The carbon-14 in the organism’s tissues is no longer replenished, and the radioactive decay that is continuously transforming carbon-14 into stable nitrogen-14 begins to reduce the concentration of the isotope.

Because the rate of this decay is known — carbon-14 has a half-life of approximately 5,730 years — measuring the remaining concentration of carbon-14 in a sample and comparing it to the expected original concentration allows researchers to calculate how long ago the organism died.

For the Trail Creek tooth, the material subjected to radiocarbon dating was the collagen within the tooth’s dentine — the protein-containing organic matrix that forms the bulk of the tooth below the enamel surface.

Collagen is formed during the period of tooth development, which in the case of the Trail Creek incisor would have been during the first year of the child’s life.