The primary method used to determine the age of ancient fossils is radiometric dating, particularly using the decay of isotopes such as carbon-14 or potassium-40. This technique involves measuring the amount of a radioactive isotope present in the fossil and comparing it to the stable isotopes. Atomic decay occurs at a predictable rate, allowing scientists to estimate the age of the fossil and understand the timeline of biological evolution. This method has been fundamental in paleontology and anthropology.