By looking at the layers beneath our feet, geologists are able to identify and describe crucial episodes in life’s history. Most important, these key events frame the chapters in the story of life on earth and the system we use to bind all these chapters together is the Geologic Time Scale.
The Geologic Time Scale is divided into four major units: Eons, Eras, Periods and Epochs. Besides that, an Eon is the longest division of geologic time. So long in fact that there have only been four Eons. Collectively the first three eons are called the Precambrian. That stretch of geological time from the formation of the Earth itself to the start of the Cambrian period. This immensely long stretch of time – some four billion years or more – saw the formation of the Earth as a planetary body, including geosphere, atmosphere, and hydrosphere, as well as the appearance of life.
Video Source – ” PBS Eons “