As every schoolchild knows, the dino-saurs were wiped out in an instant, when a rock from outer space hit what is now southern Mexico. That happened 66m years ago, at the end of the Cretaceous period. Well-informed schoolchildren also know that this mass extinction was neither unique nor the biggest. The geological record speaks of four others since animal life became complex at the beginning of the Cambrian period 541m years ago. What neither these clever schoolchildren nor anyone else knows, however, is whether these extinctions had similar causes. But evidence is accumulating that the biggest extinction of all, 252.3m years back, at the end of the Permian period, was indeed also triggered by an impact. Nevertheless, though the trigger was the same, the details are significantly different, according to Eric Tohver of the University of Western Australia.
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