The Relation between Primitive and 



Present-day Photobiological 



Processes 



C. REID 



The University of British Columbia, Canada 



The primitive photochemical processes which resulted in the syntheses of 

 simple organic compounds on the Earth are not known. In view of the many 

 possible routes it is unlikely that the pathway of molecular evolution, first 

 random and later self-sustaining, that actually led to our present forms of hfe 

 can ever be reproduced and we have no assurance that such a pathway is unique. 

 However, the comparative ease with which an array of amino acids can be 

 produced from the proposed primaeval atmospheric gases [i] makes it clear 

 that we can follow the early stages of many such simple syntheses. 



In setting up conditions for such experiments, the following postulates are 

 fairly generally accepted [2] : 



1. Little oxygen, and consequently no ozone layer was present in the 

 atmosphere. 



2. Solar energy extending to the neighbourhood of 2000 A was consequently 

 available. Since such energy is ample for the breaking of almost all chemical 

 bonds, it is in view of the very high flux of solar radiation a more likely choice 

 than atmospheric discharges as the energy source for the chosen process. 



3. In the initially reducing atmosphere NH3, CH4 and water vapour probably 

 predominated, with H2 as a minor constituent; CO2, intermediate oxidation 

 products of methane, and oxygen appeared only at a later stage. 



4. In view of the continuous 'trial and error' encounters which must have 

 been necessary, and the hydrated nature of the present-day key biological sub- 

 stances, most of the processes probably occurred in the sea, although the possi- 

 bility of atmospherically formed and subsequently dissolved primary substances 

 carmot be ruled out. 



5. In view of the number of features common to Ufe as we now know it, a 

 common origin is indicated. Any forms of life in which some of these features 

 are absent must be considered as possible representatives of very primitive forms. 

 This points to certain sulphur bacteria, in which sugars foimd in all other 

 organisms are absent, as the most Ukely candidates. 



GENERAL CONSIDERATION OF PHOTOPOLYMERIZATION 

 The continuous making and breaking of bonds under the influence of high- 

 energy radiation provides a perfectly reasonable process for the formation of 



189 



