US:Scientists are offering a new explanation for the extinction of the woolly mammoth , writes Christopher Leein Washington
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 death blow to the mammoth and more than 15 other species of large mammals, or "mega fauna," including the mastodon, the sabre-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.
"The shock wave would have spread across the whole continent," said Richard Firestone, a nuclear chemist at Lawrence Berkeley National Laboratory in California 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 glass-like 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 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 struggled as the climate warmed, that increasingly skilled and numerous human hunters dramatically thinned their numbers, and that the exploding comet finished them off.
Ian Barnes, a senior lecturer in biological sciences at Royal Holloway, part of the University of London said: "What we're now starting to think is . . . there is no single event that caused their extinction. What's important is that we seem to have the conditions for extinction set up a long time before the actual extinction occurs." - ( Los Angeles Times-Washington Post service)