PRIMITIVE PYRROLE PIGMENTS 649 



For hypotheses on the evohition of these cell catalysts and on the 

 role of pyrrole pigments in ev^olution, we have thus to go back into 

 the dim past of the history of life on earth; any theory thus becomes 

 necessarily speculative. The soundest procedure is perhaps to base 

 such a speculation on geochemical considerations, as Oparin {2078) 

 has done (without our necessarily subscribing to this author's theories 

 on the origin of life). 



3. PRIMITIVE PYRROLE PIGMENTS 



At an early period in the earth's history, according to Oparin, little 

 free oxygen was present,* the first form of living organisms being 

 anaerobic heterotrophs. The atmosphere contained, among other 

 constituents, a relatively high percentage of hydrogen. Hydrogenase, 

 which activates molecular hydrogen, occurs in present-day primitive 

 organisms. The butyric acid fermenters (anaerobic heterotrophs), in 

 which it is found, can certainly be considered primitive; they have 

 a poor energy metabolism, and contain a little-integrated mixture of 

 a variety of enzymes. Hydrogenase can thus be considered an 

 important primitive enzyme. If it can be shown to be a hematin 

 compound, it is probably the most ancient hematin compound known 

 today (c/. Chapter IX). 



In the next stage of the earth's history, abundant carbon dioxide 

 is assumed in the atmosphere, but still little oxygen. Hydrogen sul- 

 fide, as well as molecular hydrogen, may also have been atmospheric 

 constituents. It may be assumed {cf. below) that the shortage at 

 this stage of easily assimilable organic material necessitated the first 

 attempts of the primary heterotrophic organisms to use sulfur energy. 

 Hydrogenase catalyzes some reactions which may have been of 

 importance at this time, including the "Knallgas" reaction, and the 

 reduction of carbon dioxide to methane (Soehngen, cf. 2198, p. 121), 

 the latter supplying not only energy, but also cell material {169). 

 In some bacteria (purple bacteria), the "Knallgas" reaction proceeds 

 only at low oxygen pressure. 



Hydrogenase is found in some green bacteria, in Thiorhodaceae and 

 Athiorhodaceae and in primitive green algae, all of which absorb and 

 utilize light energy. 



The investigations of van Niel and GaflPron {cf. Chapter IX) lend 



* This is not generally accepted (cf. Tammann, 2736a; Wildt, 3082a), but is sup- 

 ported by Vernadsky and V. M. Goldschmidt. 



