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 hydrogen, 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. 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. At first, 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, enough oxygen accumulated for simple organisms, organisms like protozoans, amoeba, etc. which are single-celled, to come to be. 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 there was enough oxygen to breathe, and an ozone layer developed, lots of different kinds of life forms, including dinosaurs, came to be. The Earth had entered what is known as the Cambrian age.
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