The Oxygen Revolution

The Earth is roughly four billion years old, a conglomeration of leftovers from the cloud of gas and dust that gave birth to the sun. The Earth's primeval, or original, atmosphere probably consisted of carbon dioxide, nitrogen and water vapor. But as the Earth cooled, oceans formed and much of the atmospheric carbon dioxide dissolved into the water, leaving the Earth blanketed in mostly nitrogen.

Photosynthetic life - primarily in the form of phytoplankton - arose on Earth about 3.3 billion years ago. Photosynthesis is a metabolic process by which some organisms use sunlight, carbon dioxide and water to make energy. Oxygen is a waste product of photosynthesis, but it also happens to be necessary for the metabolic process used by animals. Bacteria and single-celled animals appear in the fossil record relatively soon after free oxygen began to accumulate in the atmosphere. But it was not until about 500 million years ago, not long after plants and trees had colonized the land, that larger, multi-cellular animals exploded onto the scene.

Today, oxygen makes up roughly 21% of the Earth's atmosphere and nitrogen makes up about 78%. Carbon dioxide, methane, water vapor and other compounds each account for a tiny percentage of today's atmosphere. Natural processes maintain the Earth in this delicate balance. But as we've seen, small changes can make a big difference.

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