PHOTOCHEMICAL REACTIONS 439 



The pigments which assumed the role of such sensitisers 

 in the original organisms may have been porphyrins. As we 

 showed in Chapter V, these substances, and metallic deriva- 

 tives of them, arose in the waters of the hydrosphere as a 

 result of purely organic-chemical, abiogenic synthesis, even 

 before the origin of living things. The first organisms could 

 therefore obtain them ready made, directly from the sur- 

 rounding medium, and it was only during the course of the 

 further development of life that there arose the necessity 

 to synthesise them from such simple metabolic products as 

 succinic and oxalic acids and glycine, always supposing that 

 the presence of porphyrins was beneficial to the organisms 

 of that time, giving them an advantage in the struggle for 

 existence. 



If what has been suggested is true, we must suppose that 

 porphyrins, or some similar compounds, represent one of 

 the earliest components of living matter, along with amino 

 acids, nucleotides, etc. This is suggested by their extremely 

 extensive distribution in living nature, their presence in all 

 contemporary organisms without exception. 



The classical researches of M. Nencki^^^ on the chemical 

 nature of haemoglobin and chlorophyll revealed a striking 

 similarity between these important pigments of the animal 

 and vegetable kingdoms and showed that both these king- 

 doms w^ere derived from ancestors which already possessed 

 porphyrins as necessary components of their protoplasm. This 

 was later established for microbes with a more primitive 

 organisation. 



Contemporary data on the finding of porphyrins in the 

 most diverse representatives of the living world are discussed 

 in great detail in the review of R. Lemberg and J. W. 

 Legge.^^° We reproduce here a summary of the occurrence of 

 haemoglobin in the animal kingdom borrowed from this work 

 (Table 4). Chlorophyll is equally widely distributed in the 

 plant world. All higher photosvnthetic organisms contain it, 

 while in the lower ones we find derivatives of porphyrin 

 similar to chlorophyll (bacteriochlorophyll) or compounds 

 derived from porphyrin which have a structure similar to 

 that of the bile pigments (phycocyanin and phycoerythrin). 



In tunicates, which are very ancient and primitive organ- 



