http://www.nytimes.com/2009/12/22/science/22obtundra.html
The New York Times
December 22, 2009
Observatory
DNA Shifts Timeline for Mammoths Exit
By HENRY FOUNTAIN
Thousands of years ago in northwestern North America, large animal
species, among them the woolly mammoth and the horse, became extinct.
Among the proposed explanations for this is one known as the blitzkrieg
hypothesis -- that humans entering the region rapidly wiped the animals
out through overhunting.
The validity of that explanation, and others, depends in parts on the
timing of the extinctions. How many thousands of years ago did the
animals disappear?
Until now, the answer to that question has been 13,000 to 15,000 years
ago. But those dates come from the youngest reliably dated fossils that
have been found, and who is to say there aren't even younger fossils
out there?
A new study has come up with a far different answer, using a far
different technique.
Rather than dating actual fossils, the researchers analyzed DNA found
in permanently frozen sediments at a site on the Yukon River in central
Alaska. As they report in The Proceedings of the National Academy of
Sciences, they found evidence that mammoths and horses were still
around at least until 10,500 years ago, long after humans arrived.
Earlier studies had shown that DNA from animals' waste, skin cells and
hair could be preserved in permanently frozen sediments.
James Haile and Eske Willerslev of the University of Copenhagen and
colleagues analyzed samples taken from the frozen soil at various
depths, corresponding to about 8,000 to 11,000 years ago.
Since humans were known to arrive in the region at least 14,000 years
ago, the finding casts doubt on the blitzkrieg hypothesis.
Hunting may have contributed to the decline of these animals, the
researchers write, but it "did not deliver the deathblow."