It is believed that water is the key to the origin of life on Earth. Oxygen could have been released in the atmosphere as soon as the atmosphere was formed by the process of photo-dissociation of water caused by UV from the sun. The sun breaks the hydrogen and oxygen atoms in water in a similar manner to the breakdown of oxygen molecules in the stratosphere today. However, today the oxygen in the air was produced by the action of living things (via photosynthesis).
Man is baffled as to exactly how life emerged, however, evidence suggests that UV may have been an important contributor. Ultraviolet radiation from the sun provided the necessary energy to rearrange molecules and cause chemical processes in pools of water on the Earth referred to as “primordial soup.”
In the process of photosynthesis, living organisms use energy directly from the sun and discard oxygen as a waste product. In the process of respiration, living organisms use available energy by letting oxygen react with carbon as a power supply, therefore, releasing carbon dioxide back into the environment. Human beings depend on photosynthesis by plants to provide food from which carbon is extracted to form with oxygen in the body. Without the free oxygen in the atmosphere, respiration would be impossible. Consequently, without oxygen in the atmosphere there would be no ozone layer and intense ultraviolet light would penetrate through to the ground and man would not be able to survive.