According to scientists, there have been five prior mass extinctions in the past 450 million years. The last was 65 million years ago, when the dinosaurs and tens of thousands of species disappeared, likely as a result of a comet or large asteroid hitting the Earth. Theres no great mystery about the cause of the sixth extinction. Humans have dramatically altered the ecosystems of the Earth, says Stuart Pimm, a leading conservation biologist at Duke University.
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