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The Earth is surrounded by the atmosphere.
This has looked how it does today for 200 million years, but before this it was very different. This is known as the early atmosphere.
The early atmosphere appears to have been made up of high levels of carbon dioxide, low oxygen, and some water vapour, ammonia and methane, however it is difficult to properly quantify this because of how long ago it was (it is estimated that the Earth was first formed 4.6 billion years ago).
The current theory is that volcanic activity formed the early atmosphere, and the water vapour then condensed to form oceans.
Ocean formation supported the start of plant life, including algae (algae was first formed 2.7 billion years ago). The algae would have photosynthesized, reducing the CO2 in the atmosphere and increasing the O2.
The equation for photosynthesis is shown below.
Oceans are also carbon stores, which therefore means that their formation reduced the CO2 in the atmosphere. Carbon dioxide dissolves in the ocean and reacts with ions in the water to form calcium carbonate precipitates. These then build up as sediments on the ocean bed and this forms limestone.
Also, as plants and animals die, they fall to the bottom of the seabed and sediment builds up on top of them. This sediment eventually becomes coal, oil, or gas. Coal, oil and gas are carbon stores, and are known as fossil fuels. Useful materials can be obtained from crude oil for human use via fractional distillation.
Essentially, the early atmosphere changed so that there was an increase oxygen and a decrease in carbon dioxide. This then supported the development of all other animal life.