Most scientists believe that for half of Earth's 4.6-billion-year history, the atmospherecontained almost no oxygen. Cyanobacteria or blue-green algae became the first microbes to produce oxygen by photosynthesis, perhaps as long ago as 3.5 billion years ago and certainly by 2.7 billion years ago.
this answer came from this website The Rise of Oxygen - Astrobiology Magazinewww.astrobio.net/news-exclusive/the-rise-of-oxygen/
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