The petrous bone is part of the temporal bone of the skull, a dense, extremely hard piece of bone that houses the inner ear.

Because of its density and its specific internal structure, the petrous bone tends to preserve ancient DNA better than other skeletal elements, even when the surrounding bones have degraded to the point of yielding little or no recoverable genetic material.

The decision to sample specifically the left petrous bone was a methodological choice designed to reduce the risk of sampling the same individual twice.

If remains from approximately 106 individuals were mixed or commingled in the chultún — if the bones from multiple people were intermingled in ways that made it difficult to tell which bone came from which person — then sampling randomly might risk taking two samples from the same individual and counting them as two separate people.