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"Extinction can come in the form of an asteroid strike, a volcano eruption or even a nearby supernova. But sometimes devastation follows a chain of tiny, almost imperceptible changes. Who (or what) is behind what some scientists call the sixth mass extinction?"^^<http://www.w3.org/2001/XMLSchema#string>
<http://science.howstuffworks.com/life/evolution/common-causes-extinction1.htm/printable>