Monday, February 18, 2008

MIGRANTS TO THE NEW WORLD GOT STUCK

People who migrated from Asia to the New World camped out for 20,000 years on land now submerged under the Bering Strait between Alaska and Siberia, according to a genetic analysis. A team at the University of Florida combined studies of DNA, archeological evidence, climate data and geological data to come up with their new theory, which describes a much longer migration than most other researchers have proposed. "We sort of went out onto a limb, incorporating all this nongenetic data," molecular anthropologist Connie Mulligan said.

Mulligan's team proposes that the people who left Central Asia to eventually populate the Americas passed quickly through Siberia, and then got stuck in Beringia - a former land mass that now lies under the frigid Bering Sea. There they stayed for 20,000 years, until glaciers melted about 15,000 years ago, opening a route to the Americas. "The reason there is no archeological evidence for that occupation is that the area is under water," Mulligan said.

The researchers used sequences of mitochondrial DNA taken from Asians and Native Americans for their analysis. This type of DNA is passed along virtually unchanged from mother to child. The small mutations that occur can be used as a genetic clock to track the descent and the sizes of ancient populations. "After a long period of little change in population size in greater Beringia, Amerinds (American Indians or native Americans) rapidly expanded into the
Americas less than 15,000 years ago either through an interior ice- free corridor or along the coast," they wrote in their report. "This rapid colonization of the New World was achieved by a founder group with an effective population size of 1,000 to 5,400 individuals."

The University of Florida's Michael Miyamoto said the DNA suggests a 20,000-year "waiting period" during which generations passed and genetic changes accumulated. "By looking at the kinds and frequencies of these mutations in modern populations, we can get an idea of when the mutations arose and how many people were around to carry them," he said. They were moving out of Asia and finally reached a landmass that was exposed because of lower sea levels during the last glacial maximum, but two major glaciers blocked their progress into the New World," Mulligan said in a statement. "So they basically stayed put for about 20,000 years. It wasn't paradise, but they survived. When the North American ice sheets started to melt and a passage into the New World opened, we think they left Beringia to go to a better place."

The study is available atwww.plosone.org/doi/pone.

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