Humans reached Asia in two waves

Some early migrants interbred with mysterious Neandertal sister group

DNA extracted from a 40,000-year-old pinky bone and a 100-year-old lock of hair has provided glimpses of two Stone Age human migrations to Asia, including an early foray marked by interbreeding between ancient people and some mysterious, well-traveled members of the human evolutionary family.

Denisovans, an ancient humanlike population previously identified via nuclear DNA taken from a finger bone excavated in Siberia’s Denisova Cave, contributed a small portion of genes to living New Guineans, Australian aborigines, two aboriginal groups in the Philippines and populations on several nearby islands, say geneticist David Reich of Harvard Medical School in Boston and his colleagues.

Earlier analyses of modern human mitochondrial DNA, which is maternally inherited, had suggested that a single wave of humans took a southern coastal route from Africa to Asia around 65,000 years ago. Patterns of nuclear DNA alterations in an ancient Denisovan and in living groups instead point to at least two Stone Age human migrations into Asia, Reich’s team reports in a paper published online September 22 in the American Journal of Human Genetics.

A one-two punch of human migrations into Asia, including early interbreeding with the mysterious Denisovans, also emerges from an inspection of an Australian aboriginal man’s DNA, led by geneticist Morten Rasmussen of the Natural History Museum of Denmark in Copenhagen. Those results appear in a paper published online September 22 in Science.

Both new reports advance the idea that, after leaving Africa, modern humans interacted with a greater number of humanlike groups than are documented in the fossil record, remarks anthropologist John Hawks of the University of Wisconsin–Madison. Now that scientists can isolate ancient DNA from nondescript bits of bone, “we’re discovering lost peoples whose existence we never suspected,” Hawks says.

Several Chinese sites have yielded hominid fossils that may come from Denisovans, says anthropologist Jean-Jacques Hublin of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. These fossils date to between 200,000 and 100,000 years ago and don’t appear to him to belong to Homo sapiens or Homo erectus, a possible direct ancestor of modern humans.

“Denisovan genetic material in New Guinea, Melanesia and Australia implies that this ancient group peopled a territory much broader than the southern part of Siberia where it was first identified,” Hublin says.

An initial human spread into Asia included ancestors of groups now living in eastern Oceania, some of whom interbred with Denisovans, Reich’s group concludes. Earlier evidence from the same team indicated that New Guineans and residents of neighboring Bougainville Island inherited 4 to 6 percent of their DNA from Denisovans, a sister group of Neandertals (SN: 1/15/11, p. 10). A second migratory wave brought ancestors of present-day East Asians and Indonesians, who didn’t dally with Denisovans.

Intriguingly, H. sapiens interbred with Neandertals upon reaching western Asia (SN: 6/5/10, p. 5) and with Denisovans upon reaching southeastern Asia, says molecular anthropologist Mark Stoneking of the Max Planck Institute for Evolutionary Anthropology, a study coauthor with Reich. In both instances modern human populations contained relatively few individuals, creating pressure to mate with outsiders in order to expand group numbers, Stoneking speculates.

Reich’s team compared Denisovan DNA to that of 260 people from 33 Asian populations. Denisovan genetic signatures are easily recognizable in the DNA of people today, Reich says.

Australian aborigines share as much DNA with Denisovans as do New Guineans, the researchers say. Several other island populations in eastern Oceania display lesser amounts of Denisovan genetic material. Groups in Indonesia and mainland Asia possess no Denisovan genetic remnants.

All non-Africans possess roughly the same amount of Neandertal DNA, about 1 percent to 4 percent. That’s consistent with a single human migration out of Africa about 65,000 years ago, followed by interbreeding with Neandertals in western Asia, Reich says. A trek to Southeast Asia then must have occurred. Common ancestors of New Guineans and Australian aborigines mated with Denisovans there at least 44,000 years ago, before journeying to their current homelands, the scientists estimate. Ancestors of East Asians and Indonesians arrived in a later migration.

Further genetic evidence that ancient people reached Asia in at least two waves — the first of which interbred with Denisovans — comes from a preserved lock of hair that an Australian aboriginal man donated to scientists about 100 years ago. Rasmussen’s group compared the man’s DNA, and that of three Chinese individuals, to genetic sequences of living Africans and Europeans, as well as to Denisovan DNA.

Australian aborigines trace their ancestry to a human migration into Southeast Asia sometime between 75,000 and 62,000 years ago, the investigators estimate. Denisovans interbred to a slightly lesser extent with ancestors of Australian aborigines than with ancestors of New Guineans, they find.

A second round of human arrivals gave rise to East Asians starting between 38,000 and 25,000 years ago, Rasmussen and his colleagues propose. As in Reich’s study, most current Asian populations studied by Rasmussen’s team lack Denisovan DNA.

By the time of the second human migration into Asia, early arrivers had displaced or replaced Denisovans in Southeast Asia, Hublin suggests.

Further investigations of the genetic history of Australian aborigines will reveal much about human evolution, Hawks predicts. Unlike most people today, these hardy foragers have DNA uncluttered by the relatively recent evolutionary legacy of agricultural lifestyles.

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.

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