SCOPE 13 - The Global Carbon Cycle

 ABSTRACT

Estimates on the cosmic abundance of carbon are principally derived from three sources: (i) solar atmosphere, (ii) meteorites, and (iii) Earth; lunar soils and samples tested on Mars by Viking landings have recently supplemented the information. This data is presented and used as background to test existing models on the origin of organic matter and life on Earth. It is concluded that mineralorganic interactions are the key for the synthesis and evolution of complex organic molecules such as proteins, phospholipids or the nucleic acids.

2.1 SOLAR ATMOSPHERE

It can be concluded from the relative abundance of the more common elements in the solar atmosphere (not considering noble gases) (Table 2.1), that carbon ranks third in the list of elements. The four common biogenic elements, hydrogen, oxygen, carbon, and nitrogen, are the most abundant non-noble gas elements, and two other major biogenic elements, sulfur and phosphorus, are also high on the list. It thus appears that organic matter is a more representative sample of the universe than is our dead silica-dominated Earth.

In the light of this distribution pattern, research on great interstellar clouds, initiated in 1963 by a team from the Massachusetts Institute of Technology and the Lincoln Laboratory, appears to be of great relevance. Almost 50 different molecules, including the hydroxyl radical, water, methane, formaldehyde, ammonia, hydrogen cyanide, ethyl alcohol, and carbon monoxide, have so far been discovered by radio telescope. Freezing of molecules to dust particles and their subsequent release between 4 to 25 K will promote catalysis and epitaxis. Thus, the life span of a molecule depends not only on the intensity of photo-ionization but also on the type of interstellar grains (e.g., iron, silicate, graphite, graphite covered with frozen water, ammonia, and methane) and the existing temperature. The greater abundance of complex molecules in the H⁺-regions is due to a more temperate environment. The type and size of clouds are also factors. For instance: a cosmic cloud with a radius of 1 light year and a particle density of 1000/cm ³ weakens visible light by a factor of 10 and UV radiation by a factor of about 10²⁴ (Salpeter, 1974). A complex molecule may, therefore, `survive' for millions of years in outer space.

Table 2.1 Relative abundances of the more common elements in the solar atmosphere (from Cameron, 1973)

Such a series of compounds represents a powerful tool for the synthesis of biologically interesting compounds, such as sugars, amino acids, or the bases of purines and pyrimidines. According to Weiss et al. (1970), the Ca(OH)₂ formaldehyde reaction involves: (i) self-condensation of formaldehyde (e.g. glycolaldehyde); (ii) aldol condensations (e.g. pentoses and hexoses); (iii) Cannizzaro reaction (the simultaneous reduction and oxidation of two aldehyde groups in the presence of hydroxyl ions):

        4CH₂ O + Ca(OH)₂ (HCOO)₂ Ca + 2CH₃ OH

and (iv) isomerization of hydroxy aldehydes and ketones. The complexity of the system, particularly with regard to the competitive nature of formose and Cannizzaro reactions, makes it difficult to predict equilibrium conditions. The variety of molecular species that may arise in the process of such reactions is numerous, and the resulting carbohydrate chemistry can certainly be as complex as that of crude oil.

A similar case can be made for hydrogen cyanide with respect to the synthesis of amino acids or the bases of purines and pyrimidines (Mizutani et al., 1975). Since galactic clouds are considered as a potential source for the formation of solar systems such as ours, the Earth is believed to have been condensed from a disc of gas and dust around the Sun which had conceivably already contained a wide spectrum of organics (Yoshino et al., 1971).

Many `hot' and `cold' models have been proposed on the mechanism of planetary accretion. This is not the place to critically review the formation of planets from a solar nebula, but the interested reader may refer to Anders (1977). However, independent of the viewpoint we accept, the synthesis and fractionation of complex organic molecules proceed only during the low-temperature stage of a cooling nebula and in the upper veneer of the accreted body.

