The rate at which C atoms, half of them will decay in 5730 years.
Since this rate is slow relative to the movement of carbon through food chains (from plants to animals to bacteria) all carbon in biomass at earth's surface contains atmospheric levels of C is present at atmospheric levels, the molecule must derive from a recent plant product.
Along with hydrogen, nitrogen, oxygen, phosphorus, and sulfur, carbon is a building block of biochemical molecules ranging from fats, proteins, and carbohydrates to active substances such as hormones.
Atoms of the same element that have different numbers of neutrons are called isotopes.
As a result it is always undergoing natural radioactive decay while the abundances of the other isotopes are unchanged.