The geologic time scale divides the 4.6 billion years of earths history into hierarchy of time periods. Every layer of rock corresponds to a specific time in the history of the formation of the Earth. The Precambrian era began with the formation of the Earth; it was followed by the Paleozoic, Mesozoic, and Cenozoic eras. Each of these eras is divided into periods, the periods into epochs, and epochs into ages. Paleontological studies and isotopic dating methods are used to determine the time divisions.