Figure 2.1 Total carbon (ppm) of lunar material recovered by Appollo 15 (Moore, 1974, Fig. 3. Reproduced by permission of Arizona State University)

Solar winds are another contributing source of carbon, which is implanted in materials exposed at the surface of planets and planetoids. Understanding has recently been gained of the significance of this process, by looking at meteorites and lunar soils (Gibson and Moore, 1972; Trimble, 1974; Fireman et al., 1976; Pillinger and Eglinton, 1977). With respect to lunar material, the total carbon content is approximately 100 to 200 μg C/g lunar sample (Moore, 1974; Chang et al., 1974); it appears to be derived mainly from extralunar sources, i.e. solar wind and impacting micrometeorites. The compiled data are given in Figures 2.1 and 2.2.

Figure 2.2 Total carbon (ppm) of lunar material recovered by Apollo 16 (Moore, 1974, Fig. 4. Reproduced by permission of Arizona State University) 

2.2 METEORITES

The geochemistry of meteorites is reasonably well understood (Lawless et al., 1972; Nagy, 1975) and all lines of evidence point to the conclusion that organic matter in meteorites is of extraterrestrial origin and most likely of abiotic nature. According to Wiik (1956), the carbon content in fourteen of the twenty known carbonaceous chondrites ranges from 0.19% (Ornans) to 4.83% (Ivuna). Ordinary chondrites contain, on the average, only a few tens to a few hundreds of parts per million in organic carbon. Carbonaceous chondrites represent about 1.7% in the total mass of all meteorites which have fallen on Earth, and the mass fraction of organic matter in carbonaceous chondrites is about 1.3% (see Chapter 13, this volume). This implies that about 0.06% of all meteorite material is made of organic carbon, assuming of course that the collected meteorites constitute a representative fraction of the parent body.

The total mass of the meteorite parent bodies (asteroid belt) is roughly equivalent to a sphere of terrestrial density of about 1000 km in diameter (Sagan, 1961). The mass of this sphere is less than one-thousandth of the mass of the Earth. The asteroids contain about 10¹⁴ t in organic matter, which is only about 50 times less than the total amount of terrestrial organic matter.

Carbonaceous chondritic material appears to be a primordial 'condensate' which formed at low temperatures, 400 to 500 K (Anders and Owen, 1977). Based on physical properties such as albedo, spectral reflectance and density, the moons of Mars, most asteroids, comets, interplanetary dust and, as tentatively suggested, the moons of the outer planets and the rings of Uranus are partly or entirely composed of carbonaceous chondritic material. From all we know, there is no evidence of such material on the surface of any object closer to the Sun than Mars (e.g. Wilkening, 1978; Pollack et al., 1978; Pang et al., 1978).

The forms of carbon present in different types of meteorites include: carbonate (Clayton, 1963), diamond (Urey and Mayeda, 1959), carbide, Fe₃C (Mason, 1966), extractable organics (Nooner and Oró, 1967; Nagy et al., 1961), graphite (Brett, 1966), and polymeric organics (Kaplan et al., 1963; Hayes, 1967). The subsequent oxidation of hydrocarbons introduces the formation of fatty acids:

3O + H₃CR RCOOH + H₂O

This reaction can be catalysed by silicates, due to their metal oxide nature. The presence of fatty acids in meteorites (Nagy and Bitz, 1963) may simply be a consequence of this reaction scheme.

It is known from metallurgical experience that carbon readily dissolves in melts of silicates and iron and, upon cooling, produces submicroscopic carbon and graphite particles. At higher temperatures, carbon, in contrast to other metal oxides such as those of copper, zinc or iron, cannot reduce the strong electropositive alkalis, alkaline-earths, and aluminum oxides (Cvetanivic and Amenomiya, 1967). Consequently, carbon does not oxidize to CO₂ by reduction of metals because the non-reducible elements predominate in silicates. Its affinity to iron suggests that, in the course of differentiation, much of the carbon has been extracted from the silicates and carried via iron droplets to the central core. The high abundance of graphite in iron meteorites is considered to be a consequence of such a development.

