The formation of the Earth’s atmosphere occurred during the Precambrian period. The actual composition of the Earth’s atmosphere 4 billion years ago, however, is still an area of debate (Freeman and Herron 2007). During this period, it is hypothesized that the gaseous compounds found in the atmosphere were released by volcanic eruptions and rocks as the Earth’s surface was melted and transformed in to the crust and mantle (Schlesinger 1997). The type of compounds that were emitted were either composed of methane and ammonia or a reducing mixture of gases composed of N2, CO2, and H2O (Schlesinger 1997). Other volatile elements that were supposedly in the air are believed to have entered the atmosphere from comets colliding into Earth (Schlesinger 1997). These volatile elements, carbon, sulfur, nitrogen, oxygen, chlorine, and phosphorous remained suspended in the air until the Earth’s temperature dropped (Schlesinger 1997). With the cooling of the earth around 3.8 billion years ago, water vapor condensed and formed the oceans (Schlesinger 1997). With the presence of water, life could exist on Earth and O2 could accumulate (Lower 1998). While O2 can be produced by the phytochemical decomposition of gaseous oxides, Earth acquired most of its O2 though photosynthesis (Lower 1998). Early cyanobacteria release O2 in the atmosphere as one of the products of photosynthesis of (Lower 1998). Eventually as the numbers of cyanobacteria increased, the O2 level became suitable for more advanced forms of life, marking the beginning of the Cambrian period.

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Lower, S. 1998. Origin and Evolution of the Atmosphere. Accessed January 14, 2010

Schelsinger, W. H. (1997). Origin of the Atmosphere and Oceans. In Biogeochemistry: An Analysis of Global Change (pp. 24-29). San Diego, California: Acedemic Press.