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…showed last 75 words of 918 total…time scale and determine if a certain specie evolved from another specie. Radioactive dating is the best method for determining the age of rocks and fossils on a scale of absolute time. All fossils contain isotopes of elements that accumulated in the organisms when they were alive. By determining an isotope's half-life which is the number of years it takes for 50% of the original sample to decay, it is possible to determine the fossil's age.