Carl Sagan’s Cosmic Calendar (illustrated as a comic strip here) maps the entire history of our cosmos onto a single year.
On October 29 (representing 2.4 billion years ago), life accomplishes the first major act of terraforming. The oxygenation of the atmosphere resulting from gases released by cyanobacteria created the conditions that sustain us today. It may have come at great cost to other forms of life at the time, however:
The rising oxygen levels may have wiped out a huge portion of the Earth’s anaerobic inhabitants at the time. Cyanobacteria, by producing oxygen that was toxic to anaerobic organisms, were essentially responsible for what was likely the largest extinction event 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 ever. Free oxygen has been an important constituent of the atmosphere ever since. (Wikipedia)
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