A molar from Denisova Cave, Siberia, comes from the mysterious people to which the cave gives its name, increasing the number of Denisovan fossils to just four. Moreover, while the other fossils are of similar ages, this one is substantially older. Given their role in our evolution, the tooth is one of the most precious artifacts known to science.

In March 2010, an approximately 40,000-year-old finger bone fragment was found in Denisova Cave. Although the cave was at times occupied by both Neanderthals and modern humans, DNA showed that this was a new species, or at least a previously unknown subspecies of humanity. Comparison of this DNA with that of people living today showed the Denisovans interbred with our ancestors, and Melanesians and Indigenous Australians have inherited 2-5 percent of their genome from these encounters.

That's an important part of the human heritage to come from a group about which we know so little – just one finger bone, two molars, and some DNA fragments from cave sediments. Now in Science Advances, first author Dr Viviane Slon of the Max Plank Institute for Evolutionary Anthropology and colleagues have announced the extraction of DNA from a molar found in 1984, showing it too came from a Denisovan. The different branches of humanity that occupied the cave had teeth too similar to determine their origins from shape alone.

Other Denisovan fossils were dated as between 30,000 and 50,000 years old, or lack a clear date. The new find, however, came from a much deeper layer in the cave, indicating the owner lived much earlier. Based on radiothermoluminescence dating of the layer in which it was found, the molar was 128,000-227,000 years old, a figure broadly supported by the genetic clock of its mitochondrial DNA.

This DNA indicates Denisova 2, as the tooth was named, belonged to a woman. Her genetic similarity to the later inhabitants of the cave confirms the view that Denisovans had very little genetic diversity, at least in the Altai mountains, where the cave lies. Sadly, the DNA was too degraded to answer many of the other questions we have about how Denisovans related to other humans, including the timing of their interbreeding with Neanderthals.

Denisovans were more closely related to Neanderthals than to modern humans, and are thought to have split from them sometime between 190,000 and 470,000 years ago. Although they overlapped geographically in the Altai mountains with Neanderthals and interbred with them, their genetic legacy suggests they probably extended further east and south.

Today, Denisovan genetics allow Tibetans to live at high altitudes, and strongly influence the immune systems of Pacific and southeast Asian peoples, including contributing to high rates of allergies.