Settlement of Americas a 3-act play

By Charles Q. Choi

updated 8:02 p.m. ET Feb. 13, 2008

The epic journey by which the Americas were first settled has been a great mystery for centuries. Did it happen by land or by sea? Did it happen one dozen or so millennia ago or three dozen?

The answer might be "yes."

New findings reveal the settling of the New World did not come in a single burst, as is suggested by most theories, but was, in a way, a play with three acts, each separated by thousands of generations.

The first stage of this voyage involved a gradual migration of people from Asia through Siberia starting about 40,000 years ago into Beringia, a once-habitable grassland populated with steppe bison, mammoths, horses, lions, musk oxen, sheep, wooly rhinoceros and caribou that nowadays lies submerged under the icy waters of the Bering Strait.

The second phase of the journey was basically a layover in Beringia.

"Two major glaciers blocked their progress into the New World. So they basically stayed put for about 20,000 years," said researcher Connie Mulligan, a molecular anthropologist at the University of Florida in Gainesville. The population there apparently did not grow or shrink much during this era, which suggests Beringia "wasn't paradise, but they survived."

In the final act, "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," Mulligan explained, resulting in a rapid expansion into the New World about 15,000 years ago. Their research suggests the New World was settled by approximately 1,000 to 5,000 people — a substantially higher number than the 100 or fewer individuals of some prior estimates.

The research is detailed online in the journal PLoS ONE.

How did the researchers come up with their findings? Well, DNA allows scientists to deduce the history of populations. For instance, mutations that all New World populations have in common with each other and no one else means they share a common ancestry, suggesting there was just one wave of migration into the Americas, as opposed to several unrelated waves.

However, the molecular evidence was confusing as to when this wave of migration took place. DNA accumulates mutations over time, serving like a clock, but some DNA suggested people came to the New World about 13,000 years ago, while other sequences hinted at 30,000 or more years ago.

Mulligan and her colleagues' new analysis of DNA from Native American and Asian populations seem to help resolve conflicts in past research by suggesting a long waiting period on the doorstep to the New World.

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