12 I The Process of Evolution 



strictly photosynthetic reactions. The light reaction leads to the 

 production of a reducing agent and some type of high-energy phos- 

 phate, probably adenosine triphosphate (ATP). The reducing agent 

 usually is hydrogen but occasionally is a phosphorus compound. 

 These substances then operate the carbon reduction cycle, and 

 hexose sugar molecules are produced. This energy is mobilized, in 

 ways which are imperfectly understood, so that excited chlorophyll 

 transforms other molecules to produce the reducing agent and the 

 ATP. Thus, typically, photosynthesis involves photophosphorylation, 

 i.e., the transformation of light into the "energy currency" of phos- 

 phate bonds. 



It probably never will be possible to say with certainty whether 

 or not the coupling of colored compounds with biosynthetic proc- 

 esses took place before or after the appearance of what today would 

 be called living organisms. Calvin believes that the final step in the 

 development of modern photosynthesis, the production of oxygen, 

 did not take place until relatively late in the sequence of events. 

 Therefore reactions like those of some modern organisms, which 

 are photosynthetic but do not emit oxygen, were prior. 



ORIGIN OF STRUCTURE 



In the light of the above discussion, it is not overwhelmingly diffi- 

 cult to imagine how the substances required for the processes we 

 think of as metabolism could have arisen. However, living systems 

 are not fluid structureless entities. Generally they have a character- 

 istic and complex organization of the matter comprising this energy- 

 conversion mechanism. Now the factors involved in the evolution 

 of structure as well as of function must be considered. In the sea 

 the original molecules probably were dispersed as a rather uniform 

 colloidal suspension. However, in colloids of different substances, 

 semiliquid colloidal gels or coacervates are formed, and it might be 

 expected that these may have arisen as the organic soup became in- 

 creasingly complex. From the work of physical chemists, much is 

 known about the behavior of coacervates. They often do not form 

 as a continuous layer but rather separate out of the equilibrium 

 liquid (thus left colloid-poor) in the form of discrete droplets. These 

 droplets not only concentrate organic molecules of high molecular 

 weight but also possess a definite internal structure as well as a 

 highly developed surface separating them from the equilibrium 

 liquid. In the coacervate droplet one can see the first distinct sepa- 

 ration of a structural complex of organic material from its environ- 

 ment. 



