July 11, 2012 | 16
At least 15,000 years ago intrepid Siberians crossed the newly exposed Bering land bridge to arrive in the unpeopled Americas. But was this influx the only ancient wave from East Asia? Researchers have been studying archeological, linguistic and genetic evidence for years in a quest to understand how the first Americans arrived and spread through the continents.
A new large-scale genomic study paints a much clearer picture of these early entrances and distant dispersals that led some people all the way to Tierra del Fuego in a matter of millennia. At least three major waves of migrants from Siberia hit the Americas, but not all of them have had the same reach, according to the findings, which were published online July 11 in Nature (Scientific American is part of Nature Publishing Group).
To arrive at a much-clarified historical portrait, the researchers collected genetic data from 493 people belonging to one of 52 contemporary American Indian groups from Canada to southern South America. They then compared common genetic variances with those of 245 people from 17 current Siberian groups. The key to this study was isolating the genetic variables to those that were specific to American Indian populations, which starting in the 15th century became obscured by genetic “noise” from the substantial amount of European and African influence, the researchers noted.
The initial wave, nicknamed the “First American” group, is the ancestral forebearers of every sampled group from the Yghan in southern Chile to most of Canada’s First Nations groups. But two subsequent groups, “Eskimo-Aleut” and “Chipewyan,” followed, contributing genes only to their respective Artic groups. “The Asian lineage leading to the First Americans is the most anciently diverged,” said David Rich, a geneticist at Harvard Medical School and co-author of the new paper, in a prepared statement. “The Asian lineages that contributed some of the DNA to Eskimo-Aleut speakers and the Na-Dene-speaking Chipewyan from Canada are more closely related to present-day East Asian populations.”
These later groups were apparently quick to mingle with the established populations in northern North America, so much so that Eskimo-Aleut genes sampled retained only about 43 percent of their later wave ancestor’s profiles (the rest being composed of those from the First American group)—and the Chipewyans retained only about 10 percent of their later-wave ancestry (90 percent of the variances studied belonging to the First American profile).
Based on the current analysis, very little genetic remixing appears to have occurred once groups went their separate ways throughout the continents. The exception is one group in Latin America, namely, speakers of the Chibchan languages, who live in Costa Rica, Panama and Colombia. The new genomic research shows that Chibchan speakers today carry both North and South American genetic signatures, suggesting that their ancestors subsequently moved back up the Panama isthmus.
Interestingly, the researchers also found traces of Eskimo-Aleut genetic variants in current-day coastal Siberian populations of the Naukan and Chukchi. So perhaps the Americas weren’t quite to everyone’s liking.
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