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The Archean is a geologic period; it is a somewhat antiquated term for the time span between 2500 million years before the present and 3800 million years before the present. The Archean period follows the Hadean eon and is followed by the Paleoproterozoic eon. Although a few pebbles are known that are older, the oldest rock formations exposed on the surface of the Earth are Archean or slightly older. Archean rocks are known from Greenland, the Canadian Shield, northwest Australia, and southern Africa.

The Archean atmosphere apparently lacked free oxygen. Temperatures appear to have been near modern levels, although astronomers think that the sun was about one-third dimmer. This is thought to reflect larger amounts of greenhouse gases than later in the Earth's history.

Probable remains of bacterial mats ( stromatolites) are found throughout the Archean, and a few probable bacterial fossils are known from chert beds. It is thought that life was present throughout the Archean, but was probably confined to simple non-nucleated ( prokaryotic) single-celled organisms. These organisms, Archaea, are extremophiles and form one of the three Domains of Life with the Bacteria and the EukaryotaEukaryotes are organisms with complex cells, in which the genetic material is organized into membrane-bound nuclei. They include the animals, plants, and fungi, which are mostly multicellular, as well as various other groups called protists, many of which, or nucleated cells, which include the multi-cellular lifeforms.