498 INTRODUCTION TO EVOLUTION 



adenylic-acid system of free energy transport was a very early step in the 

 origin of life (recall our discussion on pp. 90-91 ) and that this fact explains 

 why subsequent forms of life have concentrated upon that special system 

 rather than upon some other system which might have served the same 

 purpose. We may compare this to our word game; in choosing "disestab- 

 lishmentarian" at the outset we chose to concentrate on certain letters 

 to the exclusion of others. 



Again, Blum ( 1955) has pointed out the possible significance of the fact 

 that all amino acids entering into the formation of proteins are of the "left- 

 handed" variety, i.e., their solutions rotate the plane of polarized light to 

 the left. Yet both "right-handed" and "left-handed" amino acids are 

 readily synthesized in the laboratory. Why are living organisms composed 

 of "left-handed" ones only? We cannot give a definite answer but we may 

 reasonably conclude that historical events underlie that answer. Perhaps 

 the first proteins simply chanced to be formed of "left-handed" amino acids 

 only. Or perhaps the first proteins were formed under conditions in which, 

 for some reason unknown to us, "left-handed" amino acids were the ones 

 predominantly available. Lacking any reason for thinking that "right- 

 handed" amino acids could not be built into proteins, we see here another 

 probable instance of the determination by past events of subsequent ones. 



Many other examples might be given but enough has been said to indi- 

 cate that inclusion of chemical compounds in the body is determined, 

 not alone by their suitability, but also by the past history of life. This may 

 well be true of the genes themselves, composed of deoxyribose nucleic acid 

 (DNA). Because of the strategic importance of genes and their mutations 

 the particular chemical properties of DNA are of great significance to evo- 

 lution. Genes may undergo a variety of chemical changes (mutations) but 

 the variety is not endless. Limitations are set by the chemical structure of 

 DNA, just as limitations on the number of words that can be made from 

 the letters in "stamina" are set by the variety of letters in that word. And 

 not all the chemical changes possible to DNA (probably enormous in num- 

 ber) would constitute mutations capable of actual existence, just as not all 

 combinations of letters from "stamina" constitute actual words. 



Furthermore, not all the chemical changes that a gene can undergo are 

 equally likely to occur. The point is an important one but the underlying 

 reasons cannot be explained in an elementary discussion (see Blum, 1955). 

 Briefly, the reasons relate to the fact that different chemical changes have 

 different energy requirements. Changes requiring little energy of activation 

 occur more readily, and hence more frequently, than do changes requiring 

 investment of greater amounts of energy. As a result a gene will give rise 



