4.6 billion years ago, when Earth was formed, it consisted of nothing more than a rocky sphere, surrounded by an atmosphere of hydrogen and helium. Due to the lack of a magnetic field it is believed that this early atmosphere was subjected to harsh interstellar winds and radiation and it dissipated into space. The formation of many volcanoes, which accompanied the establishment of Earth’s crust, caused large amounts of gaseous compounds to be ejected into the atmosphere, such as ammonia, carbon dioxide and water vapour. The introduction of these new gases, however, would not support the modern diversity of life that we know.
3.3 billion years ago, bacteria arose, and cyanobacteria, a specialised type of bacterium began to dominate earth’s surface. Cyanobacteria are special in that they contain the same green, light-trapping pigment as plants: chlorophyll. The success of cyanobacteria set the stage for the evolution of life and the development of the atmosphere we know today. Chlorophyll is a pigment that allows an organism to harness the energy from the light of the sun by converting carbon dioxide into usable carbohydrates, a food source. Although requiring carbon dioxide, the process produced oxygen. Cyanobacteria became so prolific that they changed the entire composition of the earth’s atmosphere to an oxygen-rich one which would later be the foothold for life. Currently earth’s main atmospheric constituents are 21% oxygen and 78% nitrogen as well as smaller amounts of other gases such as water, argon and carbon dioxide.
Will life be responsible for the next major atmospheric transformation?
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