The earth’s early atmosphere did not contain oxygen. this changed dramatically once the early cells underwent photosynthesis. explain why photosynthesis could have occurred in such an atmosphere and how it eventually affected the evolution of other organisms.

Cyanobacterial species are thought to be the first photosynthetic organisms. These unicellular bacteria occur in Maine environments, the presence of water acted as a sun-screen, preventing the denaturation of photosynthetic hardware by exposure to high uv. This high volume of Cyanobacteria in the oceans produced vast quantities of oxygen - this allowed organisms to grow in size a their metabolic demands could now be met. At one point the large concentrations of oxygen lead to gigantism. following the first endosymbiotic event, where a cyanobacterial cell became enslaved in a bacterial cell, photosynthetic eukaryotes were 'born'. This lead to the formation of both terrestrial and marine chloroxygenic organisms.