The Tibetan Plateau is one of Earth's harshest environments, encompassing nearly a million square miles of land thrust two and a half miles skyward, blanketed in cold, thin air that's difficult to breathe. Now, Chinese researchers have made a remarkable find on the roof of the world: the oldest signs of human activity in this demanding landscape.

Researchers led by Xiaoling Zhang, an archaeologist at China's Institute of Vertebrate Paleontology and Paleoanthropology, have found more than 3,600 stone artifacts in a part of the central Tibetan Plateau called Nwya Devu. The site is rich with black slate—not the ideal raw material for stone tools, but the best available for miles around. Whoever the toolmakers were, they took advantage of