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 The Earth probably had an atmosphere before the Sun was fully formed. This primitive atmosphere was possibly composed principally of hydrogen and helium, the two most abundant gases in the universe, along with a few simple compounds such as ammonia and methane. Then when the temperature and pressure in the contracting center of the still-forming Sun became high enough to ignite thermonuclear reactions, our Sun was born. The blast from the Suns formation must have produced a strong outflow of charged particlesan outflow strong enough to sweep the Earth of its earliest atmosphere.
 The next stage in the formation of the atmosphere likely occurred when gases trapped in the Earths hot interior escaped through volcanoes and fissures at the Earths surface. The gases spewed out by these early eruptions were probably much like the gases found in the volcanic eruptions of todayabout 85 percent carbon dioxide, and 5 percent nitrogen, by mass. The early atmosphere had no free oxygen and therefore was inhospitable to the type of life we have today. The production of free oxygen did not occur until the primitive plants known as stromatolites and green algae appeared. Stromatolites and green algae, like all green plants use photosynthesis to convert carbon dioxide and water to hydrocarbon and free oxygen. With the production of free oxygen, an ozone (O3) layer formed in the upper atmosphere. The ozone layer acts like a filter to reduce the amount of ultraviolet radiation reaching the Earths surface. The surface therefore became more hospitable to life. The evolution of oxygen in this global envelope was a vital step in the history of Earth and its life.
 As the Earth cooled, the rich supply of water vapor condensed to form the oceans. Comet debris from interplanetary space also contributed water to the oceans. These oceans, essential to the evolution of life and ultimately to the development of the present global environment, have remained for the rest of the Earths history.
Source: Hewitt, P.G., Suchocki, J., & Hewitt, L.A. (1999). Conceptual Physical Science, 2/e., New York, © Paul G. Hewitt, John Suchocki, and Leslie A. Hewitt. An imprint of Addison Wesley Longman, pp. 642-644.