CHAP. 32] THE ORIGIN OF MFE 861 



structures to carry out their metabolism. Hence heterotrophic organisms would 

 have formed first. 



A mechanism by which heterotrophic organisms could acquire various 

 synthetic abilities was proposed by Horowitz (1945). It has been found that the 

 presence of an enzyme in an organism is often dependent on a single gene. This 

 is known as the one gene-one enzyme hypothesis. Suppose that the synthesis of 

 A involves the steps 



D-^C-^B-^A, 



where a, b and c are the enzymes and A, B and C are compounds which the 

 organism cannot synthesize. If the necessary compound A becomes exhausted 

 from the environment, then the organism must synthesize A if it is to survive. 

 But it is extremely unlikely that there would be three simultaneous mutations 

 to give the enzymes a, h and c. However, a single mutation to give enzyme a 

 would not be unlikely. If compounds D, C and B were in the environment when 

 A was exhausted, the organism with enzyme a could survive while the other 

 would die out. Similarly when compound B was exhausted, enzyme h would 

 arise by a single mutation, and organisms without this enzyme would die out. 

 By continuing this process the various steps of a biosynthetic process could be 

 developed, the last enzyme in the sequence being developed first, and the first 

 enzyme last. 



It is necessary for all living organisms to have a source of energy to drive the 

 biochemical reactions that synthesize the various structures of the organism. 

 According to the heterotrophic hypothesis the first organisms would obtain their 

 free energy from fermentation reactions. For example, the lactic acid bacteria 

 obtain their free energy from the reaction 



CeHisOe (glucose) = 2CH3CH(OH)COOH (lactic acid). 



Yeasts also ferment glucose, but they produce ethyl alcohol and CO 2 instead 

 of lactic acid, and there are bacteria which carry out many other types of 

 fermentation reactions. It is likely that organisms obtained their free energy 

 from fermentation reactions until the supply of fermentable compounds was 

 exhausted. At that point it would have been necessary for the development of 

 photosynthetic organisms which could obtain their free energy from light. 



The first type of photosynthesis was probably similar to that of the sulfur 

 bacteria and blue-green algae which carry out the reactions 



2H2S + CO2 ^ 2S + (H2CO) + H2O 



H2S + 2CO2 + 2H2O ^ H2SO4 + 2(H2CO), 



where (H2CO) means carbon on the oxidation level of formaldehyde (carbo- 

 hydrates). It is much easier to split H2S than to split H2O, and so it would seem 

 likely that organisms would develop photosynthesis with sulfur first. When the 

 H2S and S were exhausted, it would become necessary to split water and 

 evolve O2, the hydrogen being used for the reduction of CO2. 



