The biosphere of Earth developed in several stages, and oxygen appeared in it not at once. Before it reached modern 21% in the atmosphere, there passed the long time. Now without this chemical element it is difficult to imagine life on the planet in a look, habitual for us.
Instruction
1. Initially living organisms used organic chemistry of primary ocean in food. As a metabolism by-product in the atmosphere it was allocated, collecting, carbon dioxide. However stocks of "primary broth" were pretty fast exhausted.
2. Further anaerobic organisms which could synthesize organic matter from the carbon dioxide and hydrogen which was also present at the atmosphere gained broad and primary development. They with the participation of hydrogen restored carbon dioxide to methane.
3. At formation of methane the energy which living organisms used in the processes of activity was released. Getting to the atmosphere, methane under the influence of ultraviolet rays turned into the organic compounds which were again returned to water. Concentration of methane in composition of the atmosphere remained then approximately at one level.
4. So proceeded until in the atmosphere there was enough hydrogen. Over time reserves of this gaseous substance were exhausted, and metanobrazuyushchy bacteria lost a food source, without having an opportunity to transform carbon dioxide to methane. The new form which became photosynthesis was necessary for obtaining energy and a metabolism.
5. The first photosynthesizing microorganisms did not emit oxygen. There were later organisms as a result of which photosynthesis the atmosphere began to be saturated with oxygen. Gradually the composition of the atmosphere of Earth in which oxygen took more and more places changed.
6. When oxygen only appeared in the atmosphere, for living organisms of those times it was strong poison. There came ecological crisis. Life had to disappear from the face of the earth or adapt to new conditions, having found a way to neutralize this poison. And such mechanism was found.
7. Further there were living organisms which began to use oxygen for obtaining energy. So there was an oxygen breath. Thanks to photosynthesis the ozone screen which protected the planet from pernicious ultraviolet that allowed living beings to master the top layers of reservoirs subsequently was created and even to come out of the water. Energy to which the appeared breath process provided living organisms gave an impetus for their further development and complication.