From the daily exposure to radioisotopes of organisms, scientists discovered to carbon date fossils. In carbon dating, the age of organic matter can be determined from the relative proportions of C-12 and C-14 i.e. isotopes of Carbon (carbon 12 and carbon 14). This ratio changes over the years because carbon-14 decays and is not replenished as there is no exchange with the atmosphere. hope this helps you :)
