KEY CONCEPT The geologic time scale divides Earth’s history based on major past events.

Index fossils are another tool to determine the age of rock layers. Index fossils can provide the relative age of a rock layer. existed only during specific spans of time occurred in large geographic areas Index fossils include fusulinids and trilobites.

The geologic time scale organizes Earth’s history. The history of Earth is represented in the geologic time scale. 100 250 550 1000 2000 PRECAMBRIAN TIME Cyanobacteria This time span makes up the vast majority of Earth’s history. It includes the oldest known rocks and fossils, the origin of eukaryotes, and the oldest animal fossils.

Eras last tens to hundreds of millions of years. consist of two or more periods three eras: Cenozoic, Mesozoic, Paleozoic

Periods last tens of millions of years. most commonly used units of time on time scale associated with rock systems. Epochs last several million years.