New World founders small in number

About 14,000 years ago - a few hundred thousand years after our putative modern forebears spread out from Africa - descendants of archaic humans crossed the Bering land bridge from Siberia to North America. Several lines of evidence support this model, but that's where the consensus ends and the details are hotly debated. In a paper published in the premier open-access journal PLoS Biology, Jody Hey now reveals how the sizes of the first New World populations have changed since they were founded. His new approach shows that the New World was colonized by a surprisingly small population with an effective size of only about 70 individuals (those individuals likely to contribute genes to the next generation).

Hey's approach addresses shortcomings in traditional population genetic studies that rely on just one gene and that assume that population sizes have been constant over time. Studying levels of DNA sequence variation at a single genomic region, or loci, can offer insight into the history of that gene, but the stochastic nature of gene evolution means that different genes have different histories. Hey analyzed DNA sequences from nine loci, so that the population genetic history could be found despite variation among genes. He also added an additional parameter to standard models to incorporate changes in the size of the ancestral population and of each founder population through time.

His analysis suggests that only about 70 individuals left their ancestral Asian population (estimated at about 9,000 individuals) to reach America 7,000 to 14,000 years ago. Archaeological evidence places the earliest American inhabitants in the New World at around 14,000 years ago. Though Hey's estimates are more recent, they also indicate a high probability at this time period. With this new approach, researchers will be able to explore this and many other questions to fill in the details of the first American immigration.

Citation: Hey J (2005) On the number of New World founders: A population genetic portrait of the peopling of the Americas. PLoS Biol 3(6): e193.

Source : Public Library of Science