Paleontologists have made a detailed record of the evolution of life
through Earth’s history. This record is the basis for the geologic
time scale and is referred to as relative time. The geologic time
scale subdivides the 4.5 billion year history of the Earth into shorter
time periods, based on changes in fossils.
The broadest division of geologic time is the Eon. There are
two Eons, the PrePhanerozoic (or Precambrian) and Phanerozoic.
The Precambrian stretches from the formation of the Earth 4.5
billion years ago until the start of the Phanerozoic Eon, about 560
million years ago. There are many fossils in PrePhanerozoic rocks, but
they are almost all microscopic. Phanerozoic means "visible
life." The rocks of the Phanerozoic Eon, which covers about the
last 560 million years of Earth history, are characterized by abundant
Geologic Time Scale