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

Abstract

In the last few years, there have been a number of developments in origin of life studies that merit review. We will discuss primitive atmospheres, submarine vents, autotrophic versus heterotrophic origin, the RNA and pre-RNA worlds, and the time required for life to arise and evolve to cyanobacteria. Topics such as prebiotic synthesis, template polymerizations, and evolution of specific metabolic pathways will not be discussed here. There is no agreement on the composition of the primitive atmosphere, with opinion varying from strongly reducing (CH4 + N2, NH3 + H2O, or CO2 + H2 + N2) to neutral (CO2 + N2 + H2O). There is no geological evidence either way, although it is generally accepted that O2 was absent. It is beyond the scope of this review to explore this question, except to comment that atmospheric chemists mostly favor high CO2 + N2, whereas prebiotic chemists mostly favor more reducing conditions. Reducing conditions are required for the synthesis of amino acids, purines, pyrimidines, and sugars, and such syntheses are very efficient (47Stribling R. Miller S.L. Energy yields for hydrogen cyanide and formaldehyde synthesis the HCN and amino acid concentrations in the primitive ocean.Orig. Life Evol. Biosph. 1987; 17: 261-273Crossref PubMed Scopus (159) Google Scholar). The robustness of this type of chemistry is supported by the occurrence of most of these biochemical compounds in the 4.6 × 109-year-old Murchison meteorite, a carbonaceous chondrite, which comes from an asteroid. The meteorite analysis results make it plausible, but do not prove, that such syntheses also occurred on the primitive Earth. Based on what is known about prebiotic chemistry, if the Earth was not reducing, then the organic compounds would have to be brought to it by dust particles, comets, and meteorites (1Anders E. Pre-biotic organic matter from comets and asteroids.Nature. 1989; 342: 255-257Crossref PubMed Scopus (299) Google Scholar, 6Chyba C.F. Thomas P.J. Brookshaw L. Sagan C. Cometary delivery of organic molecules to the early Earth.Science. 1990; 249: 366-373Crossref PubMed Scopus (415) Google Scholar). The amounts that can be brought in this way and survive passage through the atmosphere are quite small, and may not have been sufficient for the origin of life. The temperature of the primitive Earth during the period of the origin of life is unknown. The entire planet is generally thought, without direct evidence, to have remained molten for several hundred million years after its formation 4.6 × 109 years ago (50Wetherill G.W. Formation of the Earth.Annu. Rev. Earth Planet. Sci. 1990; 18: 205-256Crossref Scopus (308) Google Scholar). The oldest sedimentary rocks in the Greenland Isua formation have been heated to 500°C, so the evidence on the conditions at that time has largely been destroyed. The sediments in the Australian Warrawoona formation 3.5 × 109 years old contain very convincing cyanobacteria-like microfossils (43Schopf J.W. Microfossils of the Early Archean Apex Chert new evidence of the antiquity of life.Science. 1993; 260: 640-646Crossref PubMed Scopus (698) Google Scholar). Some atmospheric models incorporate high partial pressures of CO2 to raise the temperature of the Earth by a greenhouse effect and thus prevent the complete freezing of the oceans (25Kasting J.F. Earth's early atmosphere.Science. 1993; 259: 920-926Crossref PubMed Scopus (944) Google Scholar). However, a frozen Earth has some advantages for prebiotic chemistry (3Bada J.L. Bigham C. Miller S.L. Impact melting of frozen oceans on the early Earth implications for the origin of life.Proc. Natl. Acad. Sci. USA. 1994; 91: 1248-1250Crossref PubMed Scopus (109) Google Scholar). But again, there is no direct evidence either way. In addition, processes relevant to the origin of life may have taken place in environments different from the terrestrial average, e.g., hot springs, eutectic sea water, or drying lagoons. Shortly after the discovery of submarine vents, or hot springs, at oceanic ridge crests (8Corliss J.B. Dymond J. Gordon L.I. Edmond J.M. von Herzen R.P. Ballard R.D. Green K. Williams D. Bainbridge A. Crane K. van Andel T.H. Submarine thermal springs on the Galapagos Rift.Science. 