Archean

The Archean (IPA: ar-KEE-ən, also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history, preceded by the Hadean Eon and followed by the Proterozoic and the Phanerozoic. The Archean represents the time period from 4,031 to 2,500 Ma (million years ago). The Late Heavy Bombardment is hypothesized to overlap with the beginning of the Archean. The oldest known glaciation occurred in the middle of the eon. The Earth during the Archean was mostly a water world: there was continental crust, but much of it was under a superocean deeper than today's oceans. Except for some rare relict crystals (Hadean zircon), today's oldest continental crust dates back to the Archean. Much of the geological detail of the Archean has been destroyed by subsequent tectonic activity. The Earth's atmosphere was also vastly different in composition from today's: the prebiotic atmosphere was a reducing atmosphere rich in methane and lacking free oxygen. The earliest known life, mostly represented by shallow-water microbial mats called stromatolites, started in the Archean and remained simple prokaryotes (archaea and bacteria) throughout the eon. The earliest photosynthetic processes, especially those by early cyanobacteria, appeared in the mid/late Archean and led to a permanent chemical change in the ocean and the atmosphere after the Archean.