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Geologists have sorted earth’s history into different geologic time scale. They just clubbed it into time scales by observing the major changes in the life forms that existed on the earth. Inferring to the fossils found in rocks the time scale was calculated.

First era single cell animals were formed then formed the complex animals.Then Mesozoic era where it is known as the era of dinosaurs. Then the evolution of different species of life during Cenozoic era. These major changes found through fossils helped them to form the GTS.