Fossil studies indicate that the Earth's early atmosphere had very little oxygen compared to its current composition; this oxygen had to "come from" somewhere. Possibilities include it being "rained" onto the earth by comets and asteroids, erupted via volcanism, or being released as a product during chemical reactions. Our current theory is that the chemical reaction pathway is the most accurate.
Also according to fossil records, it appears that photosynthesis evolved fairly early in the...
Fossil studies indicate that the Earth's early atmosphere had very little oxygen compared to its current composition; this oxygen had to "come from" somewhere. Possibilities include it being "rained" onto the earth by comets and asteroids, erupted via volcanism, or being released as a product during chemical reactions. Our current theory is that the chemical reaction pathway is the most accurate.
Also according to fossil records, it appears that photosynthesis evolved fairly early in the history of life on the bacterial level. Photosynthesis uses carbon dioxide and water to produce energy for the organism, and it releases oxygen as a waste product. The most difficult aspect here is for us to conceive of the Earth as being largely populated by photosynthetic bacteria for hundreds of millions of years. In the course of this timespan, the oxygen the bacteria released super-saturated the environment, reacting with anything it could, and then building up in the atmosphere once there was nothing left to react with. This is evidenced by distinct changes in the fossil record that show certain oxygen-related reactions taking place over time, as well as the proliferation of oxygen-metabolizing bacteria following this rise in oxygen levels.
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