During the last Ice Age, shaggy mammoths, woolly rhinos and bison lumbered across northern Siberia. Then, about 10,000 years ago — in the span of a geological heartbeat, or a few hundred years — the last of them disappeared.

Many scientists believe a dramatic shift in climate drove these giant grazers to extinction.

But two scientists who live year-round in the frigid Siberian plains say that man _either for food, fuel or fun — hunted the animals to extinction....