The cave’s preservation conditions are unusually favorable for ancient organic material.

The constant cool temperature of the interior — which stays at a relatively consistent level year-round, shielded from the surface temperature variations that accelerate the decay of biological material — has meant that DNA has survived in the cave’s sediments and bone fragments at ages that would be impossible in warmer environments.

When Russian archaeologists began systematic excavations at Denisova Cave in the 1980s, they found what they expected from a site with a long history of human use: stone tools, animal bones, traces of ancient fires.

The cultural materials spanned a remarkable time depth — tens of thousands of years of human presence, layer upon layer.