Primordial Earth had a reducing atmosphere that contained very low levels of oxygen gas
After about ~2 billion years of prokaryotic life, a form of chlorophyll evolved in prokaryotes (forming cyanobacteria)
This allowed for the process of photosynthesis to occur, creating oxygen gas as a by-product via the photolysis of water
The increase in oxygen in the atmosphere was coupled to a number of significant effects:
- There was a breakdown of chemicals in the atmosphere and oceans to produce oxidised compounds (e.g. CO2)
- There was an evolution of organisms capable of breaking down oxygen-rich oxidising agents (e.g. aerobes)
- The subsequent formation of an ozone layer restricted UV radiation to allow for the proliferation of a wider range of life forms
