Carbon 12 carbon dating
Archaeologists use the exponential, radioactive decay of carbon 14 to estimate the death dates of organic material.The stable form of carbon is carbon 12 and the radioactive isotope carbon 14 decays over time into nitrogen 14 and other particles.
So by measuring carbon 14 levels in an organism that died long ago, researchers can figure out when it died.
Carbon is naturally in all living organisms and is replenished in the tissues by eating other organisms or by breathing air that contains carbon.
At any particular time all living organisms have approximately the same ratio of carbon 12 to carbon 14 in their tissues.
The procedure of radiocarbon dating can be used for remains that are up to 50,000 years old.
Carbon-14 dating can be used on objects ranging from a few hundred years old to 50,000 years old.
The other assumption is the corollary of the first; the biosphere has the same overall concentration of radiocarbon as the atmosphere due to equilibrium.