Prokaryotes

There have been several molecular clock studies of prokaryotes that have estimated divergence times in the Precambrian. Without exception, all have found deep divergences, prior to the Neoproterozoic, of the major groups of archaebacteria and eubacteria (Doolittle et al, 1996; Feng et al, 1997; Hedges et al, 2001; Sheridan et al, 2003; Battistuzzi et al, 2004). One study (Sheridan et al, 2003) used nucleotide variation in the gene for the small subunit ribosomal RNA whereas the other studies used amino acid sequences of multiple proteins. Taxon sampling was relatively limited in the earlier protein studies (Doolittle et al., 1996; Feng et al, 1997; Hedges et al., 2001), with the recent study (Battistuzzi et al., 2004) having the largest number of taxa. Also, it is now known that there are major rate differences among groups of prokaryotes and between prokaryotes and eukaryotes (Kollman and Doolittle, 2000; Hedges et al, 2001). Some time estimates (Doolittle et al., 1996; Feng et al, 1997; Sheridan et al., 2003) were made without accounting for those rate differences, and another (Hedges et al, 2001) was made using a two rate model. The recent analysis (Battistuzzi et al, 2004) was performed using a Bayesian local clock method that permitted rate variation among branches and the results of that study are reproduced here (Fig. 1). Genes that showed obvious evidence of lateral gene transfer were avoided. As can be seen, and not surprisingly, all of the major groups of prokaryotes were extant by the onset of the Neoproterozoic and therefore their major metabolic activities (e.g., anoxygenic photosynthesis, oxygenic photosynthesis, methanogenesis, aerobic methanotrophy, etc.) were present. In most cases, this only reinforces what has already been revealed with geologic and other evidence (Knoll, 2004).

Figure 1. A timetree of prokaryotes constructed by a Bayesian analysis of proteins from complete genomes (7600 amino acids, total) (Battistuzzi et al., 2004). Gray horizontal bars are Bayesian credibility intervals.

Of potential interest to astrobiology is the clustering of three major groups (Cyanobacteria, Deinococcales, and Actinobacteria) which presumably had a common ancestor that was terrestrial approximately 3000 Ma (Fig. 1). Besides sharing photoprotective compounds, all three share a high resistance to dehydration and have species that are currently terrestrial. These three groups were named, collectively, Terrabacteria (Battistuzzi et al, 2004). This supports the paleontological and geological evidence that prokaryotes colonized the land surface in the Precambrian (Horodyski and Knauth, 1994; Watanabe et al, 2000) and therefore their metabolic and erosional activities should be considered, as well as potential interactions with terrestrial eukaryotes.

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