Tucson, Arizona Friday, 25 July 2003
THE NEW YORK TIMES
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 earlier estimates that colonization occurred 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 with the discovery of a human campsite in
Chile that 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 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 searching, a team of geneticists 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 Mark Seielstad of the Harvard School of Public Health, 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 passage.
Beringia sank beneath the waves about 11,000 years ago as the glaciers of the
last Ice Age melted. The second migration seen by the geneticists seems to have
occurred 8,000 years ago and was presumably by boat, as the land bridge had
vanished.
The date based on the new marker is important because it sets an earliest limit
on the colonization of America.