The Great Oxygenation Event

The Great Oxygenation event, also known as the Oxygen Revolution, occurred around 2.5 Billion years ago. This was a pivotal step in the development of life on Earth. Throughout the Achaean era (3.8 Billion years ago – 2.5 Billion years ago), the Earth had an immense amount of carbon dioxide in the atmosphere and an almost nonexistent amount of oxygen. The atmosphere of Achaean era Earth would be toxic to many organisms living today as well as incompatible with life for many species on the Earth today. The Great Oxygenation event could most likely be attributed to Cyanobacteria, which learned how to survive through the process of photosynthesis. As a byproduct of photosynthesis, Cyanobacteria released oxygen into the atmosphere at a rapid rate. As the population of cyanobacteria rose, so did the amount of oxygen in the atmosphere. By 2.5 billion years ago, oxygen levels spiked to a dramatic ten percent. This was the first example of organisms causing massive change in their own environments. The rise in oxygen marked the end of the Achaean era, and many of the organisms that had thrived in this era without oxygen, and brought about the development of oxygen dependent Eukaryotes, which later evolved into the dominant life forms on Earth. Oxygen levels continue to rise to a twenty-one percent, current day levels, creating the necessary environment for many organisms to survive, including humans.

References:
Beillo, D. (2009, August 19). The origin of oxygen in the Earth's atmosphere.
Retrieved from Scientific American website: http://www.scientificamerican.com/article/origin-of-oxygen-in-atmosphere/
Green, H. (2012, November 5). The history of life on Earth- CrashCourse ecology
[Video file]. Retrieved from http://www.youtube.com/watch?v=sjE-Pkjp3u4