1979; 203: 1073-1083Crossref PubMed Scopus (1102) Google Scholar), a theory of the origin of life in these vents was proposed (9Corliss J.B. Baross J.A. Hoffman S.E. An hypothesis concerning the relationship between submarine hot springs and the origin of life on Earth.Oceanologica Acta. 1981; 4: 59-69Google Scholar). Considerable attention has been given to this theory (21Holm, N.G., ed. (1992). Marine Hydrothermal Systems and the Origin of Life (Dordrecht: Klüwer Academic). Also a special issue of Orig. Life Evol. Biosph. 22, 1–241.Google Scholar) and the other possible roles of vents in the origin of life, but it seems unlikely that the vents played a role in prebiotic synthesis of organic compounds or polymers. The hot springs arise by sea water being forced down into the sediments for several kilometers, heated by magma, and pushed through the vents at 350°C. A great deal of water is involved, with the whole ocean passing through them every ten million years. The theory proposes that organic synthesis took place during the passage of vent water down the 350°C to 2°C gradient, followed by synthesis of peptides and other polymers, and the conversion of these polymers to living organisms in the temperature gradient. The steps in this theory have been examined and shown not to work (34Miller S.L. Bada J.L. Submarine hot springs and the origin of life.Nature. 1988; 334: 609-611Crossref PubMed Scopus (231) Google Scholar). For example, organic compounds are decomposed at 350°C rather than synthesized, and polymers such as peptides, RNA, and DNA are hydrolyzed rapidly rather than synthesized at vent temperatures. The submarine vents did play a role in the events leading to the origin of life, but this role was in regulating the composition of the ocean and possibly the atmosphere, and, more importantly, the destruction of organic compounds produced in the atmosphere. This means that organic compounds would not accumulate over very long periods of time, and therefore the vent destruction sets a time frame for the origin of life of approximately ten million years (47Stribling R. Miller S.L. Energy yields for hydrogen cyanide and formaldehyde synthesis the HCN and amino acid concentrations in the primitive ocean.Orig. Life Evol. Biosph. 1987; 17: 261-273Crossref PubMed Scopus (159) Google Scholar, 30Lazcano A. Miller S.L. How long did it take for life to begin and evolve to cyanobacteria?.J. Mol. Evol. 1994; 39: 546-554Crossref PubMed Scopus (106) Google Scholar). The surprising occurrence of hyperthermophiles growing at temperatures as high as 110°C (not at 350°C) near the vents (Forterre, 1996 [this issue of Cell]), as well as of tube worms and clams growing near the vents at 37°C, cannot be used as an argument for the origin of life at elevated temperatures, anymore than the present abundance of life on the Earth at 2°C in the ocean or 37°C in mammals indicates an origin at these temperatures (35Miller S.L. Lazcano A. The origin of life did it occur at high temperatures?.J. Mol. Evol. 1995; 41: 689-692Crossref PubMed Scopus (126) Google Scholar). The Oparin–Haldane heterotrophic theory of the origin of life has been widely accepted on the basis that a heterotrophic organism is simpler than an autotrophic one, and prebiotic synthesis experiments show how easy it is under reducing conditions to produce organic compounds, many of which are used in present biology. There are, however, some recent examples of autotrophic proposals made for a variety of reasons. One reason for proposing an autotrophic origin is the CO2-rich model of the primitive Earth's atmosphere (25Kasting J.F. Earth's early atmosphere.Science. 