448 FURTHER EVOLUTION 



these conditions it, as it were, reverts to a more primitive 

 form of photosynthesis, reducing CO2 by means of molecular 

 hydrogen or endogenous, organic hydrogen donors. Natur- 

 ally no oxygen is given off under these circumstances. 



Thus, under these conditions Scenedesmus reverts to a 

 form of metabolism similar to that which we described above 

 as occurring in the heterotrophic Athiorhodaceae or in the 

 autotrophic hydrogen bacteria. In the latter case the over-all 

 result of the photosynthesis carried out by Scenedesmus may 

 be expressed by the equation 



2H2 -I- C02->CH20 -f H2O 



In this reaction, which involves the oxidation of molecular 

 hydrogen, the enzyme hydrogenase plays an important part, 

 being adaptively activated under reducing conditions. When 

 oxygen is present, or when the illumination is more intense, 

 the activity of the hydrogenase is destroyed and the alga 

 reverts to its normal metabolism, photoautotrophic absorp- 

 tion of CO2 and production of O2. 



The formation of free oxygen. 



The period when autotrophic photosynthesis was coming 

 into being and leading to the formation in the atmosphere 

 of ever greater and greater amounts of molecular oxygen, 

 liberated from water by means of the energy of the long-wave 

 components of sunlight, was one of the most remarkable 

 periods in the whole history of our planet. It was a critical 

 time, separating the two important epochs of the history of 

 the surface of the Earth, the reducing and the oxidising 

 epochs. 



This period is of especial interest from the point of view 

 of the student of the evolution of metabolism because it was 

 just in this transitional epoch, when the external conditions 

 of life were radically altered, that there arose numerous and 

 diverse new forms of metabolism, there occurred what might 

 be described imaginatively as a tense search for new paths 

 for the process of life. Later on, when this revolutionary 

 period of ' Sturm und Drang ' had become a thing of the 

 past, when more or less constant oxidising conditions had 



