Around 2.4 Billion B.C.T., rising levels of oxygen in the atmosphere provided an environment in which eukaryotes (cells with nuclei) would come to flourish.
The Great Oxygenation Event (GOE), also called the Oxygen Catastrophe or the Oxygen Crisis or the Great Oxidation, was the biologically induced appearance of free oxygen (O2) in the Earth's atmosphere. Geological, isotopic, and chemical evidence suggest this major environmental change happened around 2.4 billion years ago.
Around 2.4 Billion B.C.T., cyanobacteria (see 3.5 Billion B.C.T. - Cyanobacteria Appeared) began producing oxygen by photosynthesis. Before the GOE, any free oxygen they produced was chemically captured by dissolved iron organic matter. The GOE was the point when these oxygen sinks became saturated and could not capture all of the oxygen that was produced by cyanobacterial photosynthesis. After the GOE the excess free oxygen started to accumulate in the atmosphere.
Free oxygen is toxic to anaerobic organisms and the rising concentrations may have wiped out most of the Earth's anaerobic inhabitants at the time. Cyanobacteria were therefore responsible for one of the most significant extinction events in Earth's history. Additionally the free oxygen reacted with the atmospheric methane, a greenhouse gas, triggering the Huronian glaciation, possibly the longest snowball Earth episode. Free oxygen has been an important constituent of the atmosphere ever since.
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