Earth atmosphere wasn't like that amount of oxygen present today, about 3.5 billion years ago earth atmosphere have only Nitrogen carbon dioxide and methane and oceans full of anaerobic organism including cyanobacteria ,after cyanobacteria floats in the ocean and able to photosynthesis with the help of sunlight and carbon dioxide produce their own energy and expelled oxygen to atmospher after the population growth of bacteria increase which leads to oxygen level high which causes death of other organisms around 2.5-2.1 cooling of earth forms ice age and temperature falls and up varied then , aerobic organism comes to play cause decrease the oxygen level so that the level of oxygen rise and fall and finally brings to 21% that we have today after that cyanobacteria swelled by a microbe so that the microbe itself produce energy by the help of cyanobacteria later it's give way to plant cell Now we are calling this cyanobacteria as chloroplast The d...
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