Humans cleared of killing off woolly mammoths

Climate shifts, not over-hunting,killed off the woolly mammoth andwild horse, a carbon-dating study suggests.

What caused the animals to become extinct at the end of the last Ice Age more than 10,000 years ago has been one ofprehistory's greatest whodunits. Biologists have often pointed the finger at over-hunting by expanding populations of humans.

Butnew radiocarbon dates give a more precise account of what happened at the time of the mass extinctions, and shift the focus to global warming.

Paleobiologist Dale Guthrie analyzed bone samples from bison, moose and humans, whichlived through the extinction period, andfrom wild horse and mammoth, whichdid not survive. Themore than 600 sampleswererecovered inAlaskaand the Yukon. He alsostudiedpreserved samples of pollen from the period.

He found that by the timeHomo sapiens started pushing into the region around 12,300 years ago,the wild horse had already died out and woolly mammoth werein decline.

Meanwhile,populations of bison, moose andwhite-rumped elk calledwapiti were increasing, saidGuthrie,professor emeritus withthe Institute of Arctic Biology at University of Alaska, Fairbanks.

By analyzingpollen samples, heconcluded that a naturally occurring shift in climate caused the animals to change their diet.

Like their modern cousins,the wild horses and the woolly mammoth of the past had a large intestinal pouch, or caecum, suited to feeding on low-quality forage on the steppe.

But asthe frozen landscape thawed,higher-quality grassesstarted to grow. Those grasses werefavoured by the bison and wapiti but were indigestible tothe mammoth, Guthrie suggested.

His results were published in Thursday's edition of the journal Nature.