The geological time scale is the time scale of Earth history. It is based upon the deposits of sedimentary, igneous, and metamorphic rocks throughout Earth history. No place on Earth contains a complete column of geological deposits. By the process of stratigraphy, which was developed in the early 19th century by geologist William Smith, geologists can compare the deposits in one location with those in another and, by lining them up, reconstruct a complete geological column. Dates can be assigned to these rocks by radiometric dating of igneous rocks that are found between many of the sedimentary layers.

The major divisions of the geological time scale are eons. Eons are divided into eras. The first three eons are often informally lumped into the Precambrian time. The Precambrian represents almost 90 percent of Earth history. Human civilization represents an almost unmeasurably small portion of Earth history. Eons and eras represent major events in the history of the Earth.

Precambrian time is divided into: 

- Hadean time ("hellish"; sometimes called Hadean era). The Earth formed about 4.5 billion years ago but was so hot that oceans formed only toward the end of the Hadean.

- Archaean Eon ("old"; sometimes called Archaean era). Organisms similar to modern bacteria may have lived in the oceans almost as soon as they formed. There was little oxygen gas in the atmosphere.

- Proterozoic Eon ("earlier life"; also called Proterozoic era). During the Proterozoic, oxygen gradually accumulated in the atmosphere, and complex cells evolved. In the middle of the Proterozoic, a few multicellular organisms such as seaweeds evolved. Toward the end of the Proterozoic, during the Ediacaran period, there were a few animals, as evidenced by embryos and burrows. Most of the inhabitants of the Ediacaran period were the enigmatic Ediacaran organisms. By the end of the Proterozoic, the atmosphere had its modern concentration of oxygen. At least two worldwide glaciations occurred during the Proterozoic. The Ediacaran period (defined in 2004) is the first new geological period to be defined in over a century. It lasted from about 600 to about 540 million years ago. . .

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