This discovery of the Archaea established rRNA as a molecular chronometer for prokaryotic evolution and provided new perspectives on prokaryotic diversity. The enormous diversity of prokaryotes on earth may follow from their abundance. In terms of individuals, their numbers are about 4-6 x 1030 cells. In terms of biomass, they represent about 3.5-5.5 x 1017 g of C, or about the same as the biomass of plants. Moreover, geochemical evidence suggests that large amounts of organic carbon have been present on earth for greater than three billion years. Since the eukaryotes evolved during the last two billion years, the prokaryotes must have been the major source of this ancient organic carbon. The presence of an ancient and large population of prokaryotes has important implications. One, all the modern principles of the cell, major biogeochemical cycles, and most major lineages of life evolved in a prokaryotic world. Two, prokaryotes play a major role in the chemistry of the biosphere. Three, the diversity of prokaryotes far exceeds that of eukaryotes.
This last conclusion follows from the idea that rare genetic events are common and the potential for diversification is very high within large populations. Comparisons of standard chronometers and surveys of natural populations of prokaryotes further support the hypothesis that prokaryotic diversity far exceeds that found in the eukaryotes.
Related papers:
Schidlowski, M., J.M. Hayes, and I.R. Kaplan (1983) Isotopic inferences of ancient biochemistries: carbon, sulfur, hydrogen, and nitrogen. In: J.W. Schopf (ed.) Earth's Earliest Biosphere, Princeton University Press, Princeton NJ, p. 149-186.
Whitman, W. B., D. C. Coleman, and W. J. Wiebe (1998) Prokaryotes: the unseen majority. Proc. Natl. Acad. Sci. U.S.A. 94:6578-6583.
Keswani, J., and W.B. Whitman (2001) Relationships of 16S rRNA sequence similarity to DNA hybridization in prokaryotes. Int. J. Syst. Evol. Microbiol. 51:667 678.
