When the " big bang theory" happened, all of the particles in space spun around for a very long time. Eventually, after a long time, those particles stuck together and made the earth. The earth's atmosphere then was made up of nitrogen and other elements. Oxygen was not in the earth's atmosphere until a giant meteorite collided with the earth. The impact of the meteorite took a giant chunk out of the earth. After this happened, the moon was formed. that meteor brought oxygen to the earths atmosphere, and thus life started. The atmosphere stayed as a giant storm of lightning and thunder, giant waves in the ocean, and dust and dark clouds. The air was full of all kinds of different elements, but oxygen was the most important. Then one day, the giant storm subsided and the sun shone on the earth for the first time. Eventually, the atmosphere got to be just oxygen and all the other elements went out of the air. Thus, we got our atmosphere we have today.

The above explanation is a widely accepted ''theory''. At least part of it is ...the Earth's atmosphere is still made up primarily of Nitrogen - about 78 percent, in fact. Oxygen only makes up about 21 percent of our atmosphere and the remaining percentage is a combination of other gases.

If the concentration of oxygen was very high the atmosphere would be '''extremely''' unstable because oxygen is highly flammable. I'm not even certain a pure oxygen atmosphere is possible or if it is, if it would be stable enough to sustain life for any length of time.

The atmosphere formed due to differentiation. Less dense gas molecules rose to the surface, and in Earth's original atmosphere, the main gases were hydrogen and helium. However, these gases eventually escaped Earth's gravity when the sun heated them up enough; they may have been blown away by solar wind. Out gassing produced a multitude of gases as well. Volcanic eruptions ejected methane, water vapor, carbon dioxide, nitrogen, sulfur dioxide, and ammonia. These gases reacted to the sun's radiation and some of them, like water vapor and ammonia, were broken down of the hydrogen that resulted from the breakdown escaped. The remaining oxygen formed ozone and collected to form a shield in the stratosphere that protects Earth from harmful ultraviolet rays.

Then organisms that could survive in this early atmosphere developed. Primal green plants took in carbon dioxide and went through photosynthesis, therefore producing oxygen. The amount of oxygen increased slowly and was close to the present atmosphere's amount of oxygen.

The fresh air store.

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The sky

People know adays we breath in oxygen and breath out carbon dioxide and trees breath in carbon dioxide and breath out oxygen thats how

Trees. Carbon dioxide is your exhaust from breathing. Trees leaves and all plants filter Our carbon dioxide and recycle it back into breathable oxygen.

It comes from tress by way of Photosynthesis

usually industrial and nuclear sites.

Green Giant! Ho ho ho.....