 |
|
|
|
| OVERVIEW |
Approximate
Dates: 4.0–2.5 bya |
|
|
| Defining Characteristics:
|
|
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
Department of Paleobiology Home |
National Museum of Natural History Home
Smithsonian Institution Home
|
|
