There are intriguing new clues in the mystery of how the woolly mammoth met its demise in North America more than 10,000 years ago.For decades, scientists have debated whether the giant, elephant-like beasts were driven to extinction by the arrival of overzealous human hunters or by global warming at the end of the Pleistocene era, the last great Ice Age. Some say it was a combination of the two.
Recently, a group of more than two dozen scientists offered a new explanation. They have found signs that a comet -- or multiple fragments of one -- exploded over Canada about 12,900 years ago with the force equivalent to millions of nuclear weapons. That unleashed, they said, a tremendous shock wave that destroyed much of what was in its path and ignited wildfires across North America.
Another group, with the help of DNA evidence extracted from mammoth bones, teeth and ivory, has for the first time identified two distinct genetic groups among mammoths. They found that one group had died out by 40,000 years ago for unknown reasons, leaving the second to continue until the species went extinct.
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