What is Geologic Time Scale?
Geologic Time Scale is a time line of earths history. The timeline starts from when the Earth was first formed to the present day. This timeline is split up into parts called eras and periods. Certain things happened in eras like the first plant, first mammal, first fossil, and much more. Read more about all the eras and what major things happened in them. The picture below shows all the Eras in Earths timeline. The four major Eras are Cenozoic, Mesozoic, Paleozoic and Precambrian.
Scientist use fossils to as evidence by lining them all up from oldest to youngest and if they see and major changes in them thats when they determine when a new Era is formed.