The earliest living organisms were microscopic bacteria, which show up in the fossil record as early as 3.4 billion years ago. As their numbers multiplied and supplies of their chemical fuel were eaten up, bacteria sought out an alternative energy source. New varieties began to harness the power of the sun through a biochemical process known as photosynthesis—a move that would ultimately lead to simple plants and which opened the planet up to animal life.
Some three billion years ago the Earth's atmosphere was virtually devoid of oxygen. At about 2.4 billion years ago, oxygen was released from the seas as a byproduct of photosynthesis by cyanobacteria. Levels of the gas gradually climbed, reaching about one percent around two billion years ago. About 800 million years ago, oxygen levels reached about 21 percent and began to breathe life into more complex organisms. The oxygen-rich ozone layer was also established, shielding the Earth's surface from harmful solar radiation.