Scientists can determine how long ago an organism died by measuring how much carbon-14 is left relative to the carbon-12. Carbon-14 has a half life of 5730 years, meaning that 5730 years after an organism dies, half of its carbon-14 atoms have decayed to nitrogen atoms.

Why is carbon dating used to determine the age of fossils?

Because carbon-14 decays at this constant rate, an estimate of the date at which an organism died can be made by measuring the amount of its residual radiocarbon.