Carbon is found in all of our cells, including bone cells. A very small part of the carbon in our body is radioactive C-14, which does change to a nitrogen atom.
Carbon atoms such as we have in the living body are two kinds. C-14, thus, disappears, like sand running through the hourglass.
C-14 enters the air by the action of cosmic radiation.
Cosmic rays enter our Earth's atmosphere from outer space.
He would give his bone a "date" of 15,000 to 30,000 years because of the very low C-14.
He would do so because he believed all bones contained the present amount of C-14 when they died.
When their dating methods say that something lived 30,000 years ago, they sound certain.