Photoautotrophs - had the capability of dissociating carbon dioxide into carbon and free oxygen. The carbon was then combined with other elements to permit growth. With the multiplication of the photoautotrophs, billions and billions of tiny living oxygen generators began to change the primeval nonoxygenic atmosphere to an oxygenic one. Prokaryotes - the oldest evidence for fossil life dates prior to 3.5 billion years ago. These organisms are single cells rarely exceeding 20 microns in diameter.
Modern examples are cyanobacteria.
Stromatolites are laminar organic sedimentary structures formed by matlike colonies of cyanobacteria. Theses are usually larger than 60 microns in diameter. Biologists believe that the organelles in eukaryotic cells were once independent microorganisms that entered other cells and then established symbiotic relationships. This is a period of bacteria, algae, and fungi. Amoebas are living examples of protozoans.