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Subject: [DNA] NYTimes.com Article: New World Ancestors Lose 12,000 Years
Date: Fri, 25 Jul 2003 18:59:25 -0400 (EDT)
This article from NYTimes.com
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This is a column by Nicholas Wade on the same subject as the BBC item a few days ago. Note that the technical article will appear in the American Journal of Human Genetics.
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New World Ancestors Lose 12,000 Years
July 25, 2003
By NICHOLAS WADE and JOHN NOBLE WILFORD
Scientists studying the genetic signatures of Siberians and
American Indians have found evidence that the first human
migrations to the New World from Siberia probably occurred
no earlier than 18,000 years ago.
The new estimate undermines arguments for colonization as
far back as 30,000 years ago, but reinforces archaeological
findings and a linguistic theory that most American
languages belong to a single family called Amerind.
The genetic evidence fits neatly, for example, with the
discovery of a human campsite in Chile, which is apparently
15,000 years old, and with the well-established presence of
big-game hunters in North America, starting 13,600 years
ago. The few sites with possibly older human traces have
yet to gain wide acceptance among scientists.
By studying the DNA of living Siberian and American Indian
populations, geneticists had previously been able to see
traces of at least two early migrations from Siberia. But
it has been hard to put a date on when the first people set
foot in the Americas, for lack of a suitable marker in the
Y chromosome.
After much search, a team of geneticists has now detected a
change in the DNA sequence of Siberian men's Y chromosomes
that took place just before the first of the two migrations
into the Americas. They estimate that the DNA change,
called M242, occurred 15,000 to 18,000 years ago, meaning
the Americas must first have been occupied after that date.
The DNA change is not in a gene and makes no known
difference to the men who carry it.
The new result, to be published in the American Journal of
Human Genetics, is by Dr. Mark Seielstad of the Harvard
School of Public Health, Dr. R. Spencer Wells of the
University of Oxford and other colleagues.
The migration was probably by land because at that time the
world's sea level was much lower and a land bridge, known
as Beringia, stretched across what is now the Bering Strait
between Siberia and Alaska. Also, people bearing the same
genetic marker, called M3, live on either side of the
former bridge, suggesting it was the means of passage.
Beringia sank beneath the waves some 11,000 years ago as
the glaciers of the last ice age melted. The second
migration seen by the geneticists seems to have occurred
some 8,000 years ago and was presumably by boat, as the
land bridge had long since vanished.
The date based on the new marker is important because it
sets an earliest limit on the colonization of America,
something that archaeologists find hard to do because they
cannot be sure there are not sites they may have missed.
Hitherto some archaeologists have argued that people
reached the Americas as long as 30,000 years ago. This date
received some genetical support last year in a study by Dr.
Douglas Wallace, now of the University of California at
Irvine, who matched up male migrations from Siberia with
the female migrations that he and colleagues had worked out
earlier. The female migrations are traced by analyzing a
genetic element in every cell called mitochondrial DNA.
Based on the mitochondrial DNA of the women descended from
those in the first migration, Dr. Wallace estimated it
occurred 20,000 to 30,000 years ago. Dr. Spencer said in an
e-mail message that mitochondrial DNA was hard to date
accurately and often gave dates that were too old. The Y
chromosome is a better genetic clock, if a suitable marker
can be found, he said.
Dr. Wallace did not respond to e-mail requests for
comments.
The new date derived by Dr. Seielstad and Dr. Spencer may
strengthen the hand of linguists who argue that all
American languages fall into three groups, known as
Amerind, Na-Dene and Eskimo-Aleut, with Amerind being by
far the largest. Most linguists dispute that
classification, saying languages change too fast to allow
any very ancient relationships to be discerned. But if the
first humans arrived in the Americas only 18,000 years ago,
efforts to find links between present languages may seem
more plausible.
"If they entered more recently, it is not such a stretch to
say you can see a linguistic relationship," Dr. Wells said.
The new archaeological results seem compatible with the
younger date adduced by the geneticists. Radiocarbon dating
revealed that a occupation site in Siberia was only 13,000
years old and thus too recent to be a critical link in the
first migrations, as had been supposed.
The site on the Kamchatka Peninsula of Russia, previously
dated at 16,800 years old, was thought to be a way station
at the western edge of Beringia, a point of departure for
migrants either across the frozen land or by sea along the
coasts. The new research challenges the conventional idea
that this was the specific site from which people crossed
into America, but does not exclude the possibility that
they did so from other sites.
Researchers, led by Dr. Ted Goebel of the University of
Nevada at Reno, reported the redating of the Siberian site
at Ushki Lake on the Kamchatka Peninsula in today's issue
of the journal Science. The other authors were Dr. Michael
R. Waters of Texas A&M University and Dr. Margarita Dikova,
an archaeologist and widow of Dr. Nikolai Dikov, who
discovered the site in 1964.
The initial radiocarbon analysis was apparently based on
contaminated samples, the researchers said. The
13,000-year-old date, nearly 4,000 years younger than
previously thought, effectively removed the site as a way
station for the first migrants to America, they concluded.
For most of the last century, the peopling of America was
a story of big-game hunters trekking across the Bering land
bridge in the last ice age, spreading across North America
and within 1,000 years or so reaching the tip of South
America. Those who left the most durable traces, fluted
projectile points, were the Clovis people, named for the
town in New Mexico where their artifacts were first
uncovered.
The journal quoted Dr. David J. Meltzer, an archaeologist
at Southern Methodist University, as saying the new finding
"removes what was, until now, the critical link in the
chain connecting Clovis to Siberia."
When people first occupied the Ushki Lake site, Clovis
hunters had already been killing mammoths in North America
for some 600 years and groups of hunters had left their
mark at Monte Verde, Chile, 3,000 years earlier.
Radiocarbon dates are lower than calendar dates and they
become increasingly so the farther back one goes in time.
If the Ushki site is only 13,000 years old, Dr. Goebel
said, the oldest place in the Bering region with human
traces now is Broken Mammoth, a 14,000-year-old site in
central Alaska.
"It means we have even less evidence than we had before,"
Dr. Goebel said.
http://www.nytimes.com/2003/07/25/science/25HUMA.html?ex=1060173965&ei=1&en=20e74521054cb014
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