250 Million years ago, about 90 percent of ocean life and 70 percent of land species were wiped out in a relatively short geologic time span. A new study appearing today in Science magazine suggests that this die-off was triggered by a comet or asteroid collision similar to the one that many believed later killed off the dinosaurs. The lead author of the study, Luann Becker of the University of Washington, analyzed ancient deposits in China and Japan and discovered traces of fullerenes, or "Buckyballs," a carbon that is shaped like a volleyball with a hollow cavity. Becker and her colleagues found a helium isotope believed to be of extraterrestrial origin inside that cavity. The collision of the asteroid or comet, estimated at three to seven miles across, is not believed to be directly responsible for the extinction, but researchers argue that it may have triggered a series of events, including massive lava flows and changes in the climate and sea levels, which in turn led to wholesale species extinction.