If we consider the story of life on earth, changes which led to presence of free molecular oxygen on Earth (known as GOE), is probably the most horrible event. It led to annihilation of anaerobic life forms which dominated the earth for more than a billion years.
Cyanobacteria, which appeared about 200 million years before the GOE 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. From their perspective it was a catastrophe. 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.
Basically, the planet was saturated by toxic wastes (for the major inhabitants of Earth then) which led to a global bio-catastrophe and their annihilation. Sounds familiar?!
Great Oxygenation Event, Wikipedia, the free encyclopedia. 19-Jan-2013. ↩