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Native Americans streamed into the New World in at least three waves of migration starting more than 15,000 years ago, a gene study released Wednesday suggests.

North and South America were totally empty of people until the first arrivals from Siberia crossed a land bridge into Alaska, spreading in a few thousand years to the tip of South America. The genetic study may help settle a debate between a long-held view that the populating of the continents came as one event instead of the more recently supported notion, backed by this study in the journal Nature, that the migration happened in three distinct waves.

"Our study makes clear that mixing of these three ancient populations is the story of Native American arrival," says geneticist David Reich of Harvard Medical School, lead author of the study.

It finds a "First American" wave starting more than 15,000 years ago that first moved into North and then South America from Asia. Migration halted by Ice Age glaciers resumed after that time from Siberia, with two surges: one that we know today as the Eskimo-Aleut population and the other a group that today represents the Na-Dené language speakers in Canada, Reich says. "

The study looked at genetic data from 52 Native American and 17 Siberian groups collected over the last 30 years and screened against populations from elsewhere.

DNA cannot tell exactly when the migrations occurred. ("Genes don't come with time stamps," Reich says). However, it's safe to say it happened more than 15,000 years ago, when geologists established that an Ice Age land bridge from Siberia to Alaska was opened. Some scholars have proposed the first wave arriving earlier than 30,000 years ago, on an earlier Ice Age land bridge that was blocked until the era of the two later migrations.

"This is an outstanding piece of research documenting deeply and widely the human genetic record and what it reflects about the first peopling," says anthropologist Tom Dillehay of Vanderbilt University in Nashville, who was not part of the study. A mixture of archaeology, genetics and language studies already underway will likely further resolve the exact timing and direction of each migration event, he adds.

"There is still a key missing component here," Dillehay adds. Once genes from ancient burials in Asia and the New World are compared by geneticists with the modern genes used in the new study, he says, an even more accurate picture of Native American origins will likely emerge.

Reich, who has participated in studies looking at gene maps of ancient Neanderthals, agrees but points to evidence of ancient movements of Native Americans from South America to Costa Rica, as information already available from studies of modern genes. Notably, the data suggest that Aleut and Na-Dené speakers typically trace about half their genetic ancestry to "First Wave" migrants into the New World.

A pattern of ancient people mixing, rather than completely running each other off their land, "is becoming more clear," Reich says. That's seen in studies of European, Asian and African people, and now in North and South America.

By Dan Vergano

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