Over the past decade, the Denisova Cave in Siberia has yielded some of the most fascinating fossils ever found. To the naked eye, they are not much to look at — a few teeth, bits of bone.

But the fossils contain DNA dating back tens of thousands of years. That genetic material shows that Denisovans were a distinct branch of human evolution, a lost lineage.

At some point in the distant past, the Denisovans disappeared — but not before interbreeding with modern humans. Today, people in places like East Asia and New Guinea still carry fragments of Denisovan DNA.

One of the biggest obstacles to understanding the Denisovans is their age. Standard methods for dating these fossils have left scientists perplexed.