When an organism dies, it stops incorporating new carbon.

The carbon-14 already present in its tissues begins to decay at a known, constant rate: every 5,730 years, half of the remaining carbon-14 disappears.

This decay clock — ticking reliably since the organism’s death — can be read by measuring the ratio of carbon-14 to carbon-12 in ancient organic material.

The lower the ratio, the older the material.

For the human remains at sites like Buckeye Knoll, the organic material most commonly used for radiocarbon dating is either bone collagen (the protein matrix that makes up much of bone) or tooth collagen.

Teeth are often preferred because the enamel and the proteins within the root are relatively well sealed from external contamination.