Around 13,000 years ago, the world's climate began to change. Seas rose, glaciers retreated and ecosystems began to transform. At roughly the same time, humans arrived in North America, perhaps attracted by migrating game or newly hospitable land. Over the course of the next few millennia a host of indigenous large-bodied mammals, such as the mammoth, died out. Scientists have long debated whether climate warming or human hunting brought about this megafauna extinction. New radiocarbon dating results support the environmental explanation.

Arctic biologist R. Dale Guthrie of the University of Alaska-Fairbanks, compiled radiocarbon dates for the permafrost-preserved fossils of six species--mammoths, horses, bison, moose, wapiti and humans--found in Alaska and Yukon Territory. The former two disappeared from the continent around 12,000 years ago as the latter four multiplied and spread.

He found that the horse Equus ferus had been declining long before humans arrived and disappeared a full 1,000 years before mammoths. This knocks out the so-called keystone theory, which holds that humans hunted the mammoths to extinction, causing a change in vegetation that subsequently precipitated other extinctions. And the mammoth's persistence over the next 1,000 years argues against precipitous overhunting.

A change in vegetation, however, does seem to hold the key to understanding this radical transformation, Guthrie argues. Prior to the warming, this geographic area lacked trees and provided only sparse forage. This would have given mammoths, horses and other related species a competitive advantage, because they can wrest sufficient nutrients from a high volume of low quality feed. But as the climate shifted, the so-called mammoth steppe became the environment we recognize today, characterized by shrubs, tundra and forests. This type of forage favors grazers such as bison, wapiti and moose. There are no signs of these species in the region before 13,000 years ago, but they appear to have proliferated rapidly thereafter.

"Archaeological refuse clearly illustrates the crucial role [in human colonization] of large mammal (at least bison and wapiti) resources as well as the increasing numbers of migratory waterfowl and salmonids in the Holocene," Guthrie writes in a paper published today in Nature. "These new data indicate that humans might have been not so much riding down the demise of [the] Pleistocene mammoth steppe as they were being carried into [the area] on a unique tide of resource abundance." In other words, at least in Alaska and the Yukon, climate change doomed the mammoths, but allowed humans, bison and other species to prosper.