New World Stopover: People may have entered the Americas in stages

1:34pm, February 13, 2008
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Think of it as the ultimate travel delay. Asian migrants first reached the northwestern edge of the Americas as early as 40,000 years ago but then had to wait at least 20,000 years before heading south into the continent's heart, a new genetic analysis finds.

Until now, many researchers had assumed that multiple waves of migrants rapidly settled the New World some time after 16,000 years ago.

The reason for the holdup: Two massive ice sheets on what is now Alaska blocked entry into North America from Beringia, a once-habitable region that today lies submerged in the Bering Strait. Melting of those glaciers yielded a coastal and an inland passage south by 16,000 years ago.

Between 1,000 and 5,400 people rapidly moved through those routes and settled the New World, propose anthropologist Connie J. Mulligan of the University of Florida in Gainesville and her colleagues. Previous DNA-based studies had estimated that no more than 100 individuals initially settled the Americas.

Stone Age travelers likely left a harsh environment in what is now eastern Asia and crossed the land bridge looking for greener pastures. They eventually reached Beringia, which was exposed during the last glacial maximum and contained grasslands and abundant large animals from around 36,000 to 16,000 years ago.

"Beringia wasn't paradise, but the new settlers survived," Mulligan says. "When the North American ice sheets started to melt and passage into the New World opened, we think they left Beringia to go to a better place."

Her team used new methods to assess DNA sequences obtained from eastern Asian and Native American populations.

An initial analysis, based on identifying the kinds and frequencies of mutations in mitochondrial genomes of 77 people, allowed the scientists to construct an evolutionary tree showing which sequences arose first and which came later. They then used a computer model to estimate the number of individuals who were mating and contributing to the gene pool at each branch in the tree.

This technique re-creates changes in an ancient population's size over time, rather than taking a genetic snapshot of population size at one time, as do most studies.

The scientists also combined evidence from mitochondrial DNA—which is passed solely from mothers to their children—and nuclear DNA, which contains genes from both parents, to estimate the size of the group that initially settled Beringia and the timing of its arrival from Asia.

Several hundred Asians started their trek to Beringia roughly 43,000 years ago, the researchers report in the February PLoS ONE. The population grew rapidly and peaked to several thousand by 36,000 years ago. The population was stable for about 20,000 years, and then swelled after migrating further south, the investigators conclude.

"What's most surprising is the long period of population stability in Beringia, especially with no archaeological evidence for such an occupation," Mulligan says. She and her coworkers suspect that, since Beringia now lies underwater, ancient sites of the first Americans are submerged as well.

"The idea that people were stuck in Beringia for a long time is obvious in retrospect, but it has never been promulgated," remarks anthropologist Henry C. Harpending of the University of Utah in Salt Lake City.

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