Oxygen has been present in the atmosphere for 5 billion years. It is believed that its concentration in atmosphere was infinitesimal until 2.5 billion years ago when the first photosynthetic organisms made their appearance.
Oxygen is essential for life and without oxygen we cannot survive. There should be a balance between oxygen demand and delivery in order to maintain homeostasis within the body.
A human breathes something like 9.5 tons of air in one year. Oxygen makes about 21% of the air and we extract only 1/3 of the oxygen from each breath. This means that we need some 750 kg of oxygen per year. This amount is produced by some 8 trees. This means that every human needs 8 trees to survive.
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