Geologic Time Scale Overview

Geologic Time Scale

• The geologic time scale is a timeline that organizes major events in Earth's history, from its formation to the present time.

History of Life on Earth

• Life on Earth began around 3.5 billion years ago in the oceans with simple organisms such as algae and bacteria.
• Complex organisms such as land plants and fish appeared only within the last 500 million years.
• Humans did not appear until about 200,000 years ago.

Geologic Time Divisions

• Geologic time is divided into eons, eras, periods, and epochs, with eons being the longest unit of time.
• There are four eons, and Earth history is divided into these eons.

Phanerozoic Eon

• The Phanerozoic Eon is the most recent eon, which began about 550 million years ago when the variety of life-forms suddenly increased.
• Almost all known fossils come from this eon.

Eras and Mass Extinctions

• Eons are divided into eras, with scientists using evidence of mass extinctions as the dividing line between eras.
• A mass extinction occurs when large numbers of species die out over a fairly short period of time, often due to catastrophes such as asteroid or comet crashes.

Periods and Epochs

• Eras are divided into periods, which are blocks of time during which unique rock layers were laid down.
• Periods are divided into epochs, which are the shortest divisions of geologic time.

Precambrian and Proterozoic Eons

• The Precambrian consists of three eons, including most of Earth's history, but very few fossils have been found from this period.
• The Proterozoic Eon ended with the appearance of many new life-forms.

Phanerozoic Eras

• The Phanerozoic Eon is divided into three eras: Paleozoic, Mesozoic, and Cenozoic.
• The Paleozoic era is the oldest, with its first period being the Cambrian period.
• The Cambrian period is marked by the "Cambrian explosion" due to the great variety of living things that developed during that time.

Cenozoic Era and Holocene Epoch

• Paleontologists have found more fossils from the Cenozoic era than from earlier times, making it a well-documented era.
• The Cenozoic era is still ongoing, and you are living during the Holocene epoch.