PHYLOGENY 



isms possess enzymes which are specific for the necessary reactions. But 

 these reactions are frequently quite complex, requiring a long series of 

 steps with a different enzyme controlling each step. Thus it has been 

 shown that the synthesis of arginine, an amino acid, by the mold Neuro- 

 spora requires at least seven different genically controlled enzymes. Each 

 of these is useless without all of the others. That such "useless" genes 

 should be preserved by natural selection is incomprehensible. But the 

 probability that all of them should appear simultaneously, and so become 

 useful, is equally incomprehensible. 



Horowitz has devised a very clever solution to this dilemma, namely 

 that the enzymes might have been originally acquired in a sequence oppo- 

 site to that in which they are used by existing organisms. Thus a primitive 

 organism requires a substance A, which is abundantly present in its en- 

 vironment, as are also substances B and C, from which A could be synthe- 

 sized in the presence of an appropriate enzyme. As long as A remains 

 abundantly present in the environment, presence or absence of the en- 

 zyme can have no bearing on survival. But eventually the food require- 

 ments of the growing population of primitive organisms may be expected 

 to outstrip the stock pile of organic compounds which was built up during 

 the abiotic ages. Thus when A becomes rare in the environment, those 

 organisms which have the enzyme for the production of A from B and C 

 will have a selective advantage, and will replace the original type. Such 

 preadaptafion is a common phenomenon in evolution. The enzyme has 

 now become a permanent part of the organism's biological equipment. But 

 B might also be synthesizable from D and £ in the presence of the neces- 

 sary enzyme. Then when B becomes scarce, the possession of this enzyme, 

 acquired by mutation, will also have selective advantage. There is no 

 theoretical reason why such a process should not continue until an organ- 

 ism had acquired the ability to synthesize all of its requirements from the 

 elements. This has happened in the case of the green plants. Such an 

 organism is said to be autotrophic. 



Diversification of the Bacteria. It seems probable, then, that the 

 primitive free-living viruses and bacteria graduallv used up the available 

 supply of proteins and other complex compounds in the environment, and 

 that they simultaneously, step bv step, developed the svstems of enzymes 

 necessary for the biosvnthesis of the samc^ compounds. Or it may be that 

 only the primitive bacteria were sufficientl\' complc'x to permit this devel- 

 opment, the viruses remaining completeh' dependent upon preh)rmed 

 compounds. If so, this would account for their obhgate parasitism, for 

 only within the protoplasm ol undoubted organisms could an environment 

 be found to contain all uecessar\' food substances in the completely elab- 

 orat(Hl form. The evolution of tlie bacteria made parasitism possilile; the 

 impoverislmient of the earth's supplv of complex organic compounds 

 made it desirable^ lor such organisms as could not synthesize their food 

 requirements at least in [lart. 



Respiratory Systems and Photosynthesis. The most primitive organ- 

 isms, living in an environment which included an abundance of the most 

 comp](>\ substances whicli the orgam'sms might re({uire, must hav(^ had 



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