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mitochondria to prokaryote cells is not as clear, though the mito- 

 chondria exhibit a relationship to the purple bacteria. As a whole, 

 though, the information contained in the biochemical record to date 

 strongly supports the endosymbiotic theory of the origin of eukary- 

 otic cells. Eukaryotes can be seen as evolutionary mosaics, depending 

 on contributions joined from several distinct lines of descent. 



The Prokaryotes 



Study of nucleotide sequences (16S RNA) in a large number of 

 bacteria has lately revealed their enormous biochemical diversity. 

 One group is called the eubacteria (true bacteria) and another called 

 the archeabacteria (old bacteria). Still a third group, called the 

 urkaryote, is represented by that part of the eukaryotic cell which is 

 external to the organelles. The eubacteria and archeabacteria are no 

 more closely related to each other than each is related to the third 

 component, the cytoplasm. There are a great number of extant 

 eubacterial species, but relatively few known archeabacterial species. 

 Despite the paucity of extant species, the archeabacteria appear to be 

 an ancient group which exhibit as much diversity as the eubacteria. 

 Its members are generally restricted to unusual niches in the environ- 

 ment, hinting that the group might have originated during a period of 

 Earth's history when prevailing conditions were different than they 

 are today. The two bacterial groups together reveal greater biochem- 

 ical diversity than all the extant eukaryotes. 



The Antiquity of Biochemical Traits 



The biochemical record can also yield inferences about the 

 traits or phenotypic features possessed by very early organisms. A 

 trait is an unlikely candidate for an ancient phenotype if the groups 

 of organisms which now share the trait have all diverged from ances- 

 tral lines which did not possess it. A trait is probably not very old 

 also if all extant organisms which possess it are closely related in 

 terms of their biochemical record, indicating a recent time of diver- 

 gence from ancestral lines. Conversely, if a trait is shared by groups 

 so diverse that their biochemical record indicates a very ancient 

 relative time of divergence, that trait may well have been possessed 

 by very early organisms. 



