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.

The comet blast and firestorm could have dealt that deathblow to the mammoth and more than 15 other species of large mammals, or "mega fauna," including the mastodon, the saber-toothed cat, the American camel and the giant ground sloth, the other researchers said. They presented their findings last month at a meeting of the American Geophysical Union in Acapulco.

 Source : San Francisco Chronicle

"The shock wave would have spread across the whole continent," said Richard B. Firestone, a nuclear chemist at Lawrence Berkeley National Laboratory who helped do the research. "This event was large enough to directly kill most everything instantly. Those that survived would have found their food sources devastated, their water polluted, all kinds of things that would have made it difficult to go on much longer."

In more than 20 locations from Arizona to Canada and California to the Carolinas, the scientists found glasslike carbon, microscopic diamonds, enriched iridium and other materials that they say are indicative of an extraterrestrial impact lying in a sediment layer corresponding to the time period. Just above that layer they found charcoal soot, decayed plant life and other debris consistent with widespread burning.

Above that, the remarkable thing is what they did not find: further evidence of the mammoth and the other large animals.

"The mammoths come up to the line and not beyond it," said James Kennett, a marine geologist and professor emeritus at UC Santa Barbara. "At some sites, the black layer with impact material shrouds the bones."

The explosion may also have spelled the end for the Clovis culture, the prehistoric North Americans who hunted with distinctive stone spearheads that have been found in the bones of the fossils of mammoths and other animals, researchers said. While humans as a species survived the cataclysm, the Clovis culture and its relatively advanced stone tools did not endure.

"At many Clovis sites, like in Arizona and New Mexico, you get the Clovis tools up to the impact layer, and then they never go beyond it," Kennett said.

The comet theory, while adding a new twist to the tale, is not wholly incompatible with earlier explanations for how the mammoth met its end. Researchers said it is possible that the Ice Age beasts, which stood about 9 feet tall and weighed three tons, struggled as the climate warmed, that increasingly skilled and numerous human hunters dramatically thinned their numbers, and that the exploding comet finished them off.

"Our theory is that if this event had not happened, that mammoths would still most likely -- not certainly, but most likely -- be wandering around North America now," said Allen West, a retired geophysicist who is a leader of the research team. "Almost certainly, humans hunting animals can have a major effect on populations. It seems like there was, in a sense, a perfect storm going on -- of overkill, the comet, climate change, possibly disease. I don't think this theory negates any of the other theories. It's just one more of a mix of things that were absolutely lethal to these animals."

The scientists have not published their findings, although two papers are under review by the National Academy of Sciences, Kennett said. Firestone said the lack of a distinctive impact crater -- the airborne explosion did not leave one -- has generated controversy. Even some who accept that the explosion occurred question whether it was the definitive blow, he said.

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