Comet may have doomed mammoths

There's a new extraterrestrial suspect in the mysterious, highly debated disappearance of the woolly mammoth some 12,900 years ago. A team of two dozen scientists say the culprit was likely a comet that exploded in the atmosphere above North America. The explosions sent a heat and shock wave across the continent, pelted the ground with a layer of telltale debris, ignited massive wildfires and triggered a major cooling of the climate, said nuclear analytic chemist Richard Firestone of Lawrence Berkeley Laboratory, one of the scientists who presented the controversial new theory Thursday at a conference of the American Geophysical Union in Acapulco. At least 15 species, mostly large mammals including mammoths, mastadons, giant ground sloths, camels and horses, were wiped out about the same time. Firestone and his colleagues think some may have been killed by the explosions, and the rest died off after fires burned the vegetation they depended on. "It seems awfully coincidental that the mammoths died at exactly the same moment where we find this impact layer," said Allen West, a member of the team from GeoScience consulting in Dewey, Ariz. The scientists are bracing themselves for fiery reaction to their theory.

The extinctions were already a hotly debated event with scientists split between two theories. The leading theory is that man hunted the animals into extinction soon after arriving in North America, but some scientists think climate upheaval as the earth warmed up from the last ice age was the killer. Others think it was more of a one-two punch with climate change weakening the animal populations and hunters delivering the final blow. But the impact theory has the advantage that it would explain why the Clovis hunting culture disappeared along with the animals, said archeologist Douglas Kennett of the University of Oregon.