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This graph shows that the amount of oxygen has increased as the Earth has aged.
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The slow build up of Oxygen in the Earth's Atmosphere

It took a long time for oxygen to build up in Earth's atmosphere. At first the atmosphere was made of H, then with volcanic eruptions the atmosphere was made of carbon dioxide, sulphur dioxide, ammonia, and other gases, then the atmosphere was made mostly of nitrogen. Today the atmosphere is 80% nitrogen and 20% oxygen.

In the early Earth there was very little oxygen. 3.5 billion years ago there was only 0.01% oxygen in the atmosphere. 2.5 billion years ago there was only 0.1% oxygen in the atmosphere. At the top of Mt. Everest it is very hard to breathe because there is not enough oxygen. So imagine how hard it would be to breathe in the environment of the early Earth!

The formation of life on Earth played a very large role in the build up of oxygen in the environment. As early as 3.5 billion years ago, bacteria began to produce oxygen as a waste product of their activity. That oxygen reacted with iron in the ocean to make iron ore. Later, after the iron in the ocean was gone, and the making of iron ore was finished, about 2.5 billion years ago, enough oxygen accumulated for respiration (for animals which breathe oxygen) to begin in simple organisms, organisms like protozoans, amoeba, etc. Such organisms are sophisticated, they are single-celled beings with a nucleus! In sophisticated cells with a nucleus, not only respiration, but even photosynthesis is more efficient, so the production of oxygen in the Earths early environment accelerated. Oxygen continued to build at an accelerating pace until 1% oxygen levels were in place. The more oxygen accumulated in the atmosphere, the larger became the protective ozone layer (formed from oxygen in the atmosphere). Ozone helped protect developing life from the harmful effects of the Sun's ultraviolet radiation. Then other life forms such as sponges, worms, and other organisms came to be.

Once oxygen levels of 1% were achieved, and an ozone layer developed, there seemed to be enough oxygen present for the development of many different kinds of life forms, including dinosaurs. The Earth had entered what is known as the Cambrian age.

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