The Origin and Early Evolution of Life: Prebiotic Chemistry, the Pre-RNA World, and Time

If it is assumed that life arose in a prebiotic soup containing most, if not all, of the necessary small molecules, then there was a large potential energy supply available on the primitive Earth from different fermentations. It is clear that such compounds could provide both the growth and energy supply of a large number of organisms, but this would rapidly result in the depletion of the available nutrients. Although the usual example of a primordial fermentation is that of glucose (Oparin 1938xOparin, A.I. See all ReferencesOparin 1938), it is unlikely that large quantities of this sugar were available in the primitive environment because of its instability. As noted by Clarke and Elsden 1980xThe earliest catabolic pathways. Clarke, P.H. and Elsden, S.R. J. Mol. Evol. 1980; 15: 333–338Crossref | PubMed | Scopus (11)See all ReferencesClarke and Elsden 1980, a more likely early fermentation reaction was that of glycine: The primitive ocean may have had a glycine concentration between 10−8–10−4 M, depending on the efficiency of prebiotic synthesis and whether the ultimate source of organic compounds was endogenous or not. At one mole ATP per mole of glycine, these values correspond to 1025–1028 cells. Such high numbers of cells would lead to an exponential decrease in the concentration of the available fermentable organic compounds of prebiotic origin and would bring about a metabolic crisis that could only be overcome by the evolutionary development of light-harvesting autotrophic organisms with CO2-fixing abilities (Lazcano and Miller 1994xHow long did it take for life to begin and evolve to cyanobacteria?. Lazcano, A. and Miller, S.L. J. Mol. Evol. 1994; 39: 546–554Crossref | PubMed | Scopus (74)See all ReferencesLazcano and Miller 1994).The time for evolution of the first DNA/protein organisms to oscillatorian-like cyanobacteria is usually thought to be very long, because the latter have rather large genomes of 6 × 103 kb to 8 × 103 kb (Herdman 1985xSee all ReferencesHerdman 1985) and are usually considered to be very complex. However, many of the evolutionary novelties required for the emergence of oxygenic photosynthesis are the result of duplication and divergence of genes. Assuming that Archean cells had a random rate of duplicon fixation, and a rate of spontaneous gene duplications comparable with the present values of 10−5–10−3 gene duplications (Anderson and Roth 1977xTandem genetic duplications in phage and bacteria. Anderson, R.P. and Roth, J.R. Annu. Rev. Microbiol. 1977; 31: 473–505Crossref | PubMedSee all ReferencesAnderson and Roth 1977), the time required for the development of a 100 kb genome of a DNA/protein primitive heterotroph into a 7,000-genes filamentous cyanobacteria would require only 7 × 106 years (Lazcano and Miller 1994xHow long did it take for life to begin and evolve to cyanobacteria?. Lazcano, A. and Miller, S.L. J. Mol. Evol. 1994; 39: 546–554Crossref | PubMed | Scopus (74)See all ReferencesLazcano and Miller 1994).It is well known that only a few weeks are required for the rapid spread of duplicates in bacterial populations under the stress conditions of directed evolution experiments. There appear to be no experimental measurements of the rate of formation and fixation of new enzyme activities resulting from gene duplication. However, recent results on the organophosphate and phosphonate hydrolyzing phosphotriesterase from Pseudomonas diminuta and other soil eubacteria suggest that this new enzyme diverged by duplication from the α/β barrel family and reached the diffusion limit in only 40 years (Scanlan and Reid 1995xEvolution in action. Scanlan, T.S. and Reid, R.C. Chemistry and Biology. 1995; 2: 71–75Abstract | Full Text PDF | PubMed | Scopus (40)See all ReferencesScanlan and Reid 1995). Thus, the rate of duplication and fixation of new genes can be surprisingly fast on the geological timescale.There are a number of additional mechanisms that could have increased the rate of metabolic evolution, including the modular assembly of new proteins, gene fusion events, and horizontal gene transfer as seen in extensive antibiotic resistance in bacteria. Directed evolution experiments have shown that new substrate specificities appear in a few weeks from existing enzymes by recombination events within a gene (Hall and Zuzel 1980xEvolution of a new enzymatic function by recombination within a gene. Hall, B. and Zuzel, T. Proc. Natl. Acad. Sci. USA. 1980; 77: 3529–3533Crossref | PubMedSee all ReferencesHall and Zuzel 1980). This suggests that mosaic proteins may have enhanced the catalytic repertoire of ancient organisms.It is likely that the widespread belief that the origin and early evolution of life were slow processes requiring billions and billions of years stems from the classical Darwinian approach that major changes are slow and proceed in a stepwise manner over extended periods of time. All the evidence reviewed here suggests that stability of monomers and polymers essential for the origin of life strongly limited the possibility of a slow emergence of life. After the explosive metabolic evolution that took place soon after the beginning of life, the basic genetic processes and major molecular traits have persisted essentially unchanged for more than three-and-a-half billion years, perhaps owing to the linkages of the genes involved and the complex interactions between different metabolic routes. At a macroevolutionary level, this represents a case of conservatism that is even more striking than the maintenance of the major animal body plans that appeared at the base of the Cambrian, and which have remained basically unchanged for 600 million years.