DNA degrades over time through bacterial action, chemical reactions with water and oxygen, and the physical fragmentation of the long molecular chains that carry genetic information.

In most burial environments, by the time thousands of years have passed, only short fragments of DNA survive — and most of what can be extracted is the DNA of the bacteria and fungi that colonized the remains after death, not the DNA of the human who left those remains behind.

The most reliable source of ancient human DNA has proven to be the petrous bone — the dense, compact portion of the temporal bone that houses the inner ear.

The petrous bone is the hardest bone in the human body, and its density creates a physical barrier that slows bacterial invasion and preserves the cellular material inside better than softer skeletal elements.