The Oxygen Catastrophe

As much as 2.9 billion years ago during the Paleoproterozoic Era Earth’s atmosphere was made up of nitrogen, methane, ammonia, carbon dioxide, and inert gases. The earliest life forms were anaerobic cyanobacteria, which produced oxygen as a waste product. 

Over then next half-billion years or so, the cyanobacteria produced so much oxygen that they changed the chemistry of the atmosphere, poisoning themselves in the process. This is known as the Great Oxidation Event, also called the Great Oxygenation Event, the Oxygen Crisis and the Oxygen Catastrophe (Video 1).

While almost all of them died out, some were engulfed by eukaryotes to become endosymbiotic cyanobacteria. Over hundreds of millions of years, they evolved into chloroplasts: the green parts inside of plants that we see today, responsible for photosynthesis. That is, they still produce the oxygen we and all animals need to breath while taking in the carbon dioxide that we exhale, playing a vital, life-supporting role in our existence, which depends in good part on the carbon cycle. 

Today, similar bacteria form stromatolites found in warm shallow water off the coast of Exumas in the Bahamas (top image) and Western Australia.