Most scientists believe that for half of Earth's 4.6-billion-year history, the atmosphere<span>contained almost no </span>oxygen. Cyanobacteria or blue-green algae became the first microbes to produce oxygen<span> 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 </span>The Rise of Oxygen - Astrobiology Magazine<span>www.astrobio.net/news-exclusive/the-rise-of-oxygen/ it should help</span>
