From the New York Times:
By NICHOLAS WADE
North and South America were first populated by three waves of migrants from Siberia rather than just a single migration, say researchers who have studied the whole genomes of Native Americans in South America and Canada.
Some scientists assert that the Americas were peopled in one large migration from Siberia that happened about 15,000 years ago, but the new genetic research shows that this central episode was followed by at least two smaller migrations from Siberia, one by people who became the ancestors of today’s Eskimos and Aleutians and another by people speaking Na-Dene, whose descendants are confined to North America. The research was published online Wednesday in the journal Nature.
The Na-Dene speakers include the Navajo and Apache of the American Southwest, although no U.S. tribes were included in the study because of political opposition to genetic research. It's my vague impression that Na-Dene speaking Indians tend to look more Siberian than other American Indians, which wouldn't be surprising since they had ancestors in Siberia more recently.
The finding vindicates a proposal first made on linguistic grounds by Joseph Greenberg, the great classifier of the world’s languages. He asserted in 1987 that most languages spoken in North and South America were derived from the single mother tongue of the first settlers from Siberia, which he called Amerind. Two later waves, he surmised, brought speakers of Eskimo-Aleut and of Na-Dene, the language family spoken by the Apache and Navajo.
But many linguists who specialize in American languages derided Dr. Greenberg’s proposal, saying they saw no evidence for any single ancestral language like Amerind. “American linguists made up their minds 25 year ago that they wouldn’t support Greenberg, and they haven’t changed their mind one whit,” said Merritt Ruhlen, a colleague of Dr. Greenberg, who died in 2001.
Reductionism is popular in physics, but not in the social sciences since the 1960s, or maybe not since the stock market crash of 1929. Anthropologist Robin Fox jokes that his field suffers from "ethnographic dazzle" or stamp collectoritis.
The new DNA study is based on gene chips that sample the entire genome and presents a fuller picture than earlier studies, which were based on small regions of the genome like the Y chromosome or mitochondrial DNA. Several of the mitochondrial DNA studies had pointed to a single migration.
A team headed by David Reich of the Harvard Medical School and Dr. Andres Ruiz-Linares of University College London report that there was a main migration that populated the entire Americas. They cannot date the migration from their genomic data but accept the estimate by others that the migration occurred around 15,000 years ago. This was in the window of time that occurred after the melting of great glaciers that blocked passage from Siberia to Alaska, and before the rising waters at the end of the last ice age submerged Beringia, the land bridge between them.
They also find evidence for two further waves of migration, one among Na-Dene speakers and the other among Eskimo-Aleut, again as Dr. Greenberg predicted. But whereas Dr. Greenberg’s proposal suggested that three discrete groups of people were packed into the Americas, the new genome study finds that the second and third waves mixed in with the first. Eskimos inherit about half of their DNA from the people of the first migration and half from a second migration. The Chipewyans of Canada, who speak a Na-Dene language, have 90 percent of their genes from the first migration and some 10 percent from a third.
It is not clear why the Chipewyans and others speak a Na-Dene language if most of their DNA is from Amerind speakers. Dr. Ruiz-Linares said a minority language can often dominate others in the case of conquest; an example of this is the ubiquity of Spanish in Latin America.