Photosynthetic organisms were present when earth was primarily anoxic. why was the appearance of cyanobacteria critical for the oxidation of the atmosphere? view available hint(s) photosynthetic organisms were present when earth was primarily anoxic. why was the appearance of cyanobacteria critical for the oxidation of the atmosphere? the early forms of photosynthesis did not produce oxygen as a waste product. cyanobacteria carry out anoxic photosynthesis. the early forms of photosynthesis did produce oxygen, but it was not enough to make a difference in the atmosphere. the early forms of photosynthesis were carried out in the water, and there was no way for the oxygen to move into the atmosphere.