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Archean Eon

 

Older of the two divisions of Precambrian time. The Archean begins with the formation of the Earth's crust 4 billion years ago and extends to 2.5 billion years ago, up to the start of the Proterozoic Eon, the second division of the Precambrian Period. The earliest and most-primitive forms of life (bacteria and cyanobacteria) originated more than 3.5 billion years ago in the middle of the Archean Eon (the Archean's alternative name, Archeozoic, means "ancient life").

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Archean (earlier of the two divisions of Precambrian time)
Precambrian
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Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved.  Read more