Ninety-five percent of Native Americans can trace their ancestry to six "founding mothers" who arrived in the New World about 20,000 years ago, says a study by Utah-based Sorenson Molecular Genealogy Foundation and the University of Pavia, Italy.
The date is about when humans first arrived in the Americas, according to the research.
The 95 percent figure is true whether today's Native Americans live in North, Central or South America, adds the study.
The research was published online Wednesday by the Public Library of Science, www.plos.org.
According to the study, this is the most comprehensive research ever into the genetic origins of Native Americans.
The finding does not mean that only six ancestral women reached the Americas around 20,000 years ago, says one of the co-authors, Ugo A. Perego, director of operations at the foundation based in Salt Lake City.
The research was based on studies of mitochondrial DNA, abbreviated as mtDNA, which is passed only from mothers to daughters. If a woman founder had only sons, her mtDNA would not have been passed down although she would have descendants.
Also, a press release from the foundation notes, "The study also confirms the presence of genetic subgroups of more rare, less known and geographically limited genetic groups who arrived later." Those groups are not detailed in the paper.
The scientists studied all available complete mtDNA data for Native Americans, amounting to more than 200 samples.
The mtDNA passed along from the six founding mothers is related to, but different from, mtDNA among today's northern Asians.
(The difference is because DNA changes over time. Using the known rate of change, scientists calculated the 20,000-year figure.)
The relationship reinforces the idea that the earliest peopling of the Americas happened because of a land bridge, called Beringia, that once stretched between Alaska and Russia.
The land bridge allowed people to live between today's continents. People and animals lived on Beringia probably for "a few thousands of years," Perego said.
When the climate im-
proved and the ice melted, people "found an open, free corridor to go to America," he said.
"It appears that the migration was very rapid, and they reached southern Chile very rapidly as well."
In America, they found a better climate and the population increased rapidly, as reflected in the genetic information. "In the 20,000 years between the time of arrival and today, many different sub-lineages began."
As the settlers moved south, they populated more and more of North, Central and South America.
The study is titled, "The Phylogeny of the Four Pan-American MtDNA Haplogroups: Implications for Evolutionary and Disease Studies." Its authors are Antonio Torroni, the lead author and Perego's mentor, and Alessandro Achilli and Antonio Torroni, all of the University of Pavia; Perego and Scott R. Woodward of Sorenson Molecular Genealogy Foundation; Claudio M. Bravi of Instituto Multidisciplinario de Biologia Celular, La Plata, Argentina; Michael D. Coble of the Armed Forces DNA Identification Laboratory, Rockville, Md.; Qing-Peng Kong of the Kunming Institute of Zoology, Kunming, China; Antonio Salas of the Hospital Clinic University, Alicia, Spain; and Hans-Urgent Brandel of the University of Hamburg, Germany.The nonprofit Sorenson Molecular Genealogy Foundation was founded in 2000 by the late James LeVoy Sorenson and by Ira Fulton, with Brigham Young University in Provo providing a lab for testing genetic information. It has grown tremendously since. Its Web site notes the foundation is dedicated to building the world's foremost collection of DNA and corresponding genealogical information.