Founding families: New World was settled by small tribe.

A geneticist armed with computer simulations of prehistoric populations says that only about 200 to 300 people crossed the ice age land bridge from Asia to become the founding population of North America. Of that pioneering group, there were just 70 adults of reproductive age, contends Jody Hey of Rutgers University in Piscataway, N.J.

Hey arrived at that strikingly small number after analyzing DNA from living Asians and Native Americans. Using nine specific DNA sequences as reference points, he inferred the movements and characteristics of the ancient population, including the Americas' founding fathers and mothers.

"It looks like a group that was about the size of a single tribe made the initial trip from Asia to the New World," Hey says. His findings appear in the June PLoS Biology.

To investigate ancient populations, scientists take samples from modem people and compare genetic locales that show no signs of having been shaped by natural selection, so random mutations probably have accumulated there at a regular rate. In earlier studies, researchers examined single sequences to reconstruct the initial New World population size. Those sequences reside either within mitochondrial DNA, which is passed on exclusively by the mother, or on the Y chromosome, which travels only from father to son. Previous estimates of the newcomer population have ranged from about 100 to 1,000 reproductive-age adults.

Hey, in contrast, used previously collected data to simultaneously assess differences in one mitochondrial DNA sequence and in eight stretches of DNA distributed among several chromosomes in the nucleus. Data on each genetic sequence came from 5 to 50 northeastern Asians and from comparable numbers of speakers of Amerind tongues, the oldest of three major Native American-language groups.

The Rutgers scientist fed the DNA data into a computer program that compared millions of possible scenarios for how patterns of genetic differences arose in the two populations, highlighting the most likely ones.

Hey's analysis indicates that between 14,000 and 7,000 years ago, approximately 200 to 300 people entered the New World after leaving an Asian population that was roughly 100 times as large. Geneticists typically assume that about one-third of any population is fertile adults. Hey acknowledges that the timing of the migration in his analysis is more recent than other estimates, which range from 20,000 to 16,000 years ago.

Hey's approach represents "a big step forward," remarks geneticist Michael F. Hammer of the University of Arizona in Tucson. The new results bolster the view that small, isolated populations settled the Americas, he says.

Geneticist Theodore Schurr of the University of Pennsylvania in Philadelphia calls the new study "a solid, but initial, effort." Analyses of DNA sequences from larger numbers of Asians and Native Americans could yield different estimates of the founding population in the New World, Schurr adds.

Archaeologist David J. Meltzer of Southern Methodist University in Dallas notes that, since the ice age, only limited numbers of people have inhabited northeastern Asia's harsh terrain. Says Meltzer: "That the founding population [of the New World] looks to be about 70 folks surprises me not at all."