The Evolution of Chemosynthesis 629 



confirmed the principle of unity of the fundamental features inherent in the 

 biochemical organization of living organisms, which forms the basis of com- 

 parative biochemistry [17, 27]. This unity of biochemical organization undoubt- 

 edly results from the fact that organisms with different types of metabolism 

 originate from a common root, namely, from a heterotrophic one. The simi- 

 larity of the most complex systems of biocatalysts in autotrophic and in hetero- 

 trophic organisms testifies to the fact that the former originated from hetero- 

 trophic organisms at an advanced stage of evolution of the latter, i.e. at the 

 period when the fundamental metaboHc mechanisms of primary heterotrophic 

 organisms had already in the main been developed as we know them now. This 

 is why autotrophic metabohsm is chiefly based on the biochemical mechanisms 

 of heterotrophic metabohsm, including the mechanisms of oxidation, energy 

 transfer and assimilation of CO2 [10, 28]. 



The driving force of evolutionary development from heterotrophy to auto- 

 trophy was the competition for organic nutrient substrate, the tendency of indi- 

 vidual groups of heterotrophic organisms to be freed from the necessity of 

 obtaining energy and carbon only from organic matter. The competition for 

 the organic substrate appears to have become particularly acute at the period of 

 evolution of heterotrophic micro-organisms when these minerahzed all the 

 stores of primary organic matter of abiogenic origin, converting them to H2, 

 CH4, H2S and NH3. The influence of this factor led some pigmented hetero- 

 trophic organisms to use their Hght-sensitive pigments for imparting by means 

 of the energy of Ught a high reduction potential to the hydrogen of water and 

 of some other compounds, and, in this way, to produce reducing agents of the 

 type of reduced diphosphopyridine nucleotide. These reductants participated in 

 the reduction of CO2, and their oxidation provided energy. In this way there 

 appeared the photoautotrophs, i.e. photosynthesizing bacteria and green plants. 



Chemosynthetic organisms arose later than the photoautotrophs, when, as a 

 result of photosynthesis, oxygen had appeared in the atmosphere. At any rate, 

 there is very little probabihty that the chemoautotrophs were the precursors of 

 photosynthetics assumed by van Niel [15] since, as will be seen below, they are 

 akin to the aerobic forms of bacteria. Most hkely these are two independent lines 

 of development. 



The results of research on the morphology of chemosynthetic organisms sup- 

 port the conclusion that the appearance of chemoautotrophic bacteria is the most 

 recent stage in the evolution of micro-organisms. On the ground of morphological 

 characteristics, the various types of chemosynthesizing bacteria may be placed 

 in the following systematic groups [29]: the genus Pseudomonas, the family 

 Chlamydobacteriales, and the class Actinomycetes. The following chemosynthetic 

 organisms are related to the genus Pseudomonas in a number of features : the 

 nitrifiers Nitrosomonas, Nitrobacter, the sulphur bacteria Thiobacillus thio- 

 oxidans, Thiobacillus thioparus, the hydrogen bacteria Hydrogenomonas flava, 

 and others [30]. Most filamentous chemoautotrophic bacteria, such as the 

 sulphur bacteria Thiotrix and iron bacteria of the genus Leptotrix are morpho- 

 logically close to filamentous heterotrophic bacteria of the Chlamydobacteriales 

 family [31, 32]. Unicellular iron bacteria of the genus Galionella are also classi- 



