Do you know what the Geologic Time Scale is? It is the record of of life forms and geologic events in Earth’s history. Scientists developed it by the study of rock layers and index fossils (fossils found everywhere and organisms that lived during only one short period of time) worldwide. Scientists placed rocks in order by relative age. Later, radioactive dating helped determine the absolute age of the fossils. Scientists decide when eras (Precambrian Time, Paleozoic Era, Mesozoic Era, Cenozoic Era) end and begin where there is a major change of landforms at certain times, also known as a mass extinction.