MtDNA mutates at a certain predictable rate - it's like a clock ticking, but with the clock recording every tick. So it's possible to judge when distant populations originally diverged, based on how different the mtDNA they have today is. If a group of people splits up, some going east and some going west, mtDNA mutations found in the east but not the west likely originated after the group diverged. Count up the mutations and - knowing the rate at which they would have occurred - you can figure out when the family tree branched off. "You literally have a genetic clock," explained Douglas Wallace, a professor of molecular medicine at the University of California, Irvine.

Scientists categorize mtDNA into a number of so-called haplogroups, based on their similarities and differences. You can think of them as like blood types - they don't affect the way you live, but they can be identified at the molecular level.

MitochondrialDNA (mtDNA) haplogroup testing led to the surprising hypothesis that some of the first Americans came from Europe thousands of years ago.

The conventional wisdom to explain the peopling of the Americas is that migrants crossed from northeast Asia to Alaska around 13,000 years ago to become the ancestors of today's Native Americans. But mtDNA analysis has revealed some unexpected links between Europe and North America. When scientists analyzed the mtDNA of a broad sample of living Native Americans, they found that about 97 percent had mtDNA from haplogroups A, B, C or D. These haplogroups are all present in modern day Siberia and Asia, so it makes sense that the forefathers of those Native Americans came from those regions. But the surprise was that about 3 percent of the Native Americans tested had mtDNA from a different haplogroup, called X. Some populations, such as the Ojibwa from the Great Lakes region, have a high concentration of X - 25 percent.

How did haplogroup X get to North America? Some X has been found in Mongolia, but it's definitely not common in modern Asia. It can, however, be found in about 4 percent of the present day European population. Genetic anthropologists suggest that the presence of X in North America points to an early migration westward from Europe. By looking at the various mutations within haplogroup X, scientists are able to use that "genetic clock" to estimate when those early Europeans would have arrived. Depending on how large a group they assume headed west, they come up with two time ranges - either between 36,000 and 23,000 years ago or between 17,000 and 12,000 years ago.

X definitely did not arrive in America with the European explorers of the last 500 years or so. European X and American X are different enough that scientists say they must have diverged thousands of years ago, long before the age of exploration introduced European genes to the New World. Scientists have also done some testing on pre-Columbian Native American skeletal remains from before 1300, and found haplogroup X in the same proportion it's present in modern Native American populations.

There's more work to do on mtDNA, in both the Americas and Asia. Critics argue that people with haplogroup X could have migrated eastward from Europe to Asia and then across the Beringia land bridge; their descendents in Asia could have subsequently died out, so we don't see their traces in modern populations. Scientists hope to answer this question by sequencing the Mongolian haplogroup X mtDNA to see if it's an intermediate form between European X and Native American X.

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