Microbial mats are thought to represent the earliest forms of life on Earth, and there is fossil evidence, called stromatolites, of their presence about 3.5 billion years ago. During the first 2 billion years, the atmosphere was anoxic and only anaerobic organisms were able to live.

Cyanobacteria began the oxygenation of the atmosphere. Prokaryotes existed for billions of years before plants and animals appeared.

cyanobacteria — bacteria that evolved from early phototrophs and oxygenated the atmosphere; also known as blue-green algae. stromatolite — a layered sedimentary structure formed by precipitation of minerals by prokaryotes in microbial mats. microbial mat — a multi-layered sheet of prokaryotes that may include bacteria and archaea. anaerobic — refers to organisms that grow without oxygen.