INITIAL SYSTEMS 335 



8-aminolaevulinic acid by decarboxylation. Two molecules of 

 the latter condense to form porphobilinogen and four mole- 

 cules of porphobilinogen give a porphyrin structure which 

 forms protoporphyrin by decarboxylation and dehydrogena- 

 tion of the side chains/* 



Each link in this chain of chemical transformations 

 requires the participation of specific catalysts, enzymes. It is 

 only because of this that each product of a preceding reaction 

 enters into the proper succeeding reaction in the chain and 

 does not wander off into the many other reactions which are 

 thermodynamically possible for it. 



The initial systems from which 

 living things arose. 



Something similar to this series of chemical reactions must 

 have taken place in the hydrosphere leading up to the prim- 

 ary syntheses of porphyrins and other complicated organic 

 compounds. The nature of these chains of reactions of com- 

 plicated organic substances which preceded the appearance 

 of life is therefore very important in connection with our 

 problem. 



It may now be taken as an established fact that in such 

 simple reactions occurring in the gaseous phase as the oxida- 

 tion of the louver hydrocarbons or other similar reactions 

 which took place in the primaeval atmosphere, an essential 

 part was played by the free radicals which were initially 

 brought into being by the action of radiations or electric 

 discharges and perpetuated in the course of chain reactions. 

 For example, the passage of an electric discharge through 

 water vapour leads to the formation of hydroxyl radicals 

 which can oxidise hydrocarbons according to the following 

 scheme^^: 



H.o >OH + H the initiation of the chain 



' Uhe continuation or the chain 



R + Oo > M2 + OH etc. j 



a particular example is: 



CH4 + OH -> CH3 + HoO 



CH3 -i-Oo -> HCHO + OH etc. 



