For rare or irreplaceable specimens like ancient human remains, the ability to date from a small sample is essential.

Radiocarbon dating measures the ratio of carbon-14 to carbon-12 in an organic sample.

Living organisms maintain a ratio that reflects the atmospheric abundance of carbon-14, which is produced continuously by cosmic rays.

After death, the carbon-14 in the sample decays at a known rate (with a half-life of approximately 5,730 years), while the carbon-12 remains stable.

By measuring how much carbon-14 has decayed relative to a known standard, the elapsed time since death can be estimated.

The complication is that atmospheric carbon-14 concentrations have not been constant over time.