The Origin of the Biosphere 29 



removal of CO2 from the atmosphere. The even more powerful process of 

 photosynthetic formation of free oxygen made its appearance. It is just at this 

 time that one can also observe traces of plants and other organisms, as well as 

 oxidized iron ores which provide direct evidence of an oxygenous atmosphere. 

 Associated with the formation of an oxygen-containing atmosphere, there was 

 intensive oxidation of HoS and S and an increase in the amount of sulphates 

 in the oceans, as well as oxidation of NH4 and an increase in the amount of N2 

 in the atmosphere. During the later periods of the Proterozoic and in the 

 Cambrian, there occurred no noteworthy change in the composition of the 

 oceans or atmosphere. 



Having been brought into existence by the whole course of geological history, 

 the biosphere made its appearance when the Earth was between two and three 

 thousand million years old. 



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