In the absence of life the most important gas in the atmosphere is carbon dioxyde (CO2) which makes the atmosphere toxic for higher life forms. Life on Earth evolved through the activity of bacteria which reduced CO2 levels to tolerable concentrations and produced oxygen (O2) in the process.Examples for this planetary state are Venus and Mars; both have an atmosphere with a CO2 content of 95%.
The present make-up of Earth's atmosphere is maintained by continuous interplay between reduction of CO2 and production of O2 by plants and reduction of O2 and production of CO2 by animals and humans.
Without the presence of life the atmosphere would not maintain its composition. The earth's atmosphere is therefore said to be in "dynamic equilibrium", while in the absence of life atmospheres are said to be in "dead equilibrium".
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