1993; 259: 920-926Crossref PubMed Scopus (944) Google Scholar). High pressures of CO2 (10–100 atm) imply the absence of reducing conditions and organic compound synthesis, and therefore it would be necessary for the first organisms to biosynthesize their organic compounds, or to make use of the very small amounts of organic compounds brought in by comets and meteorites. An autotrophic theory involving nonenzymatic reactions patterned after present biochemical pathways of intermediate metabolism has been proposed (19Hartmann H. Speculations on the origin and evolution of metabolism.J. Mol. Evol. 1975; 4: 359-370Crossref PubMed Scopus (133) Google Scholar). According to this scheme, the citric acid cycle started with acetyl-CoA by two CO2 fixations. The development of such a system is envisioned to require clays, transition state metals, and UV light. Although there are a few biosynthetic reactions that will proceed nonenzymatically, most do not. Cyclic pathways need to be very efficient or they will stop working. An example is the Krebs cycle, which stops unless the oxalacetate lost by nonenzymatic decarboxylation is replaced. Noncyclic pathways are less bothered by this problem, but, in any case, this idea has never been given an experimental test. 5Cairns-Smith A.G. Genetic Takeover and the Mineral Origins of Life. Cambridge University Press, Cambridge1982Google Scholar proposed a clay mineral theory in which the genetic information is contained in the pattern of ions in the clay mineral lattice, and reproduction is accomplished by crystal growth. The mineral system is converted to the present biological one by an unspecified process called genetic takeover. There has been no experimental support for this theory after 20 years, although there were some promising reports that have not been confirmed. The most elaborate autotrophic theory is that of 48Wächtershäuser G. Groundworks for an evolutionary biochemistry the iron–sulphur world.Prog. Biophys. Mol. Biol. 1992; 58: 85-201Crossref PubMed Scopus (472) Google Scholarreferences therein, in which biosynthesis and polymerization are postulated to take place on the surface of FeS and FeS2. The reaction FeS + H2S = FeS2 + H2 is a very favorable one (ΔG° = −9.23 kcal/mol; E° = −620 mV at pH 7 and 25°C), so the FeS/H2S combination is a strong reducing agent. According to this scheme, both enzymes and nucleic acids are the evolutionary outcome of such surface-contained archaic metabolism. The FeS/H2S has been used to reduce double bonds, α-ketoglutarate to glutamic acid, thiols to hydrocarbons, et cetera, (17Hafenbradl D. Keller M. Wächtershäuser G. Stetter K.O. Primordial amino acids by reductive amination of a-oxo acids in conjunction with the oxidative formation of pyrite.Tetrahedron Lett. 1995; 36: 5179-5182Crossref Scopus (34) Google Scholarreferences therein). However, the FeS/H2S system does not reduce CO2 to amino acids, purines, or pyrimidines, even though there is more than adequate free energy to do so (28Keefe A.D. Miller S.L. McDonald G. Bada J. Investigation of the prebiotic synthesis of amino acids and RNA bases from CO2 using FeS/H2S as a reducing agent.Proc. Natl. Acad. Sci. USA. 1995; 92: 11904-11906Crossref PubMed Scopus (45) Google Scholar). But the reduction of CO2 is precisely what is required of an autotrophic theory. The FeS/H2S system may have been used to reduce prebiotic compounds synthesized by other energy sources in reducing atmospheres, thereby being a component of a heterotrophic theory of the origin of life and Oparin's primitive soup. There have been so many unsuccessful attempts to produce prebiotic organic compounds with CO2+N2+H2O mixtures (in the absence of hydrogen) that one wonders whether successful prebiotic syntheses are possible under such conditions. Those who propose autotrophic theories need to provide experimental evidence of how organic compounds can be produced, and how such systems can work. This is quite a challenge, since even heterotrophic entities, which need only take their compounds from the environment, are difficult to envision. Another reason for postulating an autotrophic origin of life is that the deepest branches of the universal tree of life are occupied by anaerobic sulfur-dependent hyperthermophiles that fix CO2 by a reductive Krebs cycle. It is then assumed that this metabolism is primordial, rather than a result of extensive development (32Maden B.E.H. No soup for starters? Autotrophy and the origins of metabolism.Trends Biochem. Sci. 1995; 20: 337-341Abstract Full Text PDF PubMed Scopus (60) Google Scholar). However, it is important to distinguish between ancient and primitive. Hyperthermophiles may be cladistically ancient, but they are hardly primitive relative to the first living organisms. They contain the same elaborate protein biosynthesis and most of the enzymes of modern organisms. They