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Mega-Extinction? Blame the Weather
10 May 2006 (All day)
Climate change, not human hunters, drove mammoths and horses to extinction in Alaska and the Yukon Territory almost 12,000 years ago, according to a study in this week's issue of Nature. New dates on animal fossils nail down for the first time when horses and mammoths disappeared in the far north--and show that the mammals did not die off all at once when prehistoric hunters arrived. "There was no sudden impact," says paleobiologist Dale Guthrie of the University of Alaska in Fairbanks.
Guthrie's new report is at odds with a long-standing hypothesis that prehistoric hunters rapidly killed off mammoths and elephants at the end of the Ice Age about 11,000 years ago on several continents (Science, 8 June 2001, p. 1888 and ScienceNOW, 11 April 2005:). This "overkill" hypothesis has been difficult to test, however, because there have been insufficient numbers of well-dated fossils of large mammals to pinpoint precisely when they went extinct.
Guthrie collected 600 new dates on bones of mammoths, horses, bison, wapiti, and moose that had lived 9000 to 18,000 years ago in Alaska and the Yukon. The better resolution of new radiocarbon techniques showed that the animals did not go extinct at the same time, as would be expected if they were killed off in a human Blitzkrieg. Furthermore, the animals that humans hunted most--bison and elk--survived and even appeared to migrate with humans across the Bering land bridge into Alaska. "If the humans were super predators, why are the elk and bison surviving?" asks Guthrie.
Guthrie proposes instead that changes in the world climate at the end of the Ice Age 13,000 years ago altered the frigid landscape of the far north into a steppe covered with grasses, where mammoths, horses, and other grazers flourished for 1000 years. Just as humans first appear in the archaeological fossil record 12,000 years ago, the climate began to shift, turning the steppe grasses into plants toxic to grazers. Horses went extinct about 1000 years before the mammoths, which disappeared well after humans appeared on the scene in the far north, Guthrie reports.
The evidence convinces researchers who have long suspected that the human "overkill" hypothesis was too simplistic, at least for Alaska and the Yukon: "Guthrie shows very nicely that the horses go away, and then the mammoths go away--it is not synchronous," says archaeologist David Meltzer of Southern Methodist University in Dallas, Texas. "This is decidedly in opposition to the human overkill model."