How do geologists divide up earths long life?

In addition to the relative dating of periods in Earth's history for which we have rocks preserved, geologists are now able to assign absolute age dates to critical intervals. In the Geologic Time Scale, time is generally divided on the basis of the earth's biotic composition, with the Phanerozoic Eon (i. e. the Paleozoic, Mesozoic and Cenozoic Eras) representing the period of Earth's history with advanced life forms, and the Pre Cambrian (or Proterozoic and Hadean Eras) representing the period before advanced life. Within the context of the Phanerozoic Eon, geologists beginning in the late 1700's recognized that fossils appeared in an orderly fashion in stratigraphic units. Moreover, these geologists recognized that the fossilized biota demonstrated rather large changes in overall composition and showed both similarities with, and differences from living taxonomic groups. The majority of fossil organisms, however, did not match with modern groups; this led to the classification of three major eras within the period of time when the Earth's surface was populated with advanced life forms. These eras were referred to as the Paleozoic (meaning ancient life), the Mesozoic (meaning middle life), and the Cenozoic (meaning recent life) based on their relative similarity with modern taxa.