Living organisms incorporate carbon from the environment throughout their lives.

When they die, they stop incorporating new carbon, and the carbon-14 that is already in their tissues begins to decay at a known, consistent rate.

By measuring how much carbon-14 remains relative to the stable carbon-12, laboratories can calculate, within a statistical range of uncertainty, how long ago the organism died.

The technique was developed in the late 1940s and refined over subsequent decades.

By the 1990s, accelerator mass spectrometry had made it possible to radiocarbon date very small samples — a few milligrams of material — with high precision.