Around 250 million years ago, at the end of the Permian period, life on the earth almost disappeared completely in the most devastating mass extinction of all time. Yet despite the magnitude of the event (nearly 70 percent of terrestrial species and 95 percent of marine species vanished), its cause has proved difficult to identify. Unlike the extinction that claimed the dinosaurs some 65 million years ago, the biological and environmental changes that characterize the Permian crisis seem to have unfolded gradually. Most earth scientists thus favored scenarios involving climate and sea-level fluctuations, a dwindling supply of oxygen in the oceans and severe volcanism.

More recently, however, indications that the Permian extinction proceeded far more swiftly than previously thought have surfaced. And earlier this year, researchers reported detecting in Permian rocks signatures of extraterrestrial molecules that most likely arrived on an asteroid or comet. Now findings reported in the September issue of the journal Geology are lending additional weight to the argument that, like the dinosaur-squelching extinction after it, the Permian catastrophe resulted from an extraterrestrial impact.

Geochemical and paleontological analyses of late Permian sediments from southern China, Kunio Kaiho of Tohoku University and his colleagues report, suggest that an asteroid or comet hit the ocean at the end of the Permian. This collision, they argue, prompted a massive release of sulfur from the earth's mantle to the ocean-atmosphere system, which in turn led to oxygen consumption and strong acid rain.

"Understanding the cause of this event is important because it represents the largest mass extinction and it led to the subsequent origination of recent [life] on Earth," Kaiho remarks. "We would like to clarify the paleoenvironmental changes and causes of the end Permian mass extinction in different places and of the other mass extinctions that occurred during the past 500 million years."