90 INTRODUCTION TO EVOLUTION 



Thus by a series of steps involving enzymes and coenzymes are accom- 

 plished chemical transformations making possible transfer of free energy 

 from glucose to cell mechanisms. The sequence of chemical events is ex- 

 tremely complicated. Prominent among the intermediate substances in- 

 volved are phosphorus-containing derivatives of adenylic acid. The ele- 

 ment phosphorus plays a particularly important role in the mobilization 

 and transport of the free energy required for cellular activity. Readers with 

 some knowledge of organic chemistry will find in textbooks of biochemis- 

 try and physiology (e.g., Harrow and Mazur, 1958) details of the process. 

 Here we wish to emphasize one point. Insofar as our knowledge enables 

 us to reach a conclusion, these complex metabolic processes, with their 

 enzymes and coenzymes, are fundamentally similar in all animals. Indeed, 

 we may extend this statement to plants since, aside from their unique ca- 

 pability of photosynthesis, plants are much like animals in metabolic proc- 

 esses. There are many variations on the theme, but the theme itself under- 

 lies all variations. No more convincing evidence of the fundamental unity 

 of all life could be found. 



The Origin of Life 



Our brief discussion of the building of energy-rich compounds (e.g., 

 glucose) from energy-poor ones (e.g., carbon dioxide and water) raises an 

 interesting question concerning the origin of life on earth. We noted that 

 enzymes are necessary in the manufacture of carbohydrates by green 

 plants. We also noted that enzymes are proteins, and proteins have much 

 more complex chemical structure than do carbohydrates. Now, proteins 

 are built of amino acids, and the latter in turn are formed in part of car- 

 bohydrates. Yet in photosynthesis carbohydrates cannot be formed un- 

 less proteins are already present to serve as enzymes! Here we seem in 

 danger of becoming involved in a vicious circle of the "which came first, 

 the hen or the egg" variety. What we really wish to know is this: Back 

 in the beginning when there were no proteins to serve as enzymes how 

 were the first energy-rich compounds manufactured from energy-poor 

 ones? 



Undoubtedly the appearance of the first life on this planet was preceded 

 by a long period of chemical evolution, a period during which chemical 

 compounds were being formed and rebuilt under the influence of radia- 

 tions from the sun, of temperatures then prevailing, and of other condi- 

 tions found at the time. We may feel sure that carbon-containing com- 

 pounds were among those formed in this way. They would have been 



