Cosmic impact linked to extinctions of Earth\'s large mammals

U.S. researchers say they\'ve linked a cosmic impact in Canada 12,900 years ago to a global climate shift that wiped out many of the planet\'s large mammals. Scientists at Dartmouth College said the cataclysmic event may have prompted humans to start gathering and growing some of their food rather than solely hunting big game. The impact of an asteroid or comet at the beginning of the Younger Dryas period marks an abrupt global change to a colder, dryer climate with far-reaching effects on both animals and humans, they said. In North America large animals including mastodons, camels, giant ground sloths and saber-toothed cats vanished, the researchers said, and their human hunters, the Clovis people, set aside their hunting spears and turned to a hunter-gatherer subsistence diet of roots, berries and smaller game. \"The Younger Dryas cooling impacted human history in a profound manner,\" researcher Mukul Sharma said. The assumption of a cosmic impact goes against the prevailing view that the Younger Dryas cooling period was due to a change in ocean currents that kept warm tropical waters from moving northward. Sharma and his colleagues argue analysis of spherules -- droplets of solidified molten rock expelled by the impact of a comet or meteor -- at Younger Dryas boundary later sites in Pennsylvania and New Jersey are identical to rock found in southern Quebec, where Sharma and his colleagues say they believe the impact took place. \"We have for the first time narrowed down the region where a Younger Dryas impact did take place,\" Sharma says, \"even though we have not yet found its crater.\" \"It may well have taken multiple concurrent impacts to bring about the extensive environmental changes of the Younger Dryas,\" he said. \"However, to date no impact craters have been found and our research will help track one of them down.\"