During the first two billion years, the atmosphere was anoxic and only anaerobic organisms were able to live. Fossilized microbial mats are called stromatolites and consist of laminated organo-sedimentary structures formed by precipitation of minerals by prokaryotes.

Cyanobacteria evolved from early phototrophs and began the oxygenation o the atmosphere. Hot springs and hydrothermal vents may have been the environments in which life began.

phototroph — organism that is able to make its own food by converting solar energy to chemical energy. cyanobacteria — bacteria that evolved from early phototrophs and oxygenated the atmosphere; also known as blue-green algae. stromatolite — layered sedimentary structure formed by precipitation of minerals by prokaryotes in microbial mats. microbial mat — multi-layered sheet of prokaryotes that may include bacteria and archaea.