14 I The Process of Evolution 



The efficiency of the transmission apparatus has been a major factor 

 in determining the hmits of intricacy of Hving entities. 



In the 2 or 3 bilHon years of chemical evolution that preceded the 

 evolution of life, many systems of transmitting information may have 

 been tried and discarded in a selectional process. It is clear that the 

 hereditary system must have been coupled to the synthetic and 

 energy-converting systems; therefore it is no surprise to note that 

 the substances involved in the hereditary system, the nucleic acids, 

 have adenosine phosphate as a building block. The system found 

 in most cellular organisms is based on coding information in two 

 macromolecular nucleic acids, ribonucleic acid (RNA) and deoxy- 

 ribonucleic acid ( DNA ) . 



The assumption usually drawn is that the giant ordered molecules 

 of which living systems are composed are the evolutionary end 

 result of some process of aggregation of smaller molecules and that 

 selective forces controlled the process. With the spontaneous ran- 

 dom occurrence of a sequence of ordered molecular subunits of 

 nucleic acid, for example, a template against which further se- 

 quences are replicated becomes possible. This leaves essentially 

 unanswered and unanswerable the question of how the original 

 sequence arose. 



This is not the only possible interpretation, however, as Pattee 

 has pointed out. He suggests that the precursors of biological macro- 

 molecules were not random sequences but naturally ordered crystal 

 structures. These result from various restrictions found in crystal- 

 lization processes in general. In the growth of polymers the con- 

 figuration at a particular stage may determine which subunits are 

 added. Thus there may be feedback control of the growth of macro- 

 molecules. By using a computer model (Fig. 1.3), Pattee has shown 

 that, with feedback, simple configurations can be assembled to 

 produce elaborate, repeating, and well-ordered sequences. It is not 

 necessary to postulate, as most authors have done, a statistically 

 highly improbable preexisting sequence that must be copied. In 

 Pattee's view, the present genetic mechanisms themselves are the 

 evolutionary result of the natural occurrence of ordered macro- 

 molecular sequences. 



The exact functioning of genetic mechanisms has not been 

 elucidated entirely, but in general outlines it is as follows: The units 

 of information, known as genes, are coded into the structure of giant 

 self-replicating molecules of DNA. These molecules, reproduced 

 and passed from generation to generation, are the master blueprints 

 from which all living organisms are produced; they maintain the 

 continuity of life. Slight changes in these blueprints are also repro- 



