Geologic Time Scale Overview

The geologic time scale is a timeline that organizes major events in Earth's ______.

For more than a billion years, there were no life-forms on our planet, until life started in the oceans around 3.5 billion years ago with simple organisms such as algae and ______.

Complex organisms such as land plants and fish appeared only within the last 500 million ______.

Humans did not appear until about ______ years ago.

Almost all known fossils come from the most recent eon, the ______.

Mass extinctions have occurred many times in Earth's ______.

A sudden, severe change in conditions on Earth is called a ______.

One event that could cause a mass extinction is an asteroid or comet crashing into ______.

Periods are divided into ______, which are the shortest divisions of geologic time.

The last eon of the Precambrian is the ______ Eon.

Geologic time after the Proterozoic Eon is called the ______ Eon.

The beginning of the Cambrian is marked by the Cambrian ______, named because of the great variety of living things that developed during that time.