Did Comets Cause Ancient American Extinctions?Anne Casselman for National Geographic NewsMay 6, 2008 Debate has heated up over a controversial theory that suggests huge comet impacts wiped out North America's large mammals nearly 13,000 years ago. The hypothesis, first presented in May 2007, proposes that an onslaught of extraterrestrial bodies caused the mass extinction known as the Younger Dryas event and triggered a period of climatic cooling. The theory has been debated widely since it was introduced, but it drew new scrutiny in March at the annual meeting of the Society for American Archaeology annual meeting in Vancouver, Canada. Stuart...