ARCHEAN EON

also spelled Archaean, also called Archeozoic, the earlier of the two divisions of the Precambrian time (3.96 billion to 540 million years ago). The Archean eon began with the formation of the Earth's crust about 3.96 billion years ago and extended to about 2.5 billion years ago, up to the start of the Proterozoic eon; the latter is the second and younger division of the Precambrian period. The earliest and most primitive forms of lifebacteria and blue-green algaeoriginated about 3.5 billion years ago in the middle of the Archean eon. (The Archean's alternative name, Archeozoic, means ancient life.) Metamorphosed igneous Archean-eon rocks are found on all the continents.