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Earlier Arrival for Earliest Americans

Asian people first flocked to North America in a single wave as long as 40,000 years ago, some 10,000 years earlier than previously thought, according to a new study. The findings, reported in this month's American Journal of Human Genetics, may help reconcile a conflict between geneticists--who insist that Native Americans descended from a single wave of immigrants--and archaeologists, who avow that the New World, with its early cultural diversity, must have been settled in multiple waves.

Geneticists study human evolution using mitochondria, subcellular power plants with their own set of DNA. Unlike DNA in the nucleus, mitochondrial DNA is passed directly from mother to child, so variations in the genetic sequence accumulate over the years as mutations--not from mixing with the father's DNA. The pattern of mutations in mitochondrial DNA is often used to estimate how long ago separate populations diverged and roughly what racial group that individual belonged to. All Native American groups share four typical mutation patterns resembling those seen in some modern Asians, suggesting that a single group of Asians gave rise to all Native Americans, says Smithsonian molecular anthropologist Connie Kolman. Past estimates, which relied on comparing characteristic patterns in short snippets of mitochondrial DNA, suggest that the migration occurred 20,000 to 30,000 years ago. But the pattern comparison method, says Sandro Bonatto of the Universidade Federal do Rio Grande do Sul in Brazil, can underestimate the true amount of mitochondrial variation, because it doesn't actually decode the DNA in the snippets.

So Bonatto and his colleague Francisco Salzano decided to carry out the more painstaking task of decoding mitochondrial DNA sequences of more than 700 individuals from 20 Native American groups and tallying up all the variations. They then used computer simulations to compare the sequences and determine how they are distributed among populations. Their statistical analyses suggest that modern Native Americans share a single group of ancestors who lived in North America at least 25,000 years ago--and more likely between 30,000 to 40,000 years ago.

This estimate is "the best analysis to date" on the peopling of America, says geneticist Andrew Merriwether of the University of Michigan, Ann Arbor. But it's possible that genetic variation among Native Americans arose well before their Asian ancestors crossed the Bering land bridge that connected what is now Alaska and northeastern Russia thousands of years ago. "The timing of mutations must be earlier than the separation of populations," says Stanford geneticist Luca Cavalli-Sforza. "But it is difficult to say how much earlier."

Still, not all archaeologists are convinced. Fred West, an archaeologist with the Peabody Essex Museum in Salem, Massachusetts, points out that numerous linguistic and archaeological studies reveal several distinct Native American cultures dating back at least 10,000 years, suggesting multiple waves of settlement. But Merriwether counters that if the earliest Americans did in fact come over in a single wave 40,000 years ago, distinct Native American groups would have had time to develop.