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.
Also, petroleum geologists are mainly interested in rocks from the Mesozoic and Paleozoic Eras.
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.
If any of the fossils are unique to one of the geologic time periods, then the rock was formed during that particular time period. But these two methods only give the relative age of rocks--which are younger and which are older. Or how do we know how long ago a particular group of fossilized creatures lived?
The age of a rock in years is called its absolute age.