EARTH'S ATMOSPHERE IS unique in the solar system: of all the heavenly bodies, Earth alone has an atmosphere capable of supporting life. An appreciation of this highly specialized nature of the contemporary atmosphere is vital to understanding its evolution. The atmosphere has not always been the way it is today, and the present atmosphere is almost certainly not directly related to Earth's primordial atmosphere. The contemporary atmosphere arose from thermal and geological processes occurring some time after Earth's formation, and its evolution to a life-supporting state is intimately linked to the evolution of life itself.