XIII 



The Origin of Life and Photosynthesis 



H 



The smallest body which possesses all the characteristics of life, the 

 cell, is quite obviously an extremely complicated structure. At its 

 simplest it contains at least two or three complex and interlocking 

 cycles of activity, which are concerned with the breakdown of food 

 materials and with the formation of high-energy and other inter- 

 mediates which are used for the synthesis of the characteristic com- 

 pounds. The agents which bring about these changes are themselves 

 formed from the products. Apart from all this basic chemical activity, 

 there are also the special arrangements by which cell division occurs. 



In trying to discover how systems of such complication came 

 into existence in the first place, it is probably hopeless to look at cells 

 as they exist now, because they have an enormously lengthy history 

 behind them. Unfortunately, in our present day world, we do not find 

 any intermediate steps, except perhaps viruses (which are a special 

 case and a product of cells) which would help to bridge the gap be- 

 tween the living and the non-living. 



Yet all life must have originated and been elaborated on the 

 earth. There must have been a time when the earth was too hot for 

 any living things as we know them. The igneous rocks underlying the 

 present superficial strata bear evidence of having crystallized out 

 from a molten magma. The stratified rocks which overlie them bear 

 witness in their fossil relics to the long history of life on the earth and 

 many of them — the chalks, corals and limestones are actual residues 

 of former life on the planet. But the earliest stages are missing from 

 the fossil record, and it is probable that something approaching life 

 as we know it at present had already been evolved before any en- 

 during traces were left. 



It might have been expected that if the original processes which 

 led to living structures were completely natural, they would still be 

 taking place and that we should find, somewhere in the world, pro- 



