Organic Matter in Meteorites and Precambrian Rocks: Clues about the Origin and Development of Living Systems [and Discussion]

J. Brooks, L. J. Allamandola
1981 Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences  
Discoveries during the last decade in organic geochemistry and palaeochemistry are used to assess concepts and to evaluate current scientific views on the nature and significance of organic matter in meteorites and early Precambrian rocks and its role in the origin and development of living systems. A critical account is given of the various chemical and organic structures contained in various ancient Precambrian rocks and carbonaceous chondrites. Analysis and identification of such organic
more » ... of such organic matter contained in these rocks and meteorites are giving clues about how life originated and also about the nature and evolution of the early atmosphere, hydrosphere and early biological systems. Introduction It seems reasonably clear that the surface of planet Earth at one time, some 4.0 Ga ago (1 Ga = 109 years), was so hot that no life could exist, nor for that matter could one expect even complex macromolecules such as proteins and nucleic acids to exist in any form remotely resembling that required for life assembly. Consequently at some time there had to be a period when life arose on a sterile planet, and there are several possible ways in which this might have occurred. Modern hypotheses about the origin of life on Earth follow from suggestions made originally by Oparin (1924) and Haldane (1929) about the nature of (a) the de novo formation of the planet and its atmosphere and environment, and ( ) chemical synthesis in which building block precursors of biomolecules could be elaborated from a primitive early atmosphere by abiotic chemical reactions. Our present knowledge of the abiogenesis of organic molecules is derived from (i) identifi cation of organic molecules in astronomical spectra, (ii) the presence of organic compounds in carbonaceous chondrites, (iii) studies of "the organic contents in early Precambrian rocks, and (iv) laboratory experiments designed to test abiogenic synthesis theories. In this paper, I shall examine (ii-iv) and evaluate these results against the Oparin-Haldane hypothesis, using the ancient geological record and information contained in carbonaceous chondrites. Formation of the Earth's biosphere, ATMOSPHERE AND HYDROSPHERE The Earth, like the rest of the Solar System, came into existence 4.5-4.7 Ga ago and is considered to have had no significant atmosphere, but an environment rich in hydrogen may have been present. The Earth's outer crust, the initial atmosphere and hydrosphere were probably formed by the same differentiation and degassing processes of the mantle about 4.0 Ga ago. After quick dispersion of the initial 'primordial' hydrogen environment by diffusion, 596 J. BROOKS a secondary atmosphere was introduced by volcanic activity. From occasional outbursts of solid and fluid matter and the release of gases and vapours, an atmosphere formed that had reducing properties and contained no free oxygen. Water vapour (steam) is the most abundant constituent (97 %) of the volcanic gases and it is estimated that the quantity of water and gases released over the last 4.0 Ga more than accounts for the volumes of the oceans and for the nitrogen and other constituents in the present environment. Oxygen originated and increased in the atmosphere by dissociation of water under the action of solar energy. Several mechanisms (see Hesstvedt et al. 1974) for the dissociation of water have been proposed and the three most important are photosynthesis, photolysis and oxidation of elements of variable valence. The major process of dissociation is photosynthesis. Like any set of chemical reactions, photosyn thesis does not produce a net change in oxidation. Oxygen is produced, except in bacterial photosynthesis, together with a stoichiometric quantity of reduced carbon (photosynthesis ^ respiration/fermentation). Almost all the oxygen is eventually used to oxidize this reduced carbon and the only net gain in oxygen levels equals the amount of reduced carbon material removed from the biogeochemical system and buried as sedimentary organic matter before it can be oxidized. Thus an important mechanism for the fixation of organic carbon in the Earth's crust and the production of free oxygen into the atmosphere was the accumulation of sedimentary organic matter that escaped oxidation by being deposited. These processes have been active since the first presence of photosynthetic organisms in the early Precambrian. The oxygen now in the atmosphere and that combined with a wide range of other elements in the Earth's crust is probably mainly, if not wholly, biological in origin. As the content of free oxygen in the hydrosphere and atmosphere increased, living systems evolved suitable oxygenmediating enzymes within their cells and developed into more elaborate organisms with more sophisticated metabolic processes. The result of these and other processes is an intimate evolu tionary interaction between the biosphere, atmosphere, hydrosphere and lithosphere (Brooks !978). The hydrosphere, which was probably formed soon after 4.0 Ga ago (Watson 1976), was the main regulator of the cycle of organic matter in the biosphere and sediments. The timing of the build-up of oxygen in the atmosphere and hydrosphere is critical since many weathering and diagenetic processes involve redox reactions. The early presence of living systems on the Earth, which has now been established, implies that geological processes of sedimentation, weathering and diagenesis may have been influenced by organic activity from the earliest times (Watson 1976; Siever 1977) . The origin, composition and evolution of early Precambrian atmospheres have often been discussed (see Oparin
doi:10.1098/rsta.1981.0228 fatcat:zcsx77a6prb73m6vlgzbbmzlqi