Geologists divide Earth's history into two eons, six eras and 20 periods. The eons are the Precambrian, which ended around 542 million years ago, and the Phanerozoic, which has yet to end. The eras within the Precambrian were the Hadean, Archean and Proterozoic.
The Phanerozoic eon is divided between three eras: the Paleozoic, Mesozoic and Cenozoic. These names refer to the "early life," "middle life" and "late life" of their respective zoologies. These eras are further divided into periods by geologists. In this model, the Phanerozoic era began with the Cambrian period and has continued to the modern Quarternary period.