Roughly 12,900 years ago a global-cooling anomaly contributed to the extinction of 35 mammal species, including the mammoth. In some areas, average temperatures may have dropped as much as 15 degrees Celsius (27 degrees Fahrenheit). New evidence, in the form of diamonds several nanometers wide, supports a theory proposed last year that a comet collision or a similar explosive event threw up debris and caused the cooling.

Nanodiamonds occur only in sediment exposed to high temperatures and pressures, such as that produced by a cometary impact. Researchers uncovered them in six sites in North America: Murray Springs, Ariz.; Bull Creek, Okla.; Gainey, Mich.; Topper, S.C.; Lake Hind, Manitoba; and Chobot, Alberta. Preliminary searches in Europe, Asia and South America have also turned up similar finds in sediments of the same age, showing that the event had global reach. But it was definitely not as large as the one that wiped out the dinosaurs 65 million years ago. The study landed in the January 2 Science.