And if the temperature is high enough, these processes occur so rapidly that no DNA survives longer than a few thousand years.

Under the right conditions, though, degradation can be slowed dramatically.

Cold temperatures reduce the rate of all these chemical processes.

Dry conditions limit the water available for hydrolysis and reduce bacterial activity.

And if the DNA is physically protected — enclosed in a dense, mineral-rich matrix that keeps water and bacteria at bay — it can persist for surprisingly long periods.

The petrous bone, part of the temporal bone of the skull, is the densest bone in the human body.

It houses the cochlea and the semicircular canals of the inner ear, and it is built of some of the most compact bone tissue in the skeleton.