Men's DNA supports recent settlement of the Americas.

Genetic differences among the Y chromosomes of Central Asian and Native American men bolster the argument that people first reached the Americas less than 20,000 years ago, according to two groups of anthropologists. The new data also support the idea that a single wave of settlers gave rise to all native South Americans, they hold.

Scientists generally agree that the first people to reach the New World crossed from Siberia into North America, but just how and when this immigration unfolded remains controversial. Archaeological data indicate the presence of people in the America's by about 14,000 years ago. Yet there's evidence of a land bridge between Siberia and Alaska thousands of years earlier (see story p. 94), and some studies of DNA from cellular structures called mitochondria have suggested that an immigration occurred perhaps 30,000 years ago.

To address this disagreement, anthropologists have turned to variations in DNA on the Y chromosome, which passes from father to son. One such polymorphism, called M3, turns up among most Native American men but is absent in men on other continents. It therefore probably first arose shortly after the earliest colonization of the New World.

To limit how early colonization might have occurred, scientists needed to find and date some polymorphism that arose in Asia and then was carded into the New World.

In an upcoming American Journal of Human Genetics, Mark Seielstad of Harvard University and his colleagues describe such a genetic variation. Dubbed M242, it's present in all men with M3 and in a fraction of men in at least 24 Eurasian populations who lack M3. The M242 polymorphism therefore must predate settlement of the New World.

Seielstad's team calculates that M242 arose about 15,000 years ago. Allowing for some uncertainty in their methods, the researchers suggest that people probably arrived in the Americas no earlier than 18,000 years ago. For their calculations, the researchers estimated how many years separated generations of men and how many genetic mutations occurred per generation.

In a separate study reported in the same journal issue, Andres Ruiz-Linares of University College London and his colleagues used M242 and other genetic patterns on Y chromosomes to argue that migration to the Americas occurred in at least two waves, beginning about 14,000 years ago. In their study, the researchers compared the genetic histories of men in Mongolia, a North American native group, and 23 native groups in South America. All the South Americans seemed to stem from the same wave of migration.

The new studies suggest a relatively late arrival of people to the Americas, says molecular anthropologist Theodore G. Schurr of the University of Pennsylvania in Philadelphia. Ruiz-Linares' evidence is also consistent with linguists' hypothesis that all native South American and many native North American populations descended from a single group of early settlers, Schurr says.

However, random genetic changes rather than multiple immigrations could explain the patterns of genetic variation observed by Ruiz-Linares' team, argues Eduardo Tarazona-Santos of the University of Maryland at College Park.

Further, Tarazona-Santos says, the new estimates of the timing of colonization don't disprove the mitochondrial DNA studies that have suggested an older settlement of the Americas. Factors such as the fraction of men who fathered children or ancient fluctuations in population size might have biased the new results, he says.

More information about whether events during the settlement differently influenced procreation among men and women could help reconcile the different colonization timings now indicated by data from Y chromosomes and mitochondria, Tarazona-Santos says.