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Y Trail of the First Americans: DNA data point to late New World entry

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10:25am, October 1, 2003
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People first set foot in the Americas no earlier than about 18,000 years ago, according to an analysis of a newly identified gene variant on the Y chromosome.

This evidence supports the longstanding archaeological theory that New World settlers crossed a land bridge from Asia to North America about 14,000 years ago, say geneticist Mark Seielstad of the Genome Institute of Singapore and his colleagues.

The Y chromosome data generate a more precise estimate of colonization of the Americas than earlier DNA studies provided, the researchers contend. Some previous investigations–including analyses of genes in cells' mitochondria and nuclei–yielded settlement dates as early as 40,000 years ago.

"[Our] discovery . . . places the DNA evidence more in line with archaeological data," Seielstad and his coworkers conclude in the September American Journal of Human Genetics.

Their finding builds on a reconstruction of Y chromosome-based lineages worldwide that was published in 2000. Peter Underhill of Stanford University Medical School, a coauthor of the new study, led that analysis. Each Y lineage carries a distinctive set of gene alterations.

Another team, directed by Michael F. Hammer of the University of Arizona in Tucson, analyzed a different set of international Y chromosome data in 2001 and largely confirmed the evolutionary tree proposed by Underhill's group. Both projects determined that two Y lineages reached the Americas from Asia before European colonists arrived.

The newly discovered mutation–which now occurs in a substantial minority of men sampled throughout central Asia, India, and Siberia–appears to be a precursor of a closely related gene variant found only in Native American populations. Seielstad's group concludes that the mutation, dubbed M242, must have arisen in Asia before either of the Y lineages appeared in the New World. The mutation's spread and frequency in Asia suggest that it arose shortly before its New World relative did.

The scientists have calculated an age of about 18,000 years for M242, based on estimates of the rate at which mutations occur on the Y chromosome and the average generation span for men.

Hammer says that his own work is now confirming the M242 timeframe. "There may have been a single population containing both New World Y [lineages] that reached the Americas from Siberia between 17,000 and 18,000 years ago," he says.

In a study slated to appear in Molecular Biology and Evolution, Hammer and his colleagues trace the origin of the two Native American Y chromosome lineages to a mountainous region of southern Siberia. The New World lineages emerged from there no more than 17,200 years ago, according to their calculations.

That scenario fits with the view of many archaeologists, although they continue to disagree about where the first Americans came from and whether they arrived in a single migration (SN: 9/6/03, p. 150: Continental Survivors: Baja skulls shake up American ancestry).

However, until geneticists study larger samples of Native Americans, Hammer doesn't rule out the possibility that Asian groups trekked to the Americas 30,000 years ago or even earlier.

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