seem to be no more primitive in their replication and translation apparatus and metabolic abilities than mesophiles (35Miller S.L. Lazcano A. The origin of life did it occur at high temperatures?.J. Mol. Evol. 1995; 41: 689-692Crossref PubMed Scopus (126) Google Scholar). Truly primitive organisms would be those of the RNA world or some of their immediate descendents in which a simplified version of the DNA/protein system had already appeared. In principle, the latter could be recognized because they would branch off early in a universal tree of life, and would be endowed with simpler replication and translation machinery demonstrably not due to secondary adaptations. However, no such organisms have been found. The study of hyperthermophiles is an invaluable source of information on early biological evolution and the nature of the last common ancestor of all extant life forms, but an extrapolation into prebiotic times should not be taken for granted. Since the metabolic processes of the RNA world and the chemical reactions of prebiotic times were different from present day metabolism, phylogenetic information from extant protein sequences can not be applied to understand them. There has been a school of workers applying computer modeling to origin of life processes (26Kauffman S.A. The Origins of Order. Oxford University Press, New York1993Google Scholar). These computer simulations (referred to in some circles as experiments in silico, in contrast with in vitro or in vivo) can model Darwinian evolution and the emergence of order from chaotic systems. However, these calculations have so far not provided guidelines for origin of life studies because they do not take into account the specific properties of individual organic compounds and polymers, e.g., the base pairing of AU and GC. The discovery of catalytic RNA gave credibility to prior suggestions that the first living organisms were self-replicating RNA molecules with catalytic activity, a situation called the RNA world (16Gesteland, R.F., and Atkins, J.F., eds. (1993). The RNA World. (Cold Spring Harbor, New York: Cold Spring Harbor Laboratory Press).Google Scholar). This idea has become widely accepted, but as will be shown below, it is unlikely that RNA itself with AUGC and a ribose phosphate backbone is a prebiotic molecule. We will refer to the period when the informational macromolecule had a backbone different from ribose phosphate and possibly different bases as the pre-RNA world. The pre-RNA world is assumed to have the same essential characteristic of the RNA world—phenotype and genotype both reside in the same polymer, so no protein or related catalysts are required to be synthesized. Work on nucleic acids with hexoses, instead of pentoses and pyranoses, in place of furanoses suggest that a wide variety of informational macromolecules are possible, even when restricted to sugar phosphate backbones (11Eschenmoser A. Chemistry of potentially prebiological natural products.Orig. Life Evol. Biosph. 1994; 24: 389-423Crossref Scopus (72) Google Scholar). The most interesting nonsugar alternative is peptide nucleic acid (PNA). This has a backbone of ethylenediamine monoacetic acid, with the bases attached by an acetic acid, and it binds strongly to DNA (38Nielsen P.E. Peptide nucleic acid (PNA) a model structure for the primordial genetic material?.Orig. Life Evol. Biosph. 1993; 23: 323-327Crossref PubMed Scopus (121) Google Scholar). The monomers of PNA are likely prebiotic compounds, but it is not clear whether the polymer can be formed. Since many other alternatives are possible, RNA itself may have been the evolutionary outcome of a series of different genetic polymers. Recent results show that RNA itself is an unlikely prebiotic molecule. The first problem is that there is no prebiotic reaction that gives largely ribose rather than a mixture of many sugars, including those with branched chains (44Shapiro R. Prebiotic ribose synthesis a critical analysis.Orig. Life Evol. Biosph. 