May 24, 2005
Tuesday

A band of as few as 200 people may have made up the earliest wave of immigrants from Asia to the Americas some 14,000 years ago, a new genetic analysis suggests.

In a paper appearing in the June issue of Public Library of Science Biology, Jody Hey, a professor of genetics at Rutgers University in New Jersey, describes a new computational method that uses genetic information to model population divergence.

A new study using computer models and genetic samples from Asians and American Indians calculates that a band of as few as 200 people made up the founding population of the New World some 14,000 years ago. The actual number of founding "mothers and fathers" - adults of reproductive age - may have been only 70 or 80.

That's the scientific term for the point when a group splits from its ancestral population to pursue its own destiny.

"The estimated effective size of the founding population for the New World is about 70 individuals," Hey said, referring to the number of adults of reproductive age. Population scientists use a rule of thumb that effective size equals about a third of any total population figure, which means roughly 200 people made the trek along an ice-age land bridge across the Bering Strait.

But it's not as though this group left because Asia was getting particularly crowded. "Calculations also showed that this represents approximately 1 percent of the effective size of the estimated ancestral Asian population," Hey said, or about 9,000 adults.

Hey's is the latest of several efforts to trace the peopling of the Americas through genetics. Earlier efforts have focused on one gene and used models that assumed population sizes stayed relatively constant over time.

The Rutgers researcher used nine genes in which sequences and frequencies have been well documented. He set up a new model that allowed for changes in the size of the ancestral population and the founder population through time.

"The beauty of this new methodology is that it uses actual DNA sequences collected from Asian peoples and Native Americans, an approach that can provide a detailed portrait of historical populations," Hey said.

He focused on the genetics of Amerind-speaking populations, one of three major language groups in the New World representing the earliest migrants who moved deep into the Americas. The shared features of these people with the Mongoloid populations of northeast Asia and Siberia have been one of the stronger arguments for an Asian origin.

The other native groups, the more recent Athabascan speakers and even more recent Eskimos and Aleuts, have less comprehensive genetic information available and were not included in Hey's study.

Hey's estimates for when the gene flow from the ancestral population was cut off - 12,000 to 14,000 years ago - are a bit early, but still generally consistent with much of the archaeological record of human settlement in the Americas that puts the so-called Clovis culture into the American Southwest by about 11,000 years ago.

But the estimates are at odds with more controversial evidence of earlier settlement and other sources of immigration over the water from Polynesia, even from ice-age Europe under some scenarios, which has been reported in recent years.

In addition, recent finds along the northern Pacific coast of Canada and the United States suggest that some immigrants may have moved in along a coastal route during periods when ice blocked the land, prompting theories that there were successive waves of northern Asians arriving over thousands of years.

Hey concedes that "available data do not yet allow precise estimates of founding time, nor of whether there has been gene flow between the New World and Asia following the initial founding event."

But he argues that the new model offers an approach that scientists can use to explore how the first Americans spread out over the continents.

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