FORMATION OF FREE OXYGEN 449 



been established in the atmosphere, some of these new paths 

 became the broad highways for the development of most of 

 the living things on our planet, while others degenerated 

 into narrow side alleys along which only a very few groups 

 of specialised organisms pursue their metabolic activities. 



Let us try to imagine the circumstances which prevailed 

 on the surface of the Earth at the time which we have been 

 describing, about 700 or 800 million years ago. The exogen- 

 ous organic substances which had originally been formed, 

 and which could serve as nutrients for the heterotrophic, 

 anaerobic organisms which then inhabited the Earth, had 

 largely disappeared. The atmosphere contained an abun- 

 dance of carbon dioxide, hydrogen, methane and other 

 gaseous substances, which had been formed by various fer- 

 mentative processes. Dissolved in the water of the seas and 

 oceans there were ethyl alcohol, various organic acids and 

 the waste products of anaerobic metabolism which were of 

 no further use. Partly in solution and partly in the deposits 

 there were carbonates and a number of reduced inorganic 

 substances such as ferrous oxide; some of these had remained 

 in their original state and some, such as ammonia and hydro- 

 gen sulphide, had arisen biogenically. 



All these substances were relatively inaccessible to the 

 living things of that period in the absence of free oxygen. 

 Only the earliest photosynthetic organisms, which had already 

 arisen by that time, were able to make extensive use of, and 

 almost monopolise, the diverse organic residues of fermenta- 

 tion and such substances as methane, h, and H2S as hydrogen 

 donors for reduction of the CO2 which they fixed, and for 

 building up their structural components. Thanks to this 

 they must have obtained a considerable advantage in the 

 struggle for existence at that particular time. Their rapid 

 development and evolution, which occurred as a result of 

 this advantage, provided a basis for the emergence of the 

 extremely complicated and efficient metabolic mechanisms 

 which are characteristic of present-day photoautotrophs. 



However, in the very process of their development, these 

 organisms began to enrich the atmosphere with molecular 

 oxygen. This entailed a profound alteration in the course 

 of the evolution of life as a whole on our planet. The appear- 



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