The compounds detected are shown in Table 1. Uracil, 5-hydroxyuracil, orotic acid, 4,5-dihydroxypyrimidine, and nicotinic acid were detected. Adenine and guanine were tentatively identified by HPLC retention time, but their mass spectra could not be obtained because the yields were too low. Since CO is oxidized by proton irradiation to form CO2, energy yields of bioorganic compounds decrease with irradiation time. Under these experimental conditions, about half of the CO should have been converted into CO2 (25).
The energy yield of uracil from the 1-h irradiation was 7.1 x 10–12 mol•J–1. Taking this value and the energy flux of cosmic rays as 0.046 J•cm–2•yr–1 (26), the production rate of uracil from a CO-N2-H2O atmosphere is estimated to be 3.3 x 10–13 mol•cm–2•yr–1. The production rate allows a calculation of the steady-state concentration in the primitive ocean with a volume assumed to be 300 liter•cm–2, its current value. At steady state, the production rate equals the rate of decomposition. Using the rate constant of decomposition of uracil at pH 7 and 25°C as 4.0 x 10–7 yr–1 (28), the steady-state concentration of uracil in the primitive ocean is estimated to be 2.8 x 10–9 mol•liter–1. This concentration may be too low to produce more complex biomolecules. However, uracil could have been concentrated by evaporation or eutectic freezing, as uracil is relatively stable [t1/2 = 1.7 x 106 yr at pH 7 and 25°C (28)].
The reaction mechanisms for the syntheses of compounds detected here are uncertain. Although uracil, 5-hydroxyuracil, orotic acid, and 4,5-dihydroxypyrimidine are synthesized from a HCN polymerization (29, 30), this is not likely to be a major mechanism in this case, because yields of adenine and guanine would be much higher than that of uracil if these compounds were derived exclusively from HCN polymerization. Uracil can also be obtained from the hydrolysis of cytosine synthesized by reaction of cyanoacetaldehyde and urea (31). However, this is also not likely to be the source of uracil in this case, because only 3% of a cytosine standard was hydrolyzed to uracil under the hydrolysis conditions, and cytosine was not detected in the proton-irradiated sample. Nicotinic acid can be synthesized by a number of reaction pathways (32, 33). For example, nicotinonitrile, which hydrolyzes to nicotinic acid, can be synthesized from the reaction of cyanoacetaldehyde, propiolaldehyde, and ammonia (32), which are in turn synthesized from a spark discharge (32, 34, 35).
CO Atmospheres vs. Strongly Reducing Atmosphere. A CO-dominant atmosphere can give bioorganic compounds with yields comparable to those obtained from a strongly reducing atmosphere. The uracil yield from proton irradiation in a CO-N2-H2O mixture is slightly higher than that from proton irradiation in CH4-N2-H2O (36). Uracil synthesis from a spark discharge in CH4-N2-H2O or CO-N2-H2O has not been reported.
Adenine has been detected in a sample prepared from a spark discharge experiment using CH4-N2-NH3-H2O and frozen at –20°C for 5 yr (37). The carbon yield was 6 x 10–4 %, which is slightly lower than the 1.1 x 10–3 % of uracil synthesized from the proton irradiation of CO-N2-H2O. A room temperature electric discharge solution contained no detectable adenine.
A CO-N2-H2O atmosphere gives comparable amounts of amino acids as a CH4-N2-H2O atmosphere. The energy yield of glycine from proton irradiation of CO-N2-H2O is 2 x 10–9 mol•J–1, which is comparable to the 2 x 10–9 mol•J–1 from proton irradiation in CH4-N2-H2O (26) and the 8.7 x 10–10 mol•J–1 from a spark discharge acting on CH4-N2-NH3-H2O (15). The production rate of amino acids from proton irradiation of CO-N2-H2O is estimated to be 1.2 x 10–10 mol•cm–2•yr–1, which is comparable with the 1.5 x 10–10 mol•cm–2•yr–1 estimated from a spark discharge acting on CH4-N2-NH3-H2O mixtures.
Since it is generally held that CO2 would have been present in the primitive atmosphere, the effect of CO2 on the formation of uracil was investigated. Fig. 1 shows the energy yield of uracil with various mixing ratios of CO/CO2 in CO-CO2-N2-H2O gas mixtures (36). The yield of uracil is approximately proportional to CO/(CO+CO2). This finding suggests that CO2 does not inhibit the reaction that forms uracil. This is also true of glycine synthesis (38).
Fig 1. Energy yields of uracil with varying CO/CO2 mixing ratios in CO-CO2-15N2-H2O. The gas mixture had been irradiated for 1 h.
A large amount of molecular hydrogen may have been present in the primitive atmosphere if the oxidation state of the upper mantle were reducing. A H
2-CO-N
2-H
2O or H
2-CO
2-N
2-H
2O atmosphere is more favorable for amino acid synthesis than an atmosphere without H
2 (
15,
39,
40). For example, in case of reaction of a H
2-CO-N
2-H
2O atmosphere with a spark discharge, the carbon yields of amino acids are 0.05% and 0.9% at H
2/CO = 0 and H
2/CO = 1, respectively (
15). The amino acid yields from H
2-CH
4-N
2-H
2O are approximately independent of the H
2/CH
4 ratio. H
2 seems to inhibit the synthesis of the nucleic acid bases (
41,
42).
