SELECTED PAPERS 



organic donators must obviously have outweighed the amount con- 

 sumed. And because the methane escaped into the atmosphere, the 

 end result must have been a decrease in the concentration of organic 

 donators and an increase in that of carbon dioxide. 



As additional hydrogen donator the medium must initially also 

 have contained hydrogen sulphide. But the system H 2 S plus C0 2 does 

 not in itself present any metabolic possibilities. At this stage there must, 

 however, have occurred a startling event. At a certain moment the 

 continually changing life must have acquired the ability to synthesize 

 compounds that possess the property of not merely absorbing radiation 

 in the visible and near-infrared region of the spectrum, but of doing 

 so in a manner in which the energy of the absorbed quanta can be 

 made subservient to the metabolic process. By virtue of this mecha- 

 nism it became possible to hydrogenate carbon dioxide to cell material 

 with hydrogen sulphide as hydrogen donator, and with the concom- 

 itant formation of sulphate as dehydrogenation product of the sul- 

 phide. Even to-day the sulphur purple bacteria bear witness to the 

 efficacy of this remarkable mode of life. 



With the exploitation of solar radiation as energy source, life, in 

 its collective sense, liberated itself at once from its dependence on an 

 external supply of organic matter. For the radiant energy was, so to 

 speak, transformed into organic matter and acceptor, so that at the 

 same time the conditions were created for the existence of sulphate 

 reducing bacteria which could reconvert the system in question into 

 the raw materials for photosynthetic life. On this basis anaerobic, life 

 created the possibility for an autonomous perpetuation ad infinitum, at 

 least as long as the influx of solar energy remained assured. 



Yet the enormous significance of this development is overshadowed 

 by the next stage in biochemical evolution. In a living entity that 

 based its existence on the system, radiant energy-hydrogen sulphide- 

 carbon dioxide, random mutations led to new forms of life capable of 

 substituting water for hydrogen sulphide as hydrogen donator. This 

 entailed dramatic consequences, firstly because of the virtually un- 

 limited availability of the new donator, water. As a result of the elim- 

 ination of the need for other donators the potential area of life's exist- 

 ence immediately became enormously expanded. But of still farther- 

 reaching significance was the circumstance that oxygen was produced 

 as dehydrogenation product of the water. This gas, escaping into the 



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