Age of fossils in the past was generally determined by radiocarbon method and other methods involving radioactive elements found in the rocks. More precise methods, which were used recently and led to the revision of the evolutionary periods for different groups of organisms, includes

(1) electron spin resonance (ESR) and fossil DNA

(2) study of carbohydrates/proteins in fossils

(3) study of carbohydrates/proteins in rocks

(4) study of the conditions of fossilization