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By Tim Radford
Think of it as the day a comet set the earth on fire. Researchers have evidence of widespread and devastating forest fires around half the world—a blaze to blot out the light of the sun—and all of it at a geological boundary called the Younger Dryas, 13,500 years ago.
The evidence, they say, supports the hypothesis that planet Earth sailed through a cloud of shattered cometary dust and stones, and the atmospheric violence that followed was enough to set light to accumulated forest timber, peat and grasses across the Americas, Europe and western Asia.