According to British, U.S. and Russian scientists, woolly mammoths decreased in numbers and genetic diversity over tens of thousands of years. The results showed environmental factors, rather than human factors, caused the extinction of Mammuthus primigenius.

Woolly mammoths are believed to have first appeared on the Earth about 150,000 years ago. They were about five meters (16 feet) in length and around three meters (10 feet) in height at the shoulders. They evolved with thick and heavy shaggy hair (about 20 inches long), a layer of fat, small ears (only about one foot long), and a flap of skin across the anus, all to reduce loss of heat in order to adapt to cold weather conditions.

Mitochondrial DNA (deoxyribonucleic acid molecule outside of the cell nucleus) evidence from the frozen carcasses (including bones, teeth, and tusks) of 41 extinct mammoths found in northern Eurasia (specifically, Siberia) and northern North America (specifically, Alaska) were used.

The scientists found that the mammoths expanded from a small base some time before sixty thousand years ago. They then divided into two distinct genetic groups, one in Siberia and the other in Alaska, due to warmer climates when higher sea levels submerged the Bering Strait land bridge between Asia and the Americas. (The scientists did study one mammoth from Europe, indicating that a third distinct group was possible.). Then, about 100,000 years ago, the climate turned cold again and the land bridge became frozen again, allowing the two groups to intermingle again. About forty thousand years ago, the original Siberian group died out—leaving only the Alaskan group. Thus, a trend toward decreasing genetic diversity was largely responsible, according to the study, for the demise of the woolly mammoths.

Humans did aggressively hunt these animals. However, according to the study, the trend toward extinction was already progressing when humans came along—probably later than 20,000 years ago—and killed off the last small population of mammoths.