Evolution of the atmosphere

Phase 1 - Volcanoes gave out gases
> the earth's surface was originally molten for many millions of year
> then cooled down and a thin crust formed, volcanoes kept erupting
> volcanoes gave out lots of gases - carbon dioxide, water vapour and small amounts of other gases
> this was how oceans and the atmosphere were formed
> the early atmosphere was mostly CO2, virually no oxygen
> oceans were formed when the water vapour condensed

Phase 2 - Green plants evolved and produced oxygen
> a lot of the early carbon dioxide dissolved into the oceans
> marine organisms developed, which took in some CO2, when they died the organisms were buried under layers of sediment and the CO2 became 'locked up' in carbonate rocks > green plants eveolved over most of the Earth
> the green plants removed CO2 from the air and produced O2 by photosynthesis, this caused the oxygen level to gradually increase

Phase 3 - Ozone layer allows evolution of complex animals
> the build up of oxygen in the atmosphere killed off some early organisms that couldn't tolerate it, but allower other more complex organisms to evolve and flourish
> the oxygen also created the ozone layer (O3) which blocked harmful rays from the Sun and enbaled even more complex organisms to evolve - us, eventually
> there is very little CO2 left now