About one in a trillion carbon atoms is of the carbon-14. Although carbon-14 atom decays with a half-life rate constant, cosmic radiation gives rise to others, and thereby preserve the ratio of carbon 14 and carbon-12 within living organisms. The ratio of carbon 14 and carbon-12 is the same for all people, and this proportion is kept in the plants or animals. When a body dies, taking the carbon-14 from the atmosphere or from the plant or animal food ceases. Reasoning applies to plants, where photosynthesis does not occur. But the process of radioactive decay of carbon-14 already existing in the structure of dead bodies, whether animals or plants, continues.
If the isotope carbon-14 has a half-life of 5700 years, the amount of carbon-12 does not change even after the death of living organisms. Estimating the proportion of atoms of carbon-14 reported carbon-12 atoms in a sample taken from plant or animal remains unearthed in archaeological sites, scientists can estimate the age of the sample with a satisfactory accuracy, so the artifact or human remains, after case.