Once before in the history of life on Earth, a mass extinction was caused by life itself.

The Earth's early atmosphere was mostly carbon dioxide, with carbon monoxide and methane.  These gasses were released by volcanoes.  Three and a half billion years ago a group of species called cyanobacteria evolved that lived by photosynthesising carbon dioxide into carbon compounds for their own use, and oxygen, which was released into the atmosphere.  

For a billion years the oxygen that was released by cyanobacteria was absorbed by the environment.  The oxygen, which was dissolved in the oceans, combined with dissolved minerals, especially iron, causing the minerals to sink to the ocean floor as oxides.  Some of these ocean-floor deposits are what we now mine as iron ore in places like the Pilbara in Western Australia.  

After a billion years, all of the oxidisable minerals in the oceans had been used up and the oxygen level in the ocean rose.  The oxygen came out of solution into the atmosphere, and the oxygen level in the atmosphere also began to rise.  This event occurred about two-and-a-half billion years ago and was called the Great Oxygenation Event. 

This change had two extinction-causing consequences.

Until that time, all life on Earth had evolved in an oxygen-free environment.  he oxygen in the ocean and the atmosphere was poisonous to most of the other life forms on the Earth and killed them.  This extinction is called the Oxygen Catastrophe.  

The reduced levels of carbon dioxide and methane in the atmosphere brought about by photosynthesis and oxidation of the methane reduced the greenhouse effect and changed the climate, drastically lowering the surface temperature of the Earth, causing most, if not all, of the surface of the Earth to freeze over,  and remain frozen for hundreds-of-millions of years, greatly constraining the development of life.  This period is known as Snowball Earth.

The cyanobacteria also help set up humanity as the second life form to cause a mass extinction.  The photosynthesis process of taking carbon from the atmosphere that cyanobacteria developed to build their own bodies was the basis of later developments that sequestered vast quantities of hydrocarbon compounds in the Earth's crust.  We now use those carbon compounds as fossil fuels.  These fossil fuels have supplied the energy for humanity's massive expansion and ability to claim vast environments for our purposes, at the expense of the diversity of life, over the last one-hundred-and-fifty years. 

This page is linked from:

biodiversity loss

carbon dioxide in Earth's atmosphere

Agree? Disagree? Make a comment!  (Comments are moderated)