438 FURTHER EVOLUTION 



to obtain energy from the environment and to assimilate the 

 simplest forms of carbon compounds. 



Photochemical reactions. 



The most powerful and inexhaustible source of energy on 

 the surface of the Earth is solar radiation. As we showed 

 above, the chief photochemical activity on the primaeval 

 Earth must have been that of short-wave ultraviolet radia- 

 tions which decompose water, in particular, to hydrogen and 

 oxygen in the upper layers of the atmosphere. Although the 

 hydrogen was constantly escaping from the atmosphere into 

 space the amount of oxygen thus liberated by inorganic 

 means was very small ; in any case it was not great enough 

 to account for the transition of the atmosphere from its 

 original reducing state to an oxidising state. This was because 

 the development of even small amounts of oxygen must 

 immediately have led to the formation of an ozone screen 

 which prevented the access of short-wave ultraviolet radia- 

 tions to the lower layers of the atmosphere. 



The radiations which fell in large amounts on the first 

 organisms must therefore have been of longer wavelength, 

 but these, as is well known, cannot by themselves bring 

 about such reactions as the photolysis of water. Nevertheless, 

 it was long ago established in a number of photochemical 

 studies^" that the energy of visible light can also be used 

 for carrying out oxidoreductive processes in the presence of 

 photosensitisers, especially organic pigments, capable of 

 absorbing such light. According to A. Terenin^^* the mole- 

 cule of pigment which absorbs the light dissociates into two 

 radicals and acquires a very high degree of chemical reactiv- 

 ity which enables it to receive or give up an electron or a 

 hydrogen atom and thus to bring about oxidoreductive pro- 

 cesses which could not come about spontaneously in the dark 

 without the addition of the supplementary energy of light. 



If organisms possessed such sensitisers then, even without 

 the help of complicated supplementary chemical mechanisms, 

 they could rationalise their heterotrophic metabolism by a 

 more complete oxidation of the organic substances available 

 to them in the external medium. 



