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