402 FURTHER EVOLUTION 



the simpler substances B, C or D which were present in 

 sufficient amounts in the surrounding medium. This would 

 then be repeated for substance B when it disappeared from 

 the external medium, and so forth. 



Thus, the ability to synthesise any particular complicated 

 component of protoplasm must depend on each separate link 

 in the process having arisen successively in the course of the 

 prolonged evolution of organisms. According to Horowitz, 

 the first living things must have been completely hetero- 

 trophic in the sense that they needed ready-made, compli- 

 cated, organic compounds for the building up of their bodies. 



Even for such building up, however, energy was needed 

 and the source of energy most readily available when the 

 organisation of the living bodies was still primitive was, once 

 again, organic substances. They contain large, hidden stores 

 of potential energy which can be mobilised comparatively 

 easily in the course of their degradation and used for bio- 

 synthesis either by means of linked reactions or by the forma- 

 tion of high-energy compounds. The exploitation of any of 

 the other kinds of sources of energy in the external medium 

 would have required the presence in the organism of acces- 

 sory systems which could only have arisen in the course of 

 very prolonged evolution. 



It is quite clear that the mobilisation of energy by the first 

 organisms could only have been brought about by the 

 anaerobic degradation of organic substances, as there was no 

 molecular oxygen in the atmosphere of the Earth under the 

 reducing conditions which prevailed at the time when these 

 organisms existed. Only when free gaseous oxygen appeared 

 in the atmosphere did there arise the theoretical possibility 

 of oxidising organic substances completely, in order to use 

 the energy locked up in them. In order to realise this possi- 

 bility, however, the organisms must, in addition to their 

 primary, anaerobic, energy metabolism, have created in 

 the course of their evolution, under the new conditions of 

 the external medium, new oxidative enzymes and new 

 systems of reactions which certainly could not have arisen 

 at earlier periods in the history of life when the atmosphere 

 was of a reducing nature. 



The gradual complication and integration of both the 



