The Geologic Timescale
Earth History is divided into three eons. Two of these, the Archean and Proterozoic, are collectively referred to as the Precambrian. The Phanerozoic (the third and present eon), depicted to the left, began approximately 550 million years ago (although some researchers push this date back another 20 to 40 million years). The Phanerozoic comprises less than twenty percent of the history of the Earth, which spans an impressive 4.5 billion years. "Phanerozoic" means "visible life," since it was believed that the start of this eon marked the first signs of life. We now know that life originated in the Precambrian, however, the Phanerozoic still marks the prolific spread of life.
The Phanerozoic eon is further subdivided into three eras: the Paleozoic, Mesozoic, and Cenozoic. These subdivisions were devised on the basis of fossil and geologic evidence of global change. The Earth has changed dramatically during the Phanerozoic. At the start of the Phanerozoic, Earth was covered with partially submerged continental fragments, due to extremely high sea levels. During this time, the first shelled organisms were evolving.
Over the past 550 million years, the continents have moved together to form supercontinents, drifted apart again, and formed some of the most prominent mountain ranges that are found around the world. Catastrophic events have caused several mass extinctions and new life forms have evolved. By moving your mouse over each era you can see the divisions into the different periods and clicking on the timescale will bring you to a text on some of the major events that took place.
