The ratio of normal carbon (carbon-12) to carbon-14 in the air and in all living things at any given time is nearly constant.
Maybe one in a trillion carbon atoms are carbon-14.
The story of this great change in the conception of the history of Earth is not a simple one.
The chronicle of this great change can be broken into five periods; ran from AD 1600-1700.
The carbon-14 atoms that cosmic rays create combine with oxygen to form carbon dioxide, which plants absorb naturally and incorporate into plant fibers by photosynthesis.