An old, unusual tooth discovered in China reinforces the theory that Homo sapiens and an extinct human species, the Denisovans, swapped genetic material — and physical traits — thousands of years ago.

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The tooth, a molar with three roots, belonged to a jawbone found remarkably high in a mountain cave on the Tibetan Plateau. Anthropologists announced the discovery of the jaw, which predated modern human settlement of the region by 100,000 years, in May. A new paper, published last week in the Proceedings of the National Academy of Sciences, is the first to describe the jaw’s three-root tooth in detail.

Three roots in a lower molar are rare in modern humans. The overall prevalence in non-Asian people is about 3.5 percent. In Asian and Native American populations, though, the proportion of three-rooted molars rises to about 40 percent. Given this, scientists had predicted that the characteristic arose recently in human history, as people dispersed into Eurasia. (A person who lived in the Philippines almost 50,000 years ago had such a tooth.)