450 FURTHER EVOLUTION 



ance of molecular oxygen provided a theoretical possibility 

 for even the colourless heterotrophs to rationalise their 

 metabolism and to make use of substances which could not 

 be used before by ordinary anaerobic heterotrophs. Under 

 these circumstances the practical realisation of this possibility 

 only required very small additions to the previously existing 

 metabolic mechanisms of the organisms. In particular, as 

 we pointed out on p. 430, the transition from obligate to 

 facultative anaerobiosis could be brought about simply by 

 an alteration in a single link in the long chain of glycolytic 

 degradation. This involved the replacement of anaerobic 

 decarboxylation of pyruvic acid by its oxidative decarboxyla- 

 tion and the reaction whereby acetyl-coenzyme A is formed 

 proceeded in accordance with the equation with which we 

 are already familiar : 



CH3C0.C00H + CoA + |o2->acetyl-CoA -f CO2 + n^o. 



Thus, such facultative anaerobes as Esch. coli and Strep, 

 faccalis can, under aerobic conditions, not only break sugar 

 down to lactic acid, but can also oxidise it to acetic acid, 

 which is considerably more advantageous from the point of 

 view of acquiring energy. In the absence of oxygen they 

 form, as well as acetic acid, reduced products such as ethyl 

 alcohol which are useless to these organisms under the postu- 

 lated conditions. On the other hand, the acetic acid bacteria, 

 advancing even further, became confirmed aerobes ; they 

 can oxidise not only sugar, but also ethyl alcohol to acetic 

 acid, thus putting it back into circulation in their energy 

 metabolism and mobilising the energy of this waste product 

 of fermentation which was previously of no use whatsoever 

 to heterotrophs. 



The line of evolution which began in this way seems to 

 have been the outset of the development of various faculta- 

 tive anaerobes which effect many so-called oxidative fermenta- 

 tions."^ 



Chemosynthesis. 



It is our opinion that it was in this transitional period 

 that metabolism became differentiated, and that such special- 