Abundance	Form		Molecular Species
high	CO2	completely oxidized
	COOH		acid
	COH		alcohol
	CH		hydrocarbon
less	CC	completely reduced	graphite

Starting from carbon, a great number of high-molecular-weight hydrocarbons and oxy-hydroxy compounds of graphite, such as graphtic acid C₈O₂(OH₂), should come into existence. It is conceivable that a special role is played in such reactions by graphite because of its outstanding property of making intercalations of carbon sheets with molecular layers of halogenides, metal halogenides, metal oxides, and sulfides (Rüdorff et al., 1963). Reactions preferentially proceed from the outer molecular layers and, thereafter, from the atoms at the edges of the benzenoid layer (Dawson and Follett, 1963), resulting in the subsequent removal of chain and layer fragments. The fact that the molecular mechanism starts from lattice defects (Pauling, 1966; Rüdorff et al., 1963; Coulson et al., 1963) accounts for the formation of higher-molecular-weight aromatics and chains because lattice defects promote catalysis (Volkenstein, 1960). The hydrogenation process is, thermodynamically, rather effective (e.g. Bond and Wells, 1964; Siegel, 1966):

        C_(graphite) + 2H_2(gas) = CH_4(gas)              (AF⁰ 12 kcal) 

2.3 EARTH

The abundance of carbon in the Earth's crust is 0.27% (see Chapter 13, this volume, Table 13.3). The carbon content in mantle material and in the iron-nickel core is probably about the same as in the corresponding meteorites, i.e., less carbon in the mantle, and highly dispersed or nodular graphite in the core material.

In the continental crust the ratio of reduced to oxidized carbon is in the range of 1:4. This figure has been reached by calculating the difference in ¹³ C content between the magmatic carbon pool and the two principal end members of the oxidized and reduced species, i.e. carbonates and kerogen, respectively (Degens, 1969).

Concerning the origin of organic matter in terrestrial rocks, most researchers in this field assume a biogenic source, but some materials associated with crystalline rocks, including some hydrocarbon patches, might well have been formed inorganically (Hoyle, 1955; Robinson, 1964; Galaktinova, 1959). In spite of these few questionable cases, it is safe to conclude that the bulk of organic matter is biogenic. This statement leads immediately to the question of the origin of life, and many viewpoints have been advanced on this topic.

Since the early work of Oparin (1953) and Haldane (1954), many models have been proposed on the prebiotic synthesis of organic molecules. The majority have involved atmospheric systems and UV, ionizing particles, or electric discharges as activating energies. Their attraction and final acceptance by scientists and laymen alike rest principally on the successful synthesis of amino acids, small peptides, sugars and nitrogen heterocycles, among others. There are others (Matheja and Degens, 1971), who consider the lithosphere as a potential source of organic molecules of the kind encountered in organisms an idea which was initially put forward by Bernal (1954). To narrow down the wide range of possibilities inherent in such an `earthly' system, the following criteria should be applied; that is, the system must: (i) be thermodynamically feasible, (ii) have a high yield, and (iii) allow for chemical evolution towards a primordial cell. This treatment would have the added advantage of testing previously proposed models for their rationale.

2.4 THERMODYNAMIC CONSIDERATIONS

Chemical reactions may proceed in equilibrium and non-equilibrium, and the probability of formation of biochemical molecules under both conditions will be examined here. The energy of formation, G^o, necessary to produce a simple organic molecule from its individual elements is known within certain limits. In using the relationship

        G = G⁰_products - G⁰_reactants

one can determine the order of reaction. Under equilibrium conditions where 

        G = 0

the relationship is

G_products = G_reactants

Biochemical compounds contain principally C, H, O and N. Under equilibrium conditions we can write:

        C, H, O, N  CO₂, H₂ O, N₂, NH₃, H₂, C

Based on the relationship between the equilibrium constant (K) and Gibbs' energy of reaction (G)

        G = RT In K

it is concluded that, under conditions where G = 0, biochemical compounds such as amino acids, sugars, or nitrogen heterocycles are unstable and will yield CO₂, H₂ O, N₂, NH₃, H₂, C. Calculations by Dayhoff (1971) support this conclusion. Thus, biochemical molecules do not arise under equilibrium conditions, mainly because of the inability to separate the newly generated compounds in a quasi stationary phase.

Under conditions of chemical disequilibrium, the synthesis of the major biochemical molecules does not create a problem for:

        G= RT In K/0

Changes in G cause fluctuations in the concentration of the reaction partners. The two main determinants in controlling the rate of synthesis are the chemical composition of the reaction system and the manner in which energy is dissipated.

The number of non-equilibrium systems established in the lithosphere is great. In contrast, there is less occasion in either the hydrosphere or the atmosphere when considered as separate entities to maintain non-equilibrium conditions for extended periods of time. For this reason, models on prebiotic synthesis under atmospheric conditions have to incorporate the hydrosphere as an integral part of the system. This then, would represent a non-equilibrium environment and conform to thermodynamic requirements.

