Mammoth mystery.

What caused woolly mammoths to vanish from the planet? Many scientists blame the animals' extinction on overhunting by humans. But one scientist says humans are off the hook.

These hairy, elephantlike animals once roamed most of northern North America and Siberia. Evidence suggests that mammoths disappeared roughly 11,000 years ago, around the same time that humans first moved into North America. This fact, and other evidence, led some scientists to suggest that these people hunted the mammoth to extinction.

But Dale Guthrie, a paleontologist at the University of Alaska, thinks otherwise. Guthrie calculated the ages of hundreds of fossilized mammoth bones, and found that mammoth populations had been dwindling before humans arrived in the region.

Instead, Guthrie blames the animals' demise on a warming climate. The mammoths' extinction coincided with the end of a glacial period (time when Earth's climate was cool and large sheets of ice covered much of Earth's surface). As the climate warmed, new types of plants replaced the cold-weather grasses that woolly mammoths ate. The mammoths may have starved because their digestive systems were not adapted to break down the new plants. At the same time, better-adapted animals moved in and pushed the mammoths out.

SALAD BAR: Woolly mammoths were herbivores that ate low-lying grasses and other plants.