|MadSci Network: Earth Sciences|
This, in fact, is an excellent question! It illustrates the effect that simple microbes have on the composition of gasses in our atmosphere.
The earth's early atmosphere had little to no oxygen in it (~5 billion years ago). We know this by studying rocks >3.5 billion years old which contain many kinds of elements in their *reduced* states (reduced due to the lack of O2 to produce oxides..). The atmosphere consisted of an anaerobic mixture of water, CO2, carbon monoxide (CO), nitrogen, methane and hydrogen gasses that leaked from the cooling interior of the planet. Ozone is produced by the effects of UV radiation on oxygen gas, as occurs in the ozone layer:
singlet O (produced by ionizing radiation) + O2 --> O3No oxygen, no chance to produce ozone.
However, around 1 billion years ago, early prokaryotes, namely blue green algae, evloved the means to harness sunlight for energy (photosynthesis). The "light reactions" of photosynthesis break down water to gather electrons for later use, releasing oxygen gas. The massive release of O2 created the greatest incident of global pollution seen on the planet, and significantly altered the course of evolution.
More information can be found at NASA's tutorial on the Ozone Layer/Atmospheric Chemistry
Thanks for your question..
-L. Bry, MadSci Admin
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