2.5 YIELD OF REACTION

The second criterion concerns estimates on the yields of reaction. The geochemical environment has almost unlimited resources and possibilities for synthesizing and storing organic molecules. Less obvious is the situation in the atmosphere/hydrosphere system. Therefore, let us try to quantify the maximum yields in biochemical compounds, which one might get in atmospheric systems through ionizing energies, and estimate the amount that can eventually be collected in an oceanic sink.

Different gas mixtures have been proposed by various authors for the primordial atmosphere. For lack of an alternative solution, we assume for a moment that its composition is somewhat similar to that of volcanic gases:

and that by applying ionizing energies chemical radicals, intermediates and biochemical compounds can be generated (Miller, 1959). Using the published figures on the amount of photon energy accessible for photochemical reactions, a total of 100 cal/cm² yr is calculated (Table 2.2). In comparison, roughly 55 kcal/cm² yr of solar energy are available for photosynthesis within the atmosphere. The net primary productivity (= rate of energy-fixation by photosynthesis) for the whole earth amounts to ~140 cal/cm² yr (= 320 g/m² yr) which implies that 1/400 of the suitable sunlight energy is biologically utilized. At the energy flux estimated for chemical evolution in the atmosphere/hydrosphere system, almost 15 million years would be required to match the organic productivity of a single year. To obtain the  level of organic matter presently dissolved in the sea (1-2 mg/l), photochemically produced molecules would have to accumulate in the ocean without loss for about 1.5 billion years. Since chemical evolution proceeded for at least 1 billion years before the first organism emerged, the amount of organic matter present in the primordial sea could be, at most, that of the present ocean, if we assume that atmospheric systems were the only source of organic carbon.

Table 2.2 Data on energy flow in the atmosphere 

			Optimal	Probable
1.	Total energy <2400 Å available		10-1	10-3 kcal/cm2 yr
	for synthesis: U* = E<2400 Å. f1
2	Energy flow coefficient for transport		10-3	10-5 per cent
		U
	atmosphere ocean: f2 =
		U*
3.	Energy fixation rate available for		10-4	10-8 kcal/cm2 yr
	chemical evolution: U = f2 . U*
4.	Energy fixation rate of present photosynthesis			0.14 kcal/cm2 yr
5.	Lightning		10-4	10-6 kcal/cm2 yr
* 1	kcal = 4184 Joules
	Explanation:
1.	On the assumption that all radiant energy <2400 Å can be made available for organic synthesis, 0.1 kcal/ cm2 yr are transferred into chemical bonding energies. If only 1 per cent of this amount is extracted for photochemical reactions a figure which is already considered high 10-3 kcal/cm2 yr are utilized.
2.	The energy flow coefficient for the transport of organic molecules, synthesized in the atmosphere, to the ocean can be defined by the equation U=f2 . U*, where U represents the amount of energy that is extracted from the atmosphere in the form of organic molecules, or the amount of energy picked up by the ocean, respectively. U* is the total energy utilized in atmospheric synthesis. Assuming that: (i) every tenth absorption represents a `favourable' reaction (p1 = 1/10); (ii) every tenth reaction results in stable end products (p2 = 1/10); and (iii) every tenth molecule produced is not photodissociated (p3 = 1/10), the factor f2 =p1 • p2 • p3 = 10-3 per cent. This estimate is probably too high by about two orders of magnitude.
3.	In using the factors f1 and f2, we can calculate the flow of energy available for the chemical evolution in the atmosphere/hydrosphere system:
	U=f2 U*=f,1 .  f2 .  E<2400 Å
4.	This value for U is compared with the present energy fixation rate of photosynthesis.
5.	The energy flux associated with lightning in a primordial atmosphere such as that of Jupiter is only a fraction of a per cent of the solar ultraviolet flux; dominant forms of oxidized and reduced carbon in the deep Jovian atmosphere are CO and CH4, respectively (Prinn and Barshay, 1977).

