First photosynthesis

The use of sunlight's energy for chemical processes is undoubtedly the most important invention of life. While volcanic activity within the Earth also provides a chemical energy source for living organisms, solar energy is reliable and universally available across the planet.

It is likely that the very first living cells developed an early form of photosynthesis. During this process, oxygen was produced as a byproduct and reacted with dissolved substances in the primordial ocean. These substances included various metal salts, primarily iron salts. The reaction with oxygen produced iron oxide ("rust"), which does not dissolve in water and thus sank to the ocean floor.

This process spanned hundreds of millions of years, frequently interrupted by climatic changes due to volcanism and/or meteorite impacts. During this time, no significant amount of oxygen could dissolve in the water or escape into the atmosphere. However, this changed relatively suddenly once all the dissolved iron salts in the water had been depleted. What followed is described in the next section.