1988; 18: 71-85Crossref PubMed Scopus (178) Google Scholar), although there is one promising prebiotic process that might be feasible using glycolaldehyde phosphate as a starting reagent (37Müller D. Pitsch S. Kittaka A. Wagner E. Wintner C.E. Eschenmoser A. Chemistry of α-aminonitriles aldomerisation of glycolaldehyde phosphate to rac-hexose 2,4,6-triphosphates and (in presence of formaldehyde) rac-pentose 2,4-diphosphates: rac-allose 2,4,6-triphosphate and rac-ribose 2,4-diphosphate are the main reaction products.Helv. Chim. Acta. 1990; 73: 1410-1468Crossref Scopus (181) Google Scholar). The second problem is that sugars decompose very rapidly on the geological timescale. Thus, the half-life for ribose decomposition is 73 min at 100°C and pH 7, and 44 years at 0°C and pH 7. Other sugars are similarly unstable at 100°C and pH 7, with the rate approximately proportional to the amount of free aldehyde in the sugar. Examples are ribose 5-phosphate (t1/2 = 9 min), deoxyribose (t1/2 = 225 min), and ribose 2,4-diphosphate (t1/2 = 31 min) (29Larralde R. Robertson M.P. Miller S.L. Rates of decomposition of ribose and other sugars implications for chemical evolution.Proc. Natl. Acad. Sci. USA. 1995; 92: 8158-8160Crossref PubMed Scopus (230) Google Scholar). The instability problem could be overcome if the ribose nucleosides could have formed early, because nucleosides are quite stable owing to the absence of free aldehyde in its sugar. However, there is no efficient prebiotic synthesis of purine ribosides and no prebiotic synthesis of pyrimidine nucleosides at all. Added to these problems is the fact that any prebiotic synthesis of ribose or nucleosides would give a racemic mixture, and all template polymerization experiments so far show enantiomeric cross inhibition. This is where the presence of activated L nucleosides in a template polymerization of activated D nucleosides causes chain termination during polymerization (24Joyce G.F. Schwartz A.W. Miller S.L. Orgel L.E. The case for an ancestral genetic system involving simple analogues of the nucleotides.Proc. Natl. Acad. Sci., USA. 1987; 84: 4398-4402Crossref PubMed Scopus (303) Google Scholar). It also has become clear that polyphosphates and activated phosphates were not abundant prebiotic compounds (27Keefe A.D. Miller S.L. Are polyphosphates or phosphate esters prebiotic reagents?.J. Mol. Evol. 1995; 41: 693-702Crossref PubMed Scopus (112) Google Scholar). There is no known polyphosphate mineral known, and only a few kg of calcium pyrophosphate have been found in a deposit in New Jersey. The primitive Earth may have been different, but no one has yet shown how large amounts of polyphosphates could have been produced. Recently, it has been shown that phosphorus pentoxide (P4O10) can be produced by heating volcanic basalts to 1200°C, and small amounts of pyrophosphate and tripolyphosphate have been found in a fumarole near Mount Usa in Hokkaido, Japan (52Yamagata Y. Watanabe H. Saithoh M. Namba T. Volcanic production of polyphosphates and its relevance to prebiotic evolution.Nature. 1991; 352: 516-519Crossref PubMed Scopus (231) Google Scholar). However, the amounts of polyphosphates produced are so small that even greatly increased volcanic activity on the primitive Earth would not make polyphosphates available as useful prebiotic reagents, except by their concentration in very local areas. It could be argued that the first self-replicating systems arose in such rare environments. We consider this to be unlikely, but such a possibility can not be excluded altogether. It thus follows that polyphosphates are an unlikely prebiotic free energy source and that phosphate esters are unlikely to have been involved in the first genetic material. This is a very strong statement, because of the central role that phosphates play in the metabolism of all known organisms, but this can only be revised when a robust prebiotic process for polyphosphate synthesis or a plausible geochemical mechanism for concentrating them are found. One alternative is thioesters, which are high-energy compounds (10de Duve C. Blueprint for a Cell. N. Patterson Publishers, Burlington, North Carolina1991Google Scholar). Another possibility is the spontaneous synthesis of a polymer from high energy precursors, e.g., the polymerization of glycine nitrile to polyglycine is thermodynamically favorable, although the reaction is sluggish. It generally has been assumed that the origin of life took place after extended geological periods of time. Although it is not possible to assign a precise chronology to the events leading to the origin of life, in the last few years estimates of the available time for this to occur have been considerably reduced. There is compelling paleontological evidence that microbial communities were thriving on the primitive Earth 3.5 × 109 years ago (43Schopf J.W. Microfossils of the Early Archean Apex Chert new evidence of the antiquity of life.Science. 