CHEMICAL STRUCTURE AND METABOLISM 103 



ancestral group no longer existing. Parenthetically it is interesting to note 

 that precipitin tests (Chap. 6) also indicate that echinoderms are the in- 

 vertebrates most closely allied to Phylum Chordata (Wilhelmi, 1942). 



Chemical Mutations 



Florkin and Morgulis (1949) have called attention to the fact that phos- 

 phoarginine and phosphocreatine have similar chemical structures. One 

 could give rise to the other by chemical changes of no great magnitude. 

 Such changes could arise in evolutionary history through one or more 

 gene mutations. The sequence would be: (1) change in gene, with (2) re- 

 sulting change in the chemical compound whose nature was determined 

 by that gene. Such mutations evidently occurred in the line leading to 

 vertebrates, changing some, and eventually all, of the PA production to 

 PC production. These mutations may also have occurred in other lines, 

 as, for example, in the ancestry of the few annelid worms mentioned 

 above as possessors of PC or a closely allied compound. 



Another example of chemical mutation cited by Florkin and Morgulis is 

 that of a peculiar respiratory pigment found in the blood of certain fam- 

 ilies of annelid worms. The respiratory pigment in our own blood is hemo- 

 globin, the red substance in our red blood cells. Hemoglobin consists of 

 an iron-containing compound (heme) combined with a protein (globin). 

 Hemoglobin carries oxygen from the lungs to cells in need of it. All verte- 

 brates possess hemoglobin, as do a large number of invertebrates. In- 

 cluded in this group are many annelid worms, such as the common earth- 

 worm. But three families of annelid worms have a blood pigment which 

 is green, rather than red. It is called chlorocruorin. A member (Serpula) 

 of one of these three families possesses both chlorocruorin and hemoglo- 

 bin. Although the two substances have contrasting colors, the chemical 

 differences between the iron-containing portions of them are of a minor 

 nature. There are also differences in the amino acids comprising the pro- 

 tein portion. But so far as the iron-containing component is concerned, a 

 relatively small genetic change, perhaps even a single mutation, could have 

 produced the observed chemical alteration. 



Actually, all mutations (except chromosomal aberrations — see p. 396) 

 are "chemical mutations'" in the sense that they are chemical changes in 

 genes, resulting in chemical changes in bodily materials and processes con- 

 trolled by those genes. Even mutations having structural changes as their 

 most conspicuous effects are chemical mutations in this sense. The struc- 

 tural change has as its basis some change in chemical metabolism opera- 



