Cyanobacteria has been tremendously important in shaping the course of evolution and ecological change throughout earth's history. The oxygen atmosphere that we depend on was generated by numerous cyanobacteria photosynthesizing during the Archaean and Proterozoic Era. The earth's atmosphere before that time was of a different chemistry and unsuitable for life as we know it. Cyanobacteria was the first organisms that used H2O instead of H2S or other compounds as a source of electrons and hydrogen for fixing CO2 . They were able to synthesize organic compounds from water and CO2 By releasing Oxygen as a by-product of their photosynthesis they in affect changed the earth's atmosphere. Cyanobacteria lived in colonies that formed stromatolites, and evolved at least 2.5 BA. Early cyanobacteria inhabited marine sediments where Archean banded iron formations were deposited (consisting of reddish layers rich in iron oxide It is thought that when early cyanobacteria released oxygen, it reacted with dissolved iron ions, which precipitated as iron oxide. This reaction would have prevented any accumulation of free oxygen for possibly a few million years until precipitation exhausted the dissolved iron. This would have caused the oceans to become saturated with oxygen and gas out to accumulate in the atmosphere. This is shown by large deposits of iron rich rocks that were rusted red by oxidation with atmospheric oxygen beginning approximately 2BA. The change into an oxygen atmosphere created a crisis for Precambrian Cyanobacteria because oxygen attacks the bonds of organic molecules. The corrosive atmosphere that prokaryotes had essentially created likely caused the extinction of some of the species unable to cope. The surviving species lived in habitats that remained generally anaerobic.
