ANSWER: Radiometric dating shows the rate of decay of radioactive material present in any objects. Scientists can use that data to find the absolute age of the fossil.
The radioactive carbon method is a method used to determine the age of the fossil based on the ratio of the number of atoms of stable carbon isotope 12C and the radioactive isotope 14C. During their lifetime, plants or animals exchange carbon with their environment, so the carbon-containing it will have the same percentage of 14C as well as the atmosphere. When a plant or animal dies, it ceases to collect 14C, but 14C within its biological material at that moment will continue to decompose. So, the ratio of 14C and 12C in residues will gradually decrease. Since the rate at which the 14C decay is known, the percentage of radiocarbon can be used to determine the time period that has passed since the sample was given to stop exchanging carbon with the environment (older sample - less 14C).