1993; 260: 640-646Crossref PubMed Scopus (698) Google Scholar), and it has been suggested that life may have been killed off as late as 3.8 × 109 years ago if the Earth was undergoing impacts from large asteroids (33Maher K.A. Stevenson D.J. Impact frustration of the origin of life.Nature. 1988; 331: 612-614Crossref PubMed Scopus (283) Google Scholar, 46Sleep N.H. Zahnle K.J. Kasting J.F. Morowitz H.J. Annihilation of ecosystems by large asteroid impacts on the early Earth.Nature. 1989; 342: 139-142Crossref PubMed Scopus (344) Google Scholar). Thus, only 300 million years appear to be left for the origin and early diversification of life. It has been argued that such short periods of time make the accumulation of the prebiotic soup unlikely, and therefore should be interpreted as evidence supporting an autotrophic origin of life (32Maden B.E.H. No soup for starters? Autotrophy and the origins of metabolism.Trends Biochem. Sci. 1995; 20: 337-341Abstract Full Text PDF PubMed Scopus (60) Google Scholar). As argued below, there is no reason to assume that life required enormous periods of time to originate and evolve to the 3.5 × 109-year-old cyanobacteria-like Warrawoona microfossils. The accumulation of organic compounds of abiotic origin in the primitive oceans is balanced by destructive processes, and if prebiotic synthesis stops because of atmospheric changes, then it would not be possible for life to arise after the organic compounds decompose. On the other hand, the chemistry of prebiotic reactions is robust, and does not require extended periods of time to take place. For instance, the slow step in the Strecker synthesis of amino acids is the hydrolysis of the corresponding amino nitrile to the amide, which has a half-life of 40 years at pH 8 and 0°C (36Miller, S.L., and Van Trump, J.E. (1981). The Strecker synthesis in the primitive ocean. In Origin of Life, Y. Wolman, ed. (Dordrecht: Reidel), pp. 135–141.Google Scholar) (half-lives of 40 years or a process completed in 105 years are slow by biological standards, but rapid on the geological time scale). An example of a relatively rapid prebiotic synthesis is that of amino acids on the Murchison meteorite parent body, where it apparently occurred in less that 105 years (41Peltzer E.T. Bada J.L. Schlesinger G. Miller S.L. The chemical conditions on the parent body of the Murchison meteorite some conclusions based on amino-, hydroxy-, and dicarboxylic acids.Adv. Space Res. 1984; 4: 69-74Crossref PubMed Scopus (188) Google Scholar). Thus, although the buildup of the prebiotic soup may have involved millions of years, the individual reactions to synthesized prebiotic compounds have short half-lives, and there are no known relevant examples of slowly synthesized molecules. Whatever the nature of the first genetic polymer, it is clear that hydrolysis must have limited its accumulation in the primitive environment. An informational polymer must have a lifetime comparable with that of the organism (49Westheimer F.H. Why nature chose phosphates.Science. 1987; 235: 1173-1178Crossref PubMed Scopus (1122) Google Scholar) or, at least, with the time required for its replication. Even if a slow addition of monomers to a genetic polymer is envisioned, the rate of polymer synthesis nonetheless must be rapid compared with hydrolysis rates, especially if a significant amount of genetic information is to be contained in the polymer. Thus, a 100-base long RNA molecule needs to be synthesized at least 100 times faster than the hydrolysis rate of a single phosphodiester bond. Even if highly stable precursors to the ribose phosphate backbone of RNA are proposed for the pre-RNA world, the bases themselves will decompose over long periods of time. For example, cytosine hydrolyses to uracil with a half-life of 300 years at pH 7 and 25°C in single-stranded DNA (31Lindahl T. Instability and decay of the primary structure of DNA.Nature. 