THE ORIGIN OF LIFE 



probability of the achievement of good adaptation, particularly under 

 changing conditions, than could otherwise be obtained. 



Bacteria and Phylogeny. The phylogenetic relationships of the bac- 

 teria are by no means clear, primarily because of the great difficulties of 

 taxonomic studies on the bacteria. Figure 31 illustrates typical bacterial 

 morphology. Two major alternative theories of the phylogenetic sequence 

 within the bacteria have been presented. The first of these maintains that 

 the autotrophic bacteria are the more primitive ones, and that these have 

 given rise, through a series of intermediates, to the heterotrophic bacteria. 

 The iron bacteria have sometimes been regarded as the most primitive of 

 all, perhaps because of their low energy output. Adherents of this theory 

 believe that the most primitive bacteria must have been autotrophic be- 

 cause there could have been no source of organic compounds for a hetero- 

 trophic organism. This theory was generally held before the publication of 

 Oparin's work. But, as this demonstrated the plausibility of the thesis that 

 organic material might have been accumulated by a slow process of chem- 

 ical evolution long before the advent of life, the theory that the hetero- 

 trophic bacteria may be more primitive, with the autotrophs derived from 

 them, gained prestige. According to this idea, the several chemosynthetic 

 mechanisms represent a natural trial-and-error experiment, resulting in a 

 superior new energy source which culminated in the development of 

 photosynthesis. 



The knowledge of the relationship of the bacteria to higher organisms 

 is also unsatisfactory, but some indications of possible relationships are at 

 hand. As has been mentioned above, some of the sulfur bacteria contain 

 a green pigment, bacteriochlorophyll or bacteriochlorin. This pigment is 



.• • • •• • 



A • B 



II 



If . • f^ 



' ) c 



\ 





F • G I ' H 





K L 



Figure 31. Morphologic Types of Bacteria. A, micrococcus; B, diplococcus; C, 

 staphylococcus; D, streptococcus; E, sarcina; F, coccobacilli; G and H, commonly 

 occurring rod forms; I, fusiform bacilli; J, curved rods or vibrios; K and L, spiral 

 forms. ( From Jordan and Burrows, "Textbook of Bacteriology," 16th Ed., W. B. Saun- 

 ders Co., 1954.) 



