VIRUSES, RICKETTSIAE AND BACTERIA 23 



compounds. These substances are very much less complex than 

 the carbohydrates from which the higher animals obtain their 

 energy. The simple hypothesis is that the chemoautotrophs are a 

 side-line representing a more primitive state of development than 

 that shown by heterotrophic and photosynthetic bacteria. Though 

 this opinion is quite widely held, evidence is gradually accumula- 

 ting to indicate the opposite view; viz. that the chemoautotrophs 

 are in fact using systems that are secondarily simplified from those 

 of the heterotrophs. Thus O'Kane (1941) showed that the 

 sulphur bacterium Thiobacillus thioxidans could synthesise various 

 vitamins of the B group. These substances are used mainly in 

 normal heterocyclic heterotrophic glycolysis; thiamine is used in 

 oxidative decarboxylation; riboflavine is a coenzyme for the 

 hydrogen acceptors, nicotinic acid forms part of Coenzymes I 

 and II. It would therefore be interesting to know what role they 

 plav in Thiobacillus. It would appear that the bacterium has many 

 of the enzymes that are used in heterotrophic glycolysis but that 

 it uses special variations on the normal system. The chemo- 

 autotrophs would then have superimposed their own system upon 

 that of the heterotrophs. 



A schematic system for the development of metabolic systems is 

 shown below. If this is correct, and the chemoautotrophs are 

 less primitive than the heterotrophs, it again points the lesson 

 that the simplest explanations are not necessarily the correct 

 ones. 



Scheme for the origin of metabolic systems (after Oparin) : 



(1) Solution containing salts. 



(2) Solution containing salts and simple organic compounds. 



(3) Solution containing salts, simple and complex organic 

 compounds. 



(4) System that turns complex materials into simple organic 

 materials and so obtains energy. Also able to reproduce 

 itself '■= a living system. 



HETEROTROPHS (only glycolysis cycle) 



(5a) Living system that converts complex organic material to 

 simple material. 



HETEROTROPHS (glycolysis and citric cycles. 

 Hydrogen acceptors) 



