To be adequately preserved, an organism must be in an environment where it is protected from oxidation and bacterial decay. Fossils, starting as from far back as 3 bya, indicate that life evolved through great stretches of time and diversified. Sediments are deposited gradually in a flat layer and are spread over a large area. Stratigraphic Illustration from Berkeley Geologic Time Scale . Developed in the 1800's from the relative dating of rocks and the fossils each layer contained.
Major boundaries indicate mass extinctions.
Index fossils - an assemblage of fossils that characterize a particular rock unit. Organisms have evolved and gone extinct through time. Fossil content can be used to help determine age of rock, and to correlate rocks from different localities. This method uses naturally-occurring radioactive isotopes. The ratio of parent isotope to daughter isotope in the rock reveals the number of half-lives, or length of time in years, that has elapsed.