The Oxygen Cycle
Oxygen exists in our modern atmosphere in great amounts. Approximately 21% of the atmosphere is comprised of oxygen. This was not always the case, however.
Billions of years ago, it is believed that there was virtually no oxygen found in the atmosphere at all. What caused an increase in oxygen levels?
Most of the oxygen now found in our atmosphere was released by plants as a bi-product of photosynthesis. Over millions of years, as plants around the globe released oxygen, the levels continued to rise, until they reached a balance around 1 billion years ago.
For the last billion years, the amount of oxygen has remained relatively constant. At the same time that plants continue adding oxygen to the atmosphere, it is also being removed by various processes.
Oxygen is highly reactive. As the oxygen in our atmosphere interacts with other substances, it often bonds to them, becoming trapped. Many lifeforms also remove oxygen from the atmosphere as they breathe. This oxygen is used by these lifeforms to carry out the functions of life.