"Over the past half billion years, there have been at least twenty mass extinctions. Five of these—the so-called Big Five—were so devastating that they're usually put in their own category. The fifth, the end-Cretaceous event, which occurred sixty-five million years ago, exterminated not just the dinosaurs but seventy-five per cent of all species on earth. Once a mass extinction occurs, it takes millions of years for life to recover, and when it does it's generally with a new cast of characters. In this way, mass extinctions have played a determining role in evolution's course. It's now generally agreed among biologists that another mass extinction is under way. If current trends continue, by the end of this century as many as half of earth's species will be gone."
"The Sixth Extinction"
Source: New Yorker, 05/28/2009