May 11, 2006 — Recent evidence suggests people may not deserve the blame for the wave of animal extinctions — which included the woolly mammoth, wild horse Equus ferus, and saber-toothed tiger — that arrived with the end of the last Ice Age 10,000 years ago.

Many experts have argued that over-hunting caused by human expansion imperiled these species and others. As woolly mammoth numbers declined in the northern latitudes, the theory goes, vegetation patterns changed to favor small animals over big grazers and their predators.

But the authors of a new study say the die-offs were due to a more complex sequence of events between 13,000 and 10,000 years ago, when the chilly Pleistocene yielded to the balmy Holocene era of today.

The study, published in the current issue of the journal Nature, involves the radiocarbon dating of more than 600 bones from bison, wapiti, moose, humans, wild horses, and mammoths recovered from Alaska and the Yukon Territory.

The fossils suggest that people entered the region around 12,300 years ago, apparently crossing a land bridge over the Bering Strait.

But the wild horse Equus ferus had already died out by the time the first people arrived, and the mammoth was in decline, only surviving for 500 more years.

At the same time, bison were becoming more plentiful and moose and wapiti were appearing in big numbers.

What this and samples of preserved pollen suggest is that a naturally occurring climate shift drove changes in these animals' nutrition.

Animals such as today's elephants, horses and rhinoceroses — and in the past, mammoths, wild horses and rhinos — have a large intestinal pouch that allows them to survive by eating large quantities of low-quality forage.

In colder times, these creatures would have thrived on the region's endless sedge.

But during the Pleistocene-Holocene transition, summers grew warmer and wetter. This thawed swathes of the frozen, featureless northern landscape into lakes, bogs, forests, and tundra where tree willows and meadowland grasses grew.

The new terrain favored wapiti and bison, which became the dominant species and helped nourish the human immigrants.

The conventional wisdom is that, at the turn of the Ice Age, the presence of humans dramatically influenced large mammal species, but it was probably the other way around, argued author Dale Guthrie of the University of Alaska at Fairbanks.

"Humans might have been not so much riding down the demise of the Pleistocene mammoth steppe as they were being carried into (Alaska-Yukon Territory) on a unique tide of resource abundance," he wrote.