Ammonia could not have been more than a minor component in the primitive atmosphere, even if the oxidation state of the upper mantle were highly reducing. Ammonia is photochemically labile and its half-life in the primitive atmosphere would have been very short on a geological time scale (10). Ammonia is very soluble in water and some NH4+ may have been adsorbed on clay minerals. Bada and Miller (43) estimated that the maximum value of the partial pressure of NH3 in the primitive atmosphere would have been 7.3 x 10–6 atm at 25°C with a pH 8 ocean. They suggested that most of the ammonia would have been dissolved in the oceans and thus could have contributed to the synthesis of amino acids by the Strecker synthesis. The carbon yield of amino acids from a spark discharge acting on a CO-N2-H2O gas mixture over 0.05 M aqueous NH4Cl is 10 times higher than that without NH4Cl, although the effect of gaseous NH3 on the reaction (1 x 10–4 atm in that case) is unclear (15). The amino acid yields from spark discharge experiments on NH3-H2-CH4-N2-H2O gas mixtures are approximately equal to those from mixtures without NH3 (15). The yield of uracil from CO-N2-H2O was four times higher than that from CO-NH3-H2O in proton irradiation experiments (36).
It has been shown here that a CO-CO2-N2-H2O atmosphere can yield comparable amounts of bioorganic compounds as a strongly reducing atmosphere. Considering that a CO2-N2-H2O atmosphere does not yield bioorganic compounds efficiently, and that it is difficult to accept a strongly reducing atmosphere from models of early atmospheric chemistry and geology, a CO-CO2-N2-H2O atmosphere may be more favorable for prebiotic synthesis and the origin of life.
Other Models for the Origin of Life. Recently, Kasting and Brown (14) suggested that a CO2-N2-H2O atmosphere containing tens to hundreds of ppm of CH4 might have existed. Such amounts of CH4 could have produced a small amount of HCN and also contributed to early greenhouse warming of the Earth. If the atmospheric ratio of CH4 had been 100 ppm, the production rate of HCN can be estimated to have been 108 cm–2•s–1 (estimated from ref. 44). However, biologically important molecules, such as nucleic acid bases and amino acids, might not have been synthesized sufficiently via HCN polymerizations with such a low production rate of HCN if at least portions of the Earth were not frozen (45). The steady-state concentration of HCN in the primitive ocean at pH 8 and 25°C with the production rate of 108 cm–2•s–1 is estimated to be 8 x 10–10 M. HCN can effectively polymerize to form biologically important molecules when its concentration is higher than 0.01 M (46). At lower concentration, hydrolysis of HCN is predominant to form ammonia and formic acid (46). If the production rate of HCN were approximately 1,000 times higher, it would still be too low to produce significant amount of biologically important molecules unless portions of the Earth were frozen (45). Therefore, it seems unlikely that a CO2-N2-H2O atmosphere containing a small amount of CH4 played an important role for the synthesis of bioorganic compounds via HCN polymerizations.
If the primitive Earth had a CO2-dominant atmosphere, bioorganic compounds would not have been efficiently synthesized in the atmosphere. Hydrothermal vents may have been potential environments for the synthesis of bioorganic compounds (47). However, similar problems that affect atmospheric syntheses would have affected hydrothermal vent syntheses. If the upper mantle had been near its current oxidation state, most of the carbon available for synthesis of bioorganic compounds in the hydrothermal vents would have been in the form of CO2 and bioorganic compounds would not have been effectively synthesized. Fluids discharged from present hydrothermal vents are composed of H2O and CO2 combined with lesser amount of H2, H2S, N2, CH4, CO, and NH3 (48).
Some organic compounds, such as acetic acid and pyruvic acid, can be synthesized from CO in the presence of sulfide minerals under simulated hydrothermal conditions (49, 50). However, hydrothermal vent syntheses are not very efficient with respect to the synthesis of important biological compounds compared with atmospheric syntheses. Glycine and alanine can be synthesized from CH4 and N2 under simulated hydrothermal conditions (51), but the carbon yields are 1,000 times lower than those from simulated atmospheric syntheses. Shock (52) suggested that formation of amino acids from CO2 was favorable under disequilibrium hydrothermal conditions. However, there is as yet no experimental evidence showing that amino acids can be synthesized from CO2 under hydrothermal conditions (53).
Another potential source of prebiotic bioorganic compounds may have been delivery of extraterrestrial material by comets and carbonaceous chondrites, although it is uncertain how significant this source may have been (54). Using data from the Murchison meteorite (55, 56) and a production rate of intact exogenous organics from airbursts estimated by Chyba and Sagan (27), the delivery rates of uracil and glycine by meteorites 3.8 billion years ago are estimated to be 20 mol•yr–1 and 3 x 103 mol•yr–1, respectively. These values are much smaller than the estimated production rates of 2 x 106 mol•yr–1 for uracil and 5 x 108 mol•yr–1 for glycine synthesized from the proton irradiation in CO-N2-H2O. Assuming that the production rate of uracil from cosmic rays is proportional to CO/(CO+CO2) as shown in Fig. 1, when the CO/CO2 ratio is 10–5, its production rate is equal to that from delivery by meteorites.
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