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4.5 Billion years ago the earth was covered in volcanoes and magma flows that released ash into the sky. There was no oxygen and a lot of carbon dioxide. A layer of as covered the earth meaning the suns rays didn't reach the earth, nothing could grow.
Around 3.2 Billion years ago the earth cooled down, meaning the moisture in the air turned into rain. It rained for 20 million years, forming the oceans
600 Million years ago most of the volcanoes have become inactive and the ash cloud around the earth has disappeared. The rays from the sun have allowed photosynthetic bacteria to grow. The earths oxygen level is now 1% of its current level
400 Million years ago plant life has begun and taken most of the carbon dioxide out of the atmosphere and added oxygen. The oxygen level is now 10% of the present day.
Now plant levels have increased hugely reducing the amount of carbon dioxide and adding a lot of oxygen. The atmosphere's composition is 78% nitrogen, 21% oxygen and 1% other gases including 0.4% carbon dioxide
Oxygen levels are high enough, and carbon dioxide levels low enough to support human and animal life
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