There was very little oxygen in the early atmosphere, but today about 20% is oxygen; there is very little carbon dioxide in the atmosphere today(about 0.04%) but the early atmosphere was about 80% carbon dioxide; there was very little nitrogen in the early atmosphere (2.5%)now there is almost 80%.

There was very little oxygen in the early atmosphere. It was not suitable for humans or other animals because they need oxygen to respire.

Fossil fuels are those that have been formed from plants and animals that did not rot when they died. They have carbon locked up in them.

Microorganisms and plants developed over many millions of years and they used the carbon dioxide to produce food. The plants produced their food by the process of photosynthesis.

There is evidence that the oxygen levels increased because there are rocks where iron has combined with oxygen to form red iron oxide. These rocks date from about 2 billion years ago.

Limestone is formed when the shells of dead sea creatures fall to the bottom of the ocean and over millions of years these layers become compressed into rock.

About 3500 million years ago.