Our Environment Contents

1. Evolution

The 'Great Oxygenation Event' or 'Oxygen Revolution' 1.7 - 2.5 billion years ago made the evolution of more complex organisms possible.

Where did the oxygen come from?

Stromatolites evolved in a harsh environment and protected themselves from damaging UV radiation by excreting slime. They derived food products from the process of photosynthesis, which produced oxygen as a waste product. At first the oxygen was rapidly used up by inorganic processes, such as the oxidation of iron minerals. Eventually however these 'oxygen sinks' were saturated, and oxygen started to build up in the atmosphere. This caused ozone to form as a thin layer in the upper stratosphere, blocking the Sun's UV radiation.

This combination of reduced UV and an oxygen atmosphere made it possible for more complex organisms to evolve: the eukaryotes. These organisms would eventually displace most of the stromatolites. Oxygen is also poisonous to many anaerobic organisms, and it probably led to the extinction of many bacteria which had evolved in an oxygen free environment.

Why was oxygen so significant in the early evolution of life?