Tuesday 30 October 2007
Source: Science Daily.
Questions about human migration from Asia to the Americas have perplexed anthropologists for decades, but as scenarios about the peopling of the New World come and go, the big questions have remained. Do the ancestors of Native Americans derive from only a small number of “founders” who trekked to the Americas via the Bering land bridge? How did their migration to the New World proceed? What, if anything, did the climate have to do with their migration? And what took them so long?
A team of 21 researchers, led by Ripan Malhi, a geneticist in the department of anthropology at the University of Illinois, has a new set of ideas. One is a striking hypothesis that seems to map the peopling process during the pioneering phase and well beyond, and at the same time show that there was much more genetic diversity in the founder population than was previously thought.
«Our phylogeographic analysis of a new mitochondrial genome dataset allows us to draw several conclusions», the authors wrote. «First, before spreading across the Americas, the ancestral population paused in Beringia long enough for specific mutations to accumulate that separate the New World founder lineages from their Asian sister-clades.» 
Or, to express this first conclusion another way, the ancestors of Native Americans who first left Siberia for greener pastures perhaps as much as 30,000 years ago, came to a standstill on Beringia — a landmass that existed during the last glacial maximum that extended from Northeastern Siberia to Western Alaska, including the Bering land bridge — and they were isolated there long enough — as much as 15,000 years — to maturate and differentiate themselves genetically from their Asian sisters.
 A clade is a group of mitochondrial DNAs (mtDNAs) that share a recent common ancestor, Malhi said. Sister-clades would include two groups of mtDNAs that each share a recent common ancestor and the common ancestor for each clade is closely related.