These calculations have been made to show the ultimate potential of photochemical synthesis. To a student in geology, such a model is unrealistic, since organicinorganic interactions at work in the sea, as well as in terrestrial soils, will effectively reduce the level of organic matter present in the hydrosphere. Minerals are the principal scavengers. In view of their capacity to extract organic matter in the form of surface films or in structural positions and the ability of many organic molecules to produce metal complexes with the resultant formation of particulate organic detritus, it is highly questionable whether organic molecules could remain in the sea for extended periods of time; instead, they will finish in the sediment column.

2.6 STRUCTURAL ORGANIZATION

Although a variety of organic molecules can be synthesized pliotochemically, there are a number of biochemical compounds that cannot be generated in atmospheric systems. Ordinary fatty acids and phosphorus-containing molecules belong to this class of materials. Furthermore, water is not a medium suitable for the structural organization of the almost randomly distributed organic molecules. For example, polymerization of amino acids in the direction of peptides cannot proceed due to dipoledipole interactions.

These difficulties do not exist in the lithosphere. Paraffins are available as a convenient source for the generation of fatty acids. The element phosphorus is present in the form of phosphates which can exist as individual minerals or may occur as a covering layer on silicate surfaces. In this way, phosphates are readily accessible for various work assignments in chemical evolution (Matheja and Degens, 1971). The fact that amino acids can be brought into solid solution on mineral surfaces represents the decisive step towards their polymerization. The solid-state surface selects amino acids on the basis of side-chain properties, renders functional groups not participating in the formation of amide bonds inactive, and initiates carboxyl activation. Other organic monomers are structurally rearranged in their own distinct way when placed on mineral surfaces.

In summary, chemical evolution in the atmosphere/hydrosphere system is thermodynamically feasible. However, the reaction yield of photochemical synthesis is rather small and will not lead to significant concentrations of organic matter in the ocean. This conclusion holds true even if we assume that the volume of the hydrosphere was smaller in Precambrian time than today (Rubey, 1951) an idea which has been challenged (Fanale, 1971). Actually, the source and the release mechanism of volatiles from the Earth's crust and lithosphere has recently been a topic of lively discussion. According to Anders and Owen (1977), volatilerich material, similar in composition to C3V carbonaceous chondrites, was received by Earth in the final stages of accretion. A thin veneer around the Earth was produced and volatiles were released at different rates. Some elements and compounds accumulated in the primordial atmosphere and hydrosphere quite rapidly, while others were only gradually released in the course of differentiation and tectonism. Independent of which viewpoint one accepts on the origin of the hydrosphere, the gradual or catastrophic, the moment an organic molecule enters the hydrosphere its chances of being picked up by minerals and transferred to the crust are great.

2.7 PROTOBIOSPHERE

The partial melting of upper mantle material is responsible for the formation of an oceanic crust (Press, 1968), and processes of sea floor spreading are linked to the constant generation of new crusts. Figure 2.3 summarizes the recycling mechanisms along spreading centres, within geosynclines and on continents. To generate the present lithosphere, a mantle section of only about 700 km in depth has to differentiate.

During the initial degassing stage of the Earth which resulted in the formation of the atmosphere and hydrosphere, prebiotic CHNOPS compounds became rapidly discharged along the same channels where lower-melting material is presently moving upward. As a function of temperature gradients within the rock formation, a series of organic fronts developed with organic compounds differentiated in a fashion similar to that observed in ion exchange or gas chromatographic patterns. Since minerals could act as catalysts or as templates in the course of this development, a wide spectrum of new compounds were generated, many of which were eventually released to the hydrosphere and atmosphere (Degens, 1974). A protobiosphere was then formed in which minerals, emulsions, and water-soluble compounds coexisted. This type of substrate is considered the starting material for the generation of the first living cell.

Figure 2.3 Schematic diagram showing the underthrusting of a continent by an oceanic plate. A trench (consuming the plate margin) is first formed. This is accompanied by volcanism, metamorphism, and flysch migrating off the rising continent rim resulting eventually in the formation of a `Cordillerantype' mountain belt. In the back of the mountain belt, rapid sedimentation occurs, which in turn is overthrusted by the belt itself. The subduction process described here provides a mechanism whereby lithospheric material can be `recycled' (Wong and Degens, 1978)

The mineral-organic matter association present in carbonaceous chondrites is viewed as a `sediment facies' that formed at the primordial surface