼亚生物中心(Estonian Biocentre)的迈特·麦特斯帕卢(Mait Metspalu)领导着一个由98名科学家组成的团队，进行另一项基因组收集工作。他们找来了148个族群作为样本，大都来自欧洲和亚洲，也有一些基因组来自非洲和澳洲。他们以高解析度为483个基因组做了基因测序。
David Reich, a geneticist at Harvard Medical School, and his colleagues assembled a third database of genomes from all five continents. The Simons Genome Diversity Project, sponsored by the Simons Foundation and the National Science Foundation, contains 300 high-quality genomes from 142 populations.
哈佛医学院遗传学家戴维·赖希(David Reich)及其同事们从五大洲收集了第三个基因组数据库。西蒙斯基因组多样性计划(The Simons Genome Diversity Project)由西蒙斯基金会(Simons Foundation)与国家科学基金会(National Science Foundation)赞助，它从142个族群中获取了300个高质量的基因组。
Reich and his colleagues probed their data for the oldest evidence of human groups genetically separating from one another. They found that the ancestors of the KhoiSan, hunter-gatherers living today in southern Africa, began to split off from other living humans about 200,000 years ago and were fully isolated by 100,000 years ago.
Earlier studies had estimated that the split between living groups of humans occurred much more recently. The new findings indicate that our ancestors already had evolved behaviors seen in living humans, such as language, 200,000 years ago.
Metspalu and his colleagues ended up with a somewhat different result when they looked at the Estonian Biocentre data. They compared chunks of DNA from different genomes to see how long ago people inherited them from a common ancestor.
Almost all the DNA from non-Africans today could be traced back to one population that lived about 75,000 years ago — presumably a group of Africans who eventually left the continent and settled the rest of the world. That squares with the conclusions of the other two studies.
But in Papua New Guinea, Metspalu and his colleagues found, the story was a little different. They could trace 98 percent of each person’s DNA to that 75,000-year-old group. But the other 2 percent was much older.
Some people in Papua New Guinea — but no one else in the analyses — may carry a trace of DNA from a much older wave of Africans who left the continent as long as 140,000 years ago, and then vanished.
The second wave — the one from which the rest of the world descends — departed over 60,000 years later, the researchers suggest. The ancestors of the people of Papua New Guinea interbred with those first pioneers on their way east, which is why their descendants carry remarkable DNA.
Why leave Africa at all? Scientists have found some clues as to that mystery, too.
In a fourth paper in Nature, researchers described a computer model of Earth’s recent climatic and ecological history. It shows that changing rainfall patterns periodically opened up corridors from Africa into Eurasia that humans may have followed in search of food.