From the tundra of Alaska to the plains of central Mexico, from islands off California to the Atlantic coastline, mammoths trumpeted and bellowed across North America. Paleontologists traditionally have divided all these Ice Age pachyderms into at least three species—and perhaps as many as four or five. This division was based on differences in teeth and bone. But these aren’t the only clues; mammoths also left remnants of their genes, and this DNA tells a different story. Where there were once multiple mammoths, there may have only been one.