ZACATECAS, MEXICO—Nature News reports that a team of researchers led by Ciprian Ardelean of the Autonomous University of Zacatecas has found possible stone tools dated to between 25,000 and 32,000 years old in Chiquihuite Cave in the Astillero Mountains. If correct, the discovery indicates that people arrived in North America at least 15,000 years earlier than previously thought. Ardelean thinks the well-insulated cave probably offered occasional shelter to early North Americans. “There must have been horrible storms, hail, snow,” he said. Critics point out that the tools may have landed in deeper cave layers through geologic activity or burrowing animals, but Ardelean explained that the oldest tools were found under an intact layer of mud. These artifacts, he added, have marks that suggest they were made by students learning to make tools from experts. And although tests of dirt in the cave failed to detect any ancient human DNA, and studies of modern genomes have not found genetic evidence of people living in North America more than 15,000 years ago, Ardelean said that such early groups did not survive to contribute genes to today’s populations. For more on evidence for the peopling of the Americas, go to "America, in the Beginning."