Living things constantly consume carbon—through photosynthesis, for plants, and for animals, ingestion of those plants. The atmospheric ratio of carbon-14 to regular carbon-12 remains consistent at one part per trillion, so if something is alive, one-trillionth of its carbon atoms will be C-14. But once a plant or animal dies, its carbon-14 is no longer replenished. C-14 is radioactive and unstable, with a half-life of 5,730 years, which means that half the atoms will turn back into nitrogen over that period. That rate of decay is key to gauging age.
Story: Julie Rehmeyer, Wired | Photo: Mario Hugo, Wired