38 



The first trait about which we have evidence concerns oxygen 

 utilization by early organisms. There is some geological evidence indi- 

 cating that on the very early Earth (Archean Eon) oxygen was much 

 less abundant in the atmosphere. It is generally believed that signifi- 

 cant quantities of oxygen first accumulated as the result of biological 

 photosynthesis, a process in which free oxygen is a by-product. It is 

 therefore thought that the first organisms must have been nonoxygen 

 users, or anaerobes. The biochemical record supports this belief. 

 Based on sequencing of nucleotides, anaerobic eubacteria are ancient 

 compared with their aerobic counterparts. The major groups of the 

 eubacteria are basically anaerobic, and the aerobic phenotype 

 appears to have arisen relatively recently several times from various 

 groups of anaerobic organisms. 



Recent studies have shown that oxygen-using enzyme systems 

 in eukaryotic organisms require orders of magnitude less oxygen to 

 function than that in the present atmosphere. These findings suggest 

 that aerobic biochemical processes may have arisen earlier than wide- 

 spread aerobic life. 



Even more fundamental is the nature of the energy source of 

 the oldest organisms for which we have some clues. The biochemical 

 record of the eubacteria has been examined to seek an origin for 

 photosynthesis as a cellular trait, and suggests that photosynthesis is 

 indeed ancient. Major eubacterial groups are photosynthetic; e.g., the 

 purple photosynthetic bacteria, the cyanobacteria (blue-green algae), 

 and green sulfur photosynthetic bacteria. Furthermore, nonphoto- 

 synthetic phenotypes have arisen several times from lines already 

 photosynthetic (fig. III-6). For instance, the nonphotosynthetic, 

 common, human intestinal bacteria, E. coli, most likely arose from 

 the group of purple photosynthetic bacteria. 



The usual view has been that the oldest forms of life were nour- 

 ished not by internal biological photosynthesis but as heterotrophs 

 that were supplied with energy by a rich environment, where organic 

 nutrients that were abundant, were produced by processes which pre- 

 ceded life. We can find no sign of that early phase in the biochemical 

 record among living cells. Photosynthesis in living cells goes back as 

 far as the earliest groups of eubacteria. The lineages of the bacterial 

 cells we know do not indicate whether heterotrophic or autotrophic 

 life was first on the scene. 



