(I'm already well-versed with the basics of nuclear chemistry).$^\ce$ is formed in the upper atmosphere from $^\ce$ so that the relative amount of $^\ce$ in the atmospheric $\ce$ is reasonably constant.
Since plants get most of their carbon from atmospheric $\ce$, the relative amount of $^\ce$ to $\ce$ is thus constant. Now a living organism based on carbon (like us) always has a stable amount of $^\ce$ in his body, because he has to eat and so there is a stable balance between $^\ce$, $^\ce$ and $^\ce$.
Now if that organism dies, it does not eat anymore, so there is no new income of $^\ce$.
He was later awarded the Nobel Prize for his efforts.
Carbon is a common element found in nearly everything.
Radiocarbon decays slowly in a living organism, and the amount lost is continually replenished as long as the organism takes in air or food.