NEW YORK (AP) — Nearly all of today's Native Americans in North, Central and South America can trace part of their ancestry to six women whose descendants immigrated around 20,000 years ago, a DNA study suggests.

Those women left a particular DNA legacy that persists to today in about about 95 percent of Native Americans, researchers said.

The finding does not mean that only these six women gave rise to the migrants who crossed into North America from Asia in the initial populating of the continent, said study co-author Ugo Perego.

The women lived between 18,000 and 21,000 years ago, though not necessarily at exactly the same time, he said.

The work was published this week by the journal PLoS One. Perego is from the Sorenson Molecular Genealogy Foundation in Salt Lake City and the University of Pavia in Italy.

The work confirms previous indications of the six maternal lineages, he said. But an expert unconnected with the study said the findings left some questions unanswered.

Perego and his colleagues traced the history of a particular kind of DNA that represents just a tiny fraction of the human genetic material, and reflects only a piece of a person's ancestry.

This DNA is found in the mitochondria, the power plants of cells. Unlike the DNA found in the nucleus, mitochondrial DNA is passed along only by the mother. So it follows a lineage that connects a person to his or her mother, then the mother's mother, and so on.

The researchers created a "family tree'' that traces the different mitochondrial DNA lineages found in today's Native Americans. By noting mutations in each branch and applying a formula for how often such mutations arise, they calculated how old each branch was. That indicated when each branch arose in a single woman.

The six "founding mothers'' apparently did not live in Asia because the DNA signatures they left behind aren't found there, Perego said. They probably lived in Beringia, the now-submerged land bridge that stetched to North America, he said.

Connie Mulligan of the University of Florida, an anthropolgist who studies the colonization of the Americas but didn't participate in the new work, said it's not surprising to trace the mitochondrial DNA to six women. "It's an OK number to start with right now,'' but further work may change it slightly, she said.

That finding doesn't answer the bigger questions of where those women lived, or of how many people left Beringia to colonize the Americas, she said Thursday.

The estimate for when the women lived is open to question because it's not clear whether the researchers properly accounted for differing mutation rates in mitochondrial DNA, she said. Further work could change the estimate, "possibly dramatically,'' she said.