The pounding hooves of buffalo stampeding across the plains is an enduring symbol of the American West. Some paleontologists have long blamed humans for the beasts' precipitous decline. But new DNA evidence suggests that climate, not hunters, was to blame, at least initially.
Once numbering in the tens of millions, 1-ton, shaggy-headed bison eventually dwindled to less than 1000, hunted down for sport, hides, and meat during the 1800s. But their numbers also shrank much earlier in bison history. To check out the hypothesis that humans were the sole authors of the bison's early decline, Alan Cooper and Beth Shapiro of Oxford University in the U.K. and colleagues obtained ancient DNA from 442 bison fossils found in North America, Siberia, and China. For each specimen, they sequenced 685 bases from the fastest-mutating part of the animal's mitochondrial genome and then assessed the genetic diversity of ancient herds. The researchers also obtained radiocarbon dates on 220 samples to help piece together bison history.
Bison in North America spread southward, some as far as Mexico, 100,000 or more years ago, the researchers report in the 26 November issue of Science. Beginning approximately 37,000 years ago, the bison began to decline, perhaps because of climate and habitat changes associated with the developing ice age. To make matters worse, about 22,000 years ago, the expanding glaciers cut the northern group off from their southern kin. By the time the last glaciers receded, some 8000 years later, genetic diversity in the northern bison had plummeted. It never recovered completely--probably, the team concludes, because changes in habitat, particularly forest growth, kept populations small and isolated from the southern herds, which had less severe declines in diversity.
Such conclusions have elicited at least one strong reaction. "I think the interpretation is overblown and not supported by the data," says John Alroy, a paleobiologist at the University of California, Santa Barbara. He points out that other data suggest that bison in many places have weathered dramatic shifts in climate just fine. Therefore, Alroy claims, it must have been human intervention that caused local extinctions and an overall decline in bison. But John Pastor, an ecosystem ecologist at the University of Minnesota, Duluth, says the new work adds an important perspective to the debate: "What [Shapiro] is getting people to think about is that it's not one factor" that pushed these mammals toward extinction.