It took billions of years for Earth’s air to grow rich in oxygen, but it may have been destined to do so once photosynthetic bacteria began pumping out the gas
Source:
https://www.newscientist.com/article/2226874-why-is-earth-so-rich-in-oxygen-the-answer-is-simpler-than-we-thought/?utm_campaign=RSS%7CNSNS&utm_source=NSNS&utm_medium=RSS&utm_content=home