Radioactive carbon dating and radiometric dating
These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth's surface is moving and changing.
As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils.
This is not entirely true and it is necessary to readjust the time and make corrections. When the remains to date are very old, the nuclei of carbon-14 become so rare that the observation of their decays becomes impractical. This is done in facilities designed for this purpose, made of a mass spectrograph associated with a small accelerator.
This radioactive isotope of carbon is present in the atmosphere in trace amounts, and in chemical processes is indistinguishable from normal carbon 12.
As a result, animal and plant life regularly assimilate carbon 14 atom together with the usual carbon 12.
All living beings assimilate carbon dioxide molecules, a fixed but very small fraction of which contains carbon 14.
This assimilation stops upon the death of the organism, thus halting the absorption of any more carbon 14.