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").
For more information on Archean Eon, visit Britannica.com.