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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...

The Clovis people, known for their distinctive spear points, likely were not the first humans in the Americas, according to research placing their presence as more recent than previously believed. Using advanced radiocarbon dating techniques, researchers writing in the journal Science on Thursday said the Clovis people, hunters of large Ice Age animals like mammoths and mastodons, dated from about 13,100 to 12,900 years ago. That would make the Clovis culture, known from artifacts discovered at various sites including the town of Clovis, New Mexico, both younger and shorter-lived than previously thought. Previous estimates had dated the culture to about...