or Archaean Eon or Archeozoic Eon
Older of the two divisions of Precambrian time .
The Archean begins with the formation of the Earth's crust с 3.8 billion years ago and extends to с 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 с 3.5 billion years ago in the middle of the Archean Eon (the Archean's alternative name, Archeozoic, means "ancient life").