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The point of comparison for the environments of the cyanobacteria was during the Archaen eon (~3.6 billion years ago) and the current earth's atmosphere (~13,000 years ago).
The first living organisms were believed to have thrived in the atmosphere of the Earth 3.9 billion years ago. It was about 3.0 billion years ago when scientist found out the fossil records of the cyanobacterium, a photosynthetic organism. During those times, there is limited supply of oxygen gas in the earth crust - a very impossible for current life forms to survive.
Outline of the past and present atmosphere
Surface temp hotter cooler
Oxygen level limited ~21%
Sea high dissolved Fe Iron ore present
The Gram negative photosynthetic cyanobacteria are one of the most successful groups of organisms on earth which have successfully survived for about three billion years. The early atmosphere had no recent composition and there was a time when atmosphere did not contain oxygen. In oxygen-less atmosphere, any present kind of life is impossible but palaeontological research permits to believe Cynobacteria as the earliest plants owing to their capacity of thriving well in sulphurated atmosphere. They can grow well in a media containing H2S2, some other compounds of sulphur even in free sulphur.
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