1993; 362: 709-715Crossref PubMed Scopus (4023) Google Scholar). Adenine, which is usually thought to be very stable, deaminates to hypoxanthine with a half-life of 204 days at 100°C and pH 7 (45Shapiro R. The prebiotic role of adenine a critical analysis.Orig. Life Evol. Biosph. 1995; 25: 83-98Crossref PubMed Scopus (99) Google Scholar). This is only about ten times slower than cytosine (t1/2 = 21 days at 100°C and pH 7). Given these stability constraints, there is no reason to assume that the self-organization of prebiotic compounds into a system capable of undergoing Darwinian evolution involved extended periods of time. We envision a maximum upper limit of 5 × 106 years, because this is the half-life for destruction of organic compounds in the oceans owing to their passage through the submarine vents (30Lazcano A. Miller S.L. How long did it take for life to begin and evolve to cyanobacteria?.J. Mol. Evol. 1994; 39: 546-554Crossref PubMed Scopus (106) Google Scholar). These stability calculations can also be used to set an upper limit for the amount of time available for protein biosynthesis to appear. Although the emergence of translation was once considered the central issue in the origin of life, the discovery and characterization of ribozymes, including the specific binding of amino acids to RNA molecules (53Yarus, M. (1993). An RNA–amino acid affinity. In The RNA World, R.F. Gesteland and J.F. Atkins, eds. (Cold Spring Harbor, New York: Cold Spring Harbor Laboratory Press), pp. 205–217.Google Scholar) and the possibility that peptidyl–transferase activity resides in the RNA component of the ribosome (39Noller H.F. Hoffarth V. Zimniak L. Unusual resistance of peptidyl transferase activity to protein extraction procedures.Science. 1992; 256: 1416-1419Crossref PubMed Scopus (553) Google Scholar) have given credence to the idea that a rudimentary form of protein synthesis originated in the RNA world. How this took place is unknown, but it could not have been delayed for extended periods of time owing to the decomposition of both RNA and amino acids in aqueous solutions, which is significant even at low temperatures. Although alanine decomposes slowly by irreversible decarboxylation (t1/2 = 109 years at 25°C), other amino acids are rather unstable. Serine and threonine have half-lives of approximately 103 years at 25°C, whereas histidine and tyrosine decompose at a much faster rate. This problem would have been avoided if amino acid biosyntheses were accomplished by ribozymes. It has been suggested that this may be the case in histidine biosynthesis (51White H.B. Coenzymes as fossils of an earlier metabolic state.J. Mol. Evol. 1976; 7: 101-104Crossref PubMed Scopus (336) Google Scholar), but no evidence supporting this claim is available. There is still a gap between descriptions of prebiotic events and the last common ancestor. Intermediate stages must have involved simpler organisms with much smaller genomes. The question is whether it is possible to infer some of their major characteristics. It has long been recognized that most genetic information is not essential for cell growth and division. Statistical analysis of ∼80 randomly selected chromosomal loci for Bacillus subtilis has led to the suggestion that the minimum cellular genome size is of the order of 562 kb (22Itaya M. An estimation of minimal genome size required for life.FEBS Lett. 1995; 362: 257-260Abstract Full Text PDF PubMed Scopus (128) Google Scholar). This figure is comparable with the size of the Mycoplasma genitalium genome, which is 580 kb long and codes for 482 genes (15Fraser C.M. Gocayne J.D. White O. Adams M.D. Clayton R.A. Fleischmann R.D. Bult C.J. Kerlavage A.R. Sutton G. Kelley J.M. et al.The minimal gene complement of Mycoplasma genitalium.Science. 1995; 270: 397-403Crossref PubMed Scopus (2045) Google Scholar). The compactness of mycoplasma genomes can easily be understood in terms of their parasitic lifestyle, but it is somewhat surprising that the streamlining processes have not greatly affected the length of the genes or the number involved in protein synthesis and DNA replication (15Fraser C.M. Gocayne J.D. White O. Adams M.D. Clayton R.A. Fleischmann R.D. Bult C.J. K