85 



HCN oligomers. Of the pyrimidines, cytosine is formed directly by 

 the reaction of cyanoacetylene and cyanate, while uracil is the final 

 product using either HCN or cyanoacetylene as the starting material. 

 These are only examples from an extensive scientific literature. 



Sugars and Nucleosides 



The self-condensation of formaldehyde was a likely route to 

 sugars on the primitive Earth, the formaldehyde being formed by the 

 action of electric discharges or UV light on a mildly reducing atmo- 

 sphere. This condensation is inhibited by HCN which reacts rapidly 

 with formaldehyde. Therefore, the synthesis of sugars probably was 

 delayed until the bulk of the HCN had condensed or was hydrolyzed. 

 It is not clear how the relatively few sugars which have the central 

 role in contemporary living systems (ribose and glucose) were 

 selected from the very complex mixture of compounds which is 

 formed from formaldehyde. 



Lipids 



The prebiotic formation of lipids has not been extensively 

 investigated, but the limited experiments which have been per- 

 formed suggest that lipid-like materials might have also formed 

 spontaneously on the primitive Earth. 



Biosynthetic Pathways as a Guide to Prebiotic Chemistry 



Since the primitive metabolic pathways probably evolved from 

 prebiotic syntheses, some steps in the current metabolic pathways 

 may be "chemical fossils" of reactions which occurred on the 

 primitive Earth. Consequently, a proposed prebiotic pathway gains 

 validity if it can be correlated with a contemporary biosynthetic 

 process. 



There is a good correlation between the contemporary biosyn- 

 theses of purines and pyrimidines and some of the steps in their pro- 

 posed prebiotic syntheses. 



