The end of the Pleistocene epoch brought with it widespread extinctions of large mammals, such as saber-toothed cats and mammoths. Ancient bison, too, were threatened with elimination, but they managed to survive. The two leading theories of what caused the precipitous population drop focus on environmental shifts and pressure from human hunters. A genetic analysis published in the current issue of the journal Science lends support to the hypothesis that climate change was the culprit.

Beth Shapiro of Oxford University and her colleagues analyzed mitochondrial DNA from 352 bison fossils recovered from eastern and western Beringia (the landmass that includes Alaska, Canada and Siberia), North America, China and Russia. In addition, the scientists performed radiocarbon dating on 220 of the samples. They determined that the genetic diversity of the bison population dropped off drastically around 37,000 years ago. The timing of this decline correlates with environmental changes associated with the onset of the last glacial cycle, the team reports, whereas archaeological evidence does not support the presence of large populations of humans in eastern Beringia until more than 15,000 years later.

The authors suggest that their findings will help inform the debate about end-Pleistocene megafauna extinctions because they offer the first evidence of the initial decline of a population, rather than simply the resulting extinction event. The researchers do not rule out human intervention entirely, however, because some disputed archaeological evidence suggests a low number of humans may have been present at the time. Future studies with more samples from around the time of the Last Glacial Maximum, they say, should help clarify the course of extinction events.