the time from 3,800 million years to 2,500 million years ago; earth's crust formed; unicellular organisms are earliest forms of life used of the earliest known rocks
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 3.8 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 3.5 billion years ago in the middle of the Archean Eon (the Archean's alternative name, Archeozoic, means "ancient life")