(p. 79) While oxygen is the third most common element in the universe, we know that free oxygen was exceedingly rare in the Earth’s initial atmosphere, until roughly two billion years ago, when an ancestor of modern cyanobacteria hit upon a photosynthetic process that used the energy from the sun to extract hydrogen from the abundant supply of water on the planet. That metabolic strategy was spectacularly successful–the organism quickly covered the surface of the planet–but it had a pollution problem: it expelled free oxygen as a waste product. During this period, now known as the Proterozoic, the oxygen content of the atmosphere exploded from 0.0001 percent to 3 percent, beginning its long march to the current levels of 21 percent. (Even today, Earth’s atmosphere is actually dominated by nitrogen, which makes up 78 percent of its overall volume: other gases. like argon and carbon dioxide, constitute less than a single percent.) The massive increase of oxygen in the atmosphere triggered what has been called “by far the greatest pollution crisis the earth has ever endured,” destroying countless microbes for whom the cocktail of sunlight and oxygen was deadly.
In time, though, organisms evolved that thrived in an oxygen-heavy environment. We are their descendants.
Johnson, Steven. The Invention of Air: A Story of Science, Faith, Revolution, and the Birth of America. New York: Riverhead Books, 2008.