Some researchers have also published evidence of a much earlier human presence in North America, including stone tools dated to as long as 30,000 years ago. But others have questioned whether the discoveries were really tools shaped by humans, and whether estimates of their age are correct. The new White Sands research is different because the prints were obviously made by people, study co-author Vance Holliday, an archaeologist and geologist at the University of Arizona, tells National Geographic.