Back in the day the earth's surface wasmolten, and was so hot that any atmosphere would have just "boiled away".
Eventually it cooled, which allowed a thin layer of crust to form, but volcanoes kept on erupting.
The volcanoes gave out great amounts of gas, which helped oceans and the atmosphere to form.
The early atmosphere was mostly CO2, and there was also water vapour, methane, and ammonia.Barely any oxygen was present.
The oceans formed when the water vapour had condensed.
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Phase two
Green plants and algae evolved over the earth - they were happy with the CO2 atmosphere.
Lots of the CO2 dissolved into the oceans. The green plants and algae also absorbed some of it, and produced O2 by photosynthesis.
The plants and algae died and became buried underneath layers of sediment. The carbon and the hydrocarbons inside the organisms became "locked up" in the rocks as insoluble carbonates and fossil fuels.
When fossil fuels burn today, the trapped carbon is released, and the concentration of CO2 in the atmosphere increases.
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Phase three
The build up of oxygen in the atmosphere killed off some of the early organisms, and allowed more complex ones to evolve.
The oxygen created the ozone layer, which blocked harmful rays from the sun, which allowed even more complex organisms to evolve.
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