Information AboutArchaean |
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The Archean ('''Archaean'''; previously known as the '''Archaeozoic''' or '''Archeozoic''') is a Geologic Eon that refers to the time before the Proterozoic , 2500 Mya (million years ago). Instead of being based on Stratigraphy , this date is defined chronometrically. The lower boundary has not been officially recognized by the International Commission On Stratigraphy , but it is usually set to 3800 Mya at the end of the Hadean eon. The geologic term Archean should not be confused with the biologic term '''Archaea''', originally known as '''Archaebacteria'''. This important Phylogenetic label refers to a Domain of the Tree Of Life (or TOL) that may encompass one-third of the Biomass of the planet. Although a few stones 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 cyanobacterial mats ( Stromatolites ) are found throughout the Archean, and a few probable Bacterial fossils are known from Chert beds. In addition to the domain Bacteria (once known as Eubacteria ), microfossils of the extremophilic domain '''Archaea''' have also been identified. It is thought that life was present throughout the Archean, but was probably confined to simple non-nucleated single-celled organisms, called Prokaryota (and formerly known as Monera). However, no fossil evidence yet exists for ultramicroscopic intracellular organisms such as '''viruses'''. EXTERNAL LINKS |
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