Carbon dating for idiots
This tendency to decay, called radioactivity, is what gives radiocarbon the name radiocarbon.The atmosphere contains many stable carbon atoms and relatively few radiocarbon atoms.
After about 50,000 years, the radiocarbon concentration remaining is too small to be measured for the purpose of radiocarbon dating. This is necessary to remove errors in raw radiocarbon dates caused by fluctuations in the amount of radiocarbon in the atmosphere in the past.
Thus, the ratio of radiocarbon to stable carbon in a living plant is the same as the ratio of radiocarbon to stable carbon in the atmosphere at any given time.
Animals (and humans) get their carbon atoms primarily from what they eat (i.e., plants).
Thus the ratio of radiocarbon to stable carbon in living animal tissue is also virtually the same as the ratio of radiocarbon to stable carbon in the atmosphere at any given time.
This ratio is the same for all organisms across the globe at a given time due to the mixing of the atmosphere mentioned above.