WIRED, December 20, 2007
About 470 million years ago, back when our ancestors were jawless fishes and the land was ruled by insects, Earth was pounded by a series of enormous meteorites. The traces of that hammering still survive today in ancient rocks in southern Sweden and central China, where scientists have found exotic mineral grains found only in meteorites.
By measuring the amount of the grains in the rocks, the scientists calculated the rate of meteorite impacts jumped by a factor of 100 around 470 million years ago. A number of the impacts were big enough to leave 20-mile craters. The energy unleashed was 10 million times greater than the energy in the nuclear bomb dropped on Hiroshima.
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