Founding fathers & mothers: How many crossed the land bridge?
Programs on the Discovery Channel and PBS have sparked fresh interest in the prehistoric peopling of the New World. Now, for the first time, we have a realistic estimate of how many ancients made that ice age trek across the long-lost land bridge from Asia to become the first Native Americans. Jody Hey, a professor of genetics at Rutgers, The State University of New Jersey, has developed a computational method that uses genetic information to create models of population divergence - where a group has split off from its ancestral population to pursue its own destiny.
In a paper appearing in PLoS (Public Library of Science) Biology, Hey disclosed his findings. "The estimated effective size of the founding population for the New World is about 70 individuals," Hey said. "Calculations also showed that this represents approximately 1 percent of the effective size of the estimated ancestral Asian population."
"Effective size" in population genetics is often thought of as the number of adults of reproductive age. One rule of thumb is the effective size might be about one third of the 'census population size' which, in this case, comes out to about 200 people. In addition to population size, Hey's rigorous and complex methodology also generated historical estimates of when the divergence occurred. His dates are consistent with much of the archaeological record - in the range of 12,000-14,000 years ago. He was also able to discern changes in population size and the extent of gene flow between populations, potentially representing renewed contact. Hey used nine genes in which sequences and frequencies were well documented in the scientific literature.
"The beauty of the 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. The method doesn't use summary statistics or averages as some approaches do, but gleans as much information as possible directly from the genetic data.
Hey focused on the genetics of Amerind-speaking populations, one of three major language groups in the New World representing the earliest migrants who extended deep into the Americas. The other groups, the more recent Athabascan speakers and the even more recent Eskimos and Aleuts, had less comprehensive genetic information available and were not included in Hey's study.
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