A comet or an asteroid strike about 65 million years ago is the favored explanation for why dinosaurs vanished from the earth so abruptly after having dominated the planet for the preceding 70 million years. The tremendous plume of smoke and dust thrown into the atmosphere by the space intruder's impact, equivalent to perhaps a million hydrogen bombs, would have blocked out sunlight for months. The globe would have gone into a dark, deep freeze, killing first the plants and then the giant lizards that fed on them, directly or indirectly -- paving the way for mammals, and eventually humans, to...
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