The fundamental principle behind radiocarbon dating is simple: plants – and by extension animals – take in carbon from the world around them, including a natural amount of radioactive carbon-14 along with carbon-12.
When a creature dies, that absorption stops, and the carbon-14 starts to decay and become nitrogen-14 while the amount of carbon-12 does not change.
By measuring the amount of carbon-14 left relative to carbon-12, researchers can calculate how long ago the creature died.
The technique is used on everything from human bones to clothing, wood and seashells.
In The Apple Tree 