Smithsonian National Museum of Natural History Geologic Time The Story of a Changing Earth
Presented by the Department of Paleobiology.
The Archean
Eon Overview
Early Continents and Oceans
The First Life on Earth
Changes in the Atmosphere
Acasta gneiss, one of the oldest known rocks on Earth.
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Banded Iron Formation
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Foundational Concepts
Dating Methods
Earth Processes
Life Processes
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Approximate Dates: 4.0–2.5 bya
Defining Characteristics:
  • • first life appears
Secondary Characteristics:
  • • plate tectonics established
  • • oxygen-poor atmosphere

The term Archean means “ancient” and was originally used to refer to the oldest known rocks. Rocks of Archean age contain the earliest fossils of life on Earth. Because these rocks were formed very long ago—between 2.5 to 4.0 billion years ago—most have long since been covered by younger sediments, eroded, or subducted into the Earth's mantle. Nevertheless, some Archean strata survived in the central parts of continents. These Archean “shields” lie in the heart of Canada, Australia, Africa, India, and Siberia.

By the start of the Archean Eon, the Earth's crust had cooled. The atmosphere was composed of volcanic gases, including nitrogen, hydrogen, carbon, and possibly low levels of oxygen. Water vapor was abundant and the first oceans had formed. A complex set of chemical reactions in these early oceans transformed carbon-containing molecules into simple, single-celled life forms. Where these chemical reactions occurred is unclear--hydrothermal vents are one possibility. All life today is descended from these simple organisms. By the end of the Archean the first photosynthesizing organisms had evolved and begun to produce oxygen, which was released into the oceans and atmosphere. This process would dramatically change life on Earth during the following Proterozoic Eon.

Eon Overview | Early Continents and Oceans | The First Life on Earth
Changes in the Atmosphere

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