DALLAS, TEXAS—A team led by David Meltzer of Southern Methodist University has checked the accuracy of the dates obtained for 29 archaeological sites said to provide evidence of a cosmic collision thought to have triggered the Younger Dryas, a 1,300-year-long period of freezing temperatures that began at the end of the last Ice Age, some 12,800 years ago. Meltzer found that the dates for only three of the 29 sites, which include Clovis sites in North America, plant-cultivating hunter-gatherers in Syria, and sites in Greenland, Germany, and Belgium, fall within the onset of the Younger Dryas. “The supposed Younger Dryas impact fails on both theoretical and empirical grounds,” Melzer told Science Now.