We keep track of time by hours, days, months, even years. But when we study the Earth's history, these time periods aren't large enough. Scientists estimate that the Earth formed about 4.5 - 4.6 billion years ago. This is based largely upon radiometric dating of meteorites. There have been many significant events on Earth over the last few billion years. Continents, oceans, and the atmosphere formed and changed. Life arose and evolved. Geologists mark the major changes in Earth's history with the geologic time scale.

The geologic time scale is a timeline of the major events on Earth from its formation to present day. Scientists study rocks and fossils to date these major events in Earth's history. An example of the geologic time scale is shown below.

Geologic time is vast, so simply dividing into years isn't very useful. Instead, the geologic time scale has been divided into units of time based on significant changes in lifeforms or geologic events. A hierarchy of time frames is used to mark geologic time. From largest to smallest, the times are: Eons, Eras, Periods, and Epochs. Some scales will further divide epochs into ages.

Precambrian Time is largest division of geologic time. It covers about the first 90% of Earth's history. The earliest forms of life developed during this time. Fossil evidence is poorly preserved for the Precambrian. This is partly because of the natural recycling of rocks though the rock cycle. Also, organisms during this time were soft-bodied, which don't preserve well as fossils. 

Around 542 million years ago there was a sharp increase in the diversity of lifeforms. Hard-shelled organisms like trilobites appeared. This period of rapid development of life is known as the Cambrian Explosion. Organisms continued to evolve, but periodically died off in large numbers during times of mass extinctions. The mass extinction event that ended the dinosaurs marks the end of the Cretaceous Period and Mesozoic Era. Although over half of Earth's species went extinct at that time, it was not the largest mass extinction. The earlier mass extinction at the end of the Permian Period killed off a majority of ocean and land species.

Today we live during the Holocene Epoch, of the Quaternary Period, of the Cenozoic Era, of the Phanerozoic Eon. The Holocene began around 11,500 years ago when the last major ice age ended. As technology advances, so does our understanding of geologic time. Improvements in radiometric dating of rocks and fossils continue to lead to further revision of the geologic time scale.