Prebiotic synthesis from CO atmospheres: Implications for the origins of life
Shin Miyakawa * 
, Hiroto Yamanashi *, Kensei Kobayashi *, H. James Cleaves 
, and Stanley L. Miller
*Department of Chemistry and Biotechnology, Faculty of Engineering, Yokohama National University, Yokohama 240-8501, Japan; and Department of Chemistry and Biochemistry, University of California at San Diego, La Jolla, CA 92093-0506
Abstract
Most models of the primitive atmosphere around the time life originated suggest that the atmosphere was dominated by carbon dioxide, largely based on the notion that the atmosphere was derived via volcanic outgassing, and that those gases were similar to those found in modern volcanic effluent. These models tend to downplay the possibility of a strongly reducing atmosphere, which had been thought to be important for prebiotic synthesis and thus the origin of life. However, there is no definitive geologic evidence for the oxidation state of the early atmosphere and bioorganic compounds are not efficiently synthesized from CO2 atmospheres. In the present study, it was shown that a CO-CO2-N2-H2O atmosphere can give a variety of bioorganic compounds with yields comparable to those obtained from a strongly reducing atmosphere. Atmospheres containing carbon monoxide might therefore have been conducive to prebiotic synthesis and perhaps the origin of life. CO-dominant atmospheres could have existed if the production rate of CO from impacts of extraterrestrial materials were high or if the upper mantle had been more reduced than today.
PNAS, vol. 99, no. 23, 14628-14631, November 12, 2002
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Based on putative microfossil (1) and light carbon evidence in ancient sedimentary rocks (2) and the possible sterilizing consequences of the late heavy bombardment suggested by lunar records (3), the ancestor of modern life is thought to have originated about 3.8 billion years ago, although recent findings have questioned some of these data (4–7). The composition of the primitive atmosphere around this time remains uncertain, but volcanic outgassing could have been a major source of atmospheric gases. The oxidation state of volcanic gases would have depended on the oxidation state of the upper mantle (8). If the upper mantle had been in a reduced oxidation state, the gases would have been composed mainly of H2, H2O, CO, and N2. At more moderate temperatures, CO would have reacted with H2 to yield CH4 and H2O in the presence of catalysts, and N2 would have reacted with H2 to yield NH3 (9). The most reducing atmosphere possible would have been composed of CH4, NH3, H2, and H2O (herein referred to as a strongly reducing atmosphere), although it is difficult to reach this state because of factors such as photo-decomposition (10).
The oxidation state of the upper mantle 3.8 billion years ago is generally thought to have been near its current value (11, 12). Modern volcanic gases are composed mainly of H2O and CO2 (8). Walker suggested that the partial pressure of CO2 in the early atmosphere could have been as high as 10 bars (13). The dominant view in recent years has thus been that the atmosphere when life originated was composed of CO2, N2, and H2O combined with lesser amounts of CO, CH4, and H2 (11–14).
Strongly reducing atmospheres are more favorable for the synthesis of bioorganic compounds than CO2-N2-H2O atmospheres. Schlesinger and Miller (15) reported that carbon yields of amino acids from CH4-N2-H2O, and CO2-N2-H2O were 1% and 0.0006%, respectively. It was thus suggested that a strongly reducing atmosphere would have been required for the origin of life, which is contrary to the conclusions drawn from atmospheric modeling.
Recent research suggests that the continental crust and oceans could have formed by 4.3 billion years ago (16, 17). This notion suggests that CO2 precipitation as CaCO3 or MgCO3 could have started well before life originated (13). The partial pressure of CO2 in the primitive atmosphere when life originated may then have been much lower than previously thought, and a N2-dominant atmosphere may have existed. Sleep and Zahnle (18) suggested that the atmospheric mixing ratio of CO2 in the Hadean era would have been low because of vigorous mantle cycling of CO2 and the reaction of CO2 with impact ejecta. Geochemical data from Paleosols suggest that the atmospheric CO2 concentration at 2.8 billion years ago would have been a factor of 20 or more lower than those needed to keep the Earth's surface from freezing (19).
Since the lunar impact record suggests a high bolide flux on the Earth until 3.8 billion years ago (3), a large amount of CO may have been supplied by the impact of comets and asteroids. There would have been three possible extraterrestrial sources of CO. First, comets, which contain large amounts of CO (20), could have delivered CO directly. Second, organic carbon in comets or carbonaceous asteroids may have been oxidized by oxygen derived from silicates to form CO in impact plumes (21). Third, hot metallic iron produced by impacts of ordinary chondrites could have reduced atmospheric CO2 to CO (21). Thus, a CO-dominant atmosphere could have been built up from impacts. A CO-dominant atmosphere, however, could not have existed long, since CO is relatively unstable. One sink for CO could have been reaction to produce organic molecules. Another important sink is oxidation by OH radicals produced from water vapor photolysis to produce CO2 and hydration to produce formic acid (22).
A CO-dominant atmosphere might have existed intermittently around the time life originated and might have contributed to formation of bioorganic compounds. Abelson (23) pointed out that a CO atmosphere might have played an important role in the formation of bioorganic compounds and demonstrated the synthesis of HCN from such an atmosphere.
The syntheses of bioorganic compounds from a CO atmosphere is difficult because of the strong triple bond of CO. Bioorganic compounds are not effectively synthesized from a spark discharge in CO-N2-H2O, possibly because the CO is not efficiently dissociated by a spark discharge (24). High energy sources, such as cosmic rays and postimpact plumes, may be required to form bioorganic compounds.
We show here that with a high energy source CO-dominant atmospheres can give a variety of biologically important molecules with yields comparable to those obtained from a strongly reducing atmosphere. Protons of 2.5–3.0 MeV were used as the high energy source in this study. High energy protons are a major component of cosmic rays, and there was likely significant flux of cosmic rays on the early Earth.
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