Ever wonder about earth’s history? If this is you pay close attention. Earths history is shown on something called the Geologic Time Scale. It shows the Periods, Eras, and Eons of earths history. It also shows when plant and animal life where alive. The specific times of animal life is shown. Scientists searched rock layers globally to create the Geologic Time Scale. They also studied index fossils globally to determine where the ancient animals would go. Thats everything you need to know about how and where things were put on the Geologic Time Scale. Now lets talk about how scientists determined when an Eras or Eons end. Scientists know when an era ends when theres a mass extinction and when animals evolve. They can also determine the ending of an era by new species of plants and animals. Thats every important bit of information of the Geologic Time Scale.