carbon-14 dating. Article #14. Vol 2, pg 850.
Here’s how it works. Without knowing it, we’re all breathing in Carbon-14, a radioactive isotope of Carbon. While we’re alive, the amount of Carbon-14 in our bodies, as a percentage of the total Carbon, stays the same. After we die (and stop breathing), the Carbon-14 starts disappearing at a known rate while the plain old Carbon just sits there.
So if we grab a specimen of some dead plant or animal and look at the amount of Carbon-14 left in the sample, compared with the regular Carbon, we can figure out the percentage of Carbon-14 in his body that has decayed and then know exactly when the plant or animal died.
For example, if we look at Ötzi the Iceman and see that only 55% of his Carbon-14 is remaining, then we know that he died 5312 years ago, i.e. in 3300 BC.
“carbon-14 dating.” Encyclopedia Britannica. 15th ed. 2010. Vol 2, pg 850.