Stanley L. Miller

East pacific rise: hot springs and geophysical experiments.

Hydrothermal vents jetting out water at 380� � 30�C have been discovered on the axis of the East Pacific Rise. The hottest waters issue from mineralized chimneys and are blackened by sulfide precipitates. These hydrothermal springs are the sites of actively forming massive sulfide mineral deposits. Cooler springs are clear to milky and support exotic benthic communities of giant tube worms, clams, and crabs similar to those found at the Gal�pagos spreading center. Four prototype geophysical experiments were successfully conducted in and near the vent area: seismic refraction measurements with both source (thumper) and receivers on the sea floor, on-bottom gravity measurements, in situ magnetic gradiometer measurements from the submersible Alvin over a sea-floor magnetic reversal boundary, and an active electrical sounding experiment. These high-resolution determinations of crustal properties along the spreading center were made to gain knowledge of the source of new oceanic crust and marine magnetic anomalies, the nature of the axial magma chamber, and the depth of hydrothermal circulation.

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.

Prebiotic synthesis in atmospheres containing CH4, CO, and CO2. I - Amino acids

SummaryThe prebiotic synthesis of organic compounds using a spark discharge on various simulated primitive earth atmospheres at 25°C has been studied. Methane mixtures contained H2+CH4+H2O+N2+NH3 with H2/CH4 molar ratios from 0 to 4 and pNH3=0.1 torr. A similar set of experiments without added NH3 was performed. The yields of amino acids (1.2 to 4.7% based on the carbon) are approximately independent of the H2/CH4 ratio and whether NH3 was present, and a wide variety of amino acids are obtained. Mixtures of H2+CO+H2O+N2 and H2+CO2+H2O+N2, with and without added NH3, all gave about 2% yields of amino acids at H2/CO and H2/CO2 ratios of 2 to 4. For a H2/CO2 ratio of 0, the yield of amino acids is extremely low (10−3%). Glycine is almost the only amino acid produced from CO and CO2 model atmospheres. These results show that the maximum yield is about the same for the three carbon sources at high H2/carbon ratios, but that CH4 is superior at low H2/carbon ratios. In addition, CH4 gives a much greater variety of amino acids than either CO or CO2. If it is assumed that an abundance of amino acids more complex than glycine was required for the origin of life, then these results indicate the requirement for CH4 in the primitive atmosphere.

The chemical conditions on the parent body of the murchison meteorite: Some conclusions based on amino, hydroxy and dicarboxylic acids

Amino and hydroxy acids have been identified in the Murchison meteorite. Their presence is consistent with a synthetic pathway involving aldehydes, hydrogen cyanide and ammonia in an aqueous environment (Strecker-cyanohydrin synthesis). From the various equilibrium and rate constants involved in this synthesis, four independent estimates of the ammonium ion concentrations on the parent body at the time of compound synthesis are obtained; all values are about 2 x 10(-3) M. Succinic acid and beta-alanine have also been detected in the Murchison meteorite. Their presence is consistent with a synthesis from acrylonitrile, hydrogen cyanide and ammonia. Using the equilibrium and rate constants for this synthetic pathway, and the succinic acid/beta-alanine ratio measured in the Murchison meteorite, an estimate of the hydrogen cyanide concentration of 10(-3) to 10(-2) M is obtained. Since hydrogen cyanide hydrolyzes relatively rapidly in an aqueous environment (t1/2 < 10(4) yrs) this high concentration implies a period of synthesis of organic compounds as short as 10(4) years on the Murchison meteorite parent body.

Reasons for the occurrence of the twenty coded protein amino acids

SummaryFactors involved in the selection of the 20 protein L-α-amino acids during chemical evolution and the early stages of Darwinian evolution are discussed. The selection is considered on the basis of the availability in the primitive ocean, function in proteins, the stability of the amino acid and its peptides, stability to racemization, and stability on the transfer RNA. We conclude that aspartic acid, glutamic acid, arginine, lysine, serine and possibly threonine are the best choices for acidic, basic and hydroxy amino acids. The hydrophobic amino acids are reasonable choices, except for the puzzling absences ofα-amino-n-butyric acid, norvaline and norleucine. The choices of the sulfur and aromatic amino acids seem reasonable, but are not compelling. Asparagine and glutamine are apparently not primitive. If life were to arise on another planet, we would expect that the catalysts would be poly-α-amino acids and that about 75% of the amino acids would be the same as on the earth.

Energy yields for hydrogen cyanide and formaldehyde syntheses: The hcn and amino acid concentrations in the primitive ocean

Prebiotic electric discharge and ultraviolet light experiments are usually reported in terms of carbon yields and involve a large input of energy to maximize yields. Experiments using lower energy inputs are more realistic prebiotic models and give energy yields which can be used to estimate the relative importance of the different energy sources on the primitive earth.Simulated prebiotic atmospheres containing either CH4, CO or CO2 with N2, H2O and variable amounts of H2 were subjected to the spark from a high frequency Tesla coil. The energy yields for the synthesis of HCN and H2CO were estimated. CH4 mixtures give the highest yields of HCN while H2CO is most efficiently produced with the CO mixtures. These results are a model for atmospheric corona discharges, which are more abundant than lightning and different in character. Preliminary experiments using artificial lightning are also reported.The energy yields from these experiments combined with the corona discharge available on the earth, allows a yearly production rate to be estimated. These are compared with other experiments and model calculations. From these production rates of HCN (e.g. 100 nmoles cm−2 yr−1) and the experimental hydrolysis rates, the steady state concentration in the primitive ocean can be calculated (e.g., 4 × 10−6 M at pH 8 and 0°). A steady state amino acid concentration of 3 × 10−4 M is estimated from the HCN production rate and the rate of decomposition of the amino acids by passage through the submarine vents.

Carbon Dioxide Clathrate in the Martian Ice Cap

Measurements of the dissociation pressure of carbon dioxide hydrate show that this hydrate (CO2 � 6H2O) is stable relative to solid CO2 and water ice at temperatures above about 121�K. Since this hydrate forms from finely divided ice and gaseous CO2 in several hours at 150�K, it is likely to be present in the martian ice cap. The ice cap can consist of water ice, water ice + CO2 hydrate, or CO2 hydrate + solid CO2, but not water ice + solid CO2.

A Reassessment of Prebiotic Organic Synthesis in Neutral Planetary Atmospheres

The action of an electric discharge on reduced gas mixtures such as H2O, CH4 and NH3 (or N2) results in the production of several biologically important organic compounds including amino acids. However, it is now generally held that the early Earth’s atmosphere was likely not reducing, but was dominated by N2 and CO2. The synthesis of organic compounds by the action of electric discharges on neutral gas mixtures has been shown to be much less efficient. We show here that contrary to previous reports, significant amounts of amino acids are produced from neutral gas mixtures. The low yields previously reported appear to be the outcome of oxidation of the organic compounds during hydrolytic workup by nitrite and nitrate produced in the reactions. The yield of amino acids is greatly increased when oxidation inhibitors, such as ferrous iron, are added prior to hydrolysis. Organic synthesis from neutral atmospheres may have depended on the oceanic availability of oxidation inhibitors as well as on the nature of the primitive atmosphere itself. The results reported here suggest that endogenous synthesis from neutral atmospheres may be more important than previously thought.

The origin of life—did it occur at high temperatures?

A high-temperature origin of life has been proposed, largely for the reason that the hyperthermophiles are claimed to be the last common ancestor of modern organisms. Even if they are the oldest extant organisms, which is in dispute, their existence can say nothing about the temperatures of the origin of life, the RNA world, and organisms preceding the hyperthermophiles. There is no geological evidence for the physical setting of the origin of life because there are no unmetamorphosed rocks from that period. Prebiotic chemistry points to a low-temperature origin because most biochemicals decompose rather rapidly at temperatures of 100°C (e.g., half-lives are 73 min for ribose, 21 days for cytosine, and 204 days for adenine). Hyperthermophiles may appear at the base of some phylogenetic trees because they outcompeted the mesophiles when they adapted to lower temperatures, possibly due to enhanced production of heat-shock proteins.

On the Origin of Metabolic Pathways

Abstract. The heterotrophic theory of the origin of life is the only proposal available with experimental support. This comes from the ease of prebiotic synthesis under strongly reducing conditions. The prebiotic synthesis of organic compounds by reduction of CO2 to monomers used by the first organisms would also be considered an heterotrophic origin. Autotrophy means that the first organisms biosynthesized their cell constituents as well as assembling them. Prebiotic synthetic pathways are all different from the biosynthetic pathways of the last common ancestor (LCA). The steps leading to the origin of the metabolic pathways are closer to prebiotic chemistry than to those in the LCA. There may have been different biosynthetic routes between the prebiotic and the LCAs that played an early role in metabolism but have disappeared from extant organisms. The semienzymatic theory of the origin of metabolism proposed here is similar to the Horowitz hypothesis but includes the use of compounds leaking from preexisting pathways as well as prebiotic compounds from the environment.