March 14, 2008
Native Americans Traced Back to Six ‘Founding Mothers’
A new study has revealed that nearly all of the Native Americans in the western hemisphere can be traced through DNA to six women whose descendants first immigrated from Asia 20,000 years ago.
Researchers said these women left a DNA signature that can be found in about 95 percent of today's Native Americans.Ugo Perego from the Sorenson Molecular Genealogy Foundation in Salt Lake City and the University of Pavia in Italy is the study's co-author. He told Associated Press the discovery does not mean that only these six women gave rise to the migrants who crossed into North America in the initial populating of the continent.
He explained that the women had lived between 18,000 and 21,000 years ago, though not necessarily at exactly the same time.
The research confirms previous implications of the six maternal lineages, Perego said. But an expert not involved with the study said the work left some questions unanswered.
Perego and his colleagues traced the history of a particular kind of mitochondrial DNA that represents a very small fraction of the human genetic material, and indicates only a portion of a person's ancestry.
As the name suggests, mitochondrial DNA is found in the mitochondria, which function as the cells' batteries. But unlike DNA found in the cell's nucleus, mitochondrial DNA is passed along only by the mother. So it provides a way of tracking lineage to a person's mother, maternal grandmother, and so on through the generations.
Using this information, researchers created a "family tree" tracing different mitochondrial DNA lineages found in today's Native Americans. By denoting mutations in each branch and using a formula for how often such mutations arise, they were able to determine the age of each branch and the timeline during which each branch arose in a single woman.
Interestingly, Perego said the six "founding mothers" did not appear to live in Asia because the DNA signatures they left behind aren't found there. He suspected they might have lived in Beringia, the now-submerged land bridge connecting Asia to North America.
University of Florida's Connie Mulligan, an anthropologist who studies the colonization of the Americas but was not affiliated with the new research, was not surprised the mitochondrial DNA was traced back to just six women.
"It's an OK number to start with right now," but further work may change the results slightly, she said. The findings do not address the larger questions of where those women lived, or of how many people left Beringia to colonize the Americas, she told Associated Press Thursday.
The precise timelines the women lived is an open question at this point because it's not clear whether the researchers properly accounted for differing mutation rates in mitochondrial DNA, Mulligan said, adding that further research could change the estimate, "possibly dramatically."
The work was published this week by the journal PLoS One. The full report can be viewed at http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0001764
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