THE FOSSIL RECORD IS clear. Time and again during the nearly 4 billion years that life has existed on Earth, it has been assailed by global catastrophes that have caused the wholesale extinction of animals and plants. Over the past decade evidence has been mounting that many of these calamities were caused . not by long-term climatic changes, volcanism or disease, but by large asteroids or comets smashing into Earth.
These impacts blasted enough dust into the atmosphere to shroud the entire globe for months on end, blocking sunlight and causing temperatures to plummet. In the cold and dark, plants and...
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