If it were not for the oxygen in the Earth's atmosphere, we humans would not be able to live. Today, oxygen makes up almost one fourth of our atmosphere.

However, at one time  our atmosphere had almost no oxygen, and oxygen was poison to the creatures that lived on our planet.

Early in the Earth's history, after its crust cooled and solidified, a spate of volcanic activity occurred, creating an atmosphere comprised mainly of nitrogen, ammonia, methane and carbon dioxide.

Living creatures on Earth probably obtained energy by taking in nitrogen-rich amino acids. Today, there are still some living things on Earth that do not breathe oxygen and survive in extreme environments, such as deep sea vents, where conditions are similar to that of early Earth. These creatures are known as extremophiles.

The evolution of photosynthesis as a means of obtaining energy completely changed the atmosphere's composition.  Oxygen is produced during the process of photosynthesis.

Now our atmosphere is made up of 78% nitrogen, 21% oxygen and small amounts of other gases, such as carbon dioxide and helium.

The atmosphere does more for us than just provide us with the oxygen that we need to breathe.  It protects the Earth from harmful radiation and prevents objects from space, such as meteorites, from striking the Earth.

A layer of ozone in the atmosphere filters out ultraviolet light.  A molecule of ozone contains three atoms of oxygen. It differs from a molecule of that we breathe, which contains two oxygen atoms.

The ozone layer is located between about 12 and 25 miles (20 and 40 kilometers) above the Earth's surface, in a region of the atmosphere known as the stratosphere. Ozone is created when ultraviolet light in the stratosphere breaks up some oxygen molecules. Free oxygen atoms combine with unbroken oxygen molecules to form ozone.

The stratosphere is located just above the troposphere, the section of the atmosphere just above the Earth's surface. The troposphere is the part of the atmosphere that experiences weather. Almost all of the Earth's water vapor is located in the troposphere.

Above the stratosphere is the ionosphere, a region that is bombarded by radiation. The radiation in the ionosphere causes the gases there to become ionized. The Aurora Borealis and the Aurora Australis (the Northern and Southern Lights) originate in the ionosphere.

Because we depend on our atmosphere for survival, any changes to our atmosphere are cause for concern.

Since the 1970s, scientists have noticed that the ozone layer is being depleted. Much of this depletion has been attributed to the use of chlorofluorocarbons (CFCs) and similar chemicals in manmade products. Ozone is unstable, and these chemicals catalyze the breakdown of ozone into oxygen molecules and free oxygen.  Countries in North America and Europe have been working toward the elimination of CFCs.

The warming of the Earth as the result of increasing amounts of greenhouse gases in the atmosphere is also a cause of concern.  Greenhouse gases, which include carbon dioxide, methane and water vapor, absorb and emit heat.  Many people are concerned with the effect that human activity has had on the production of greenhouse gases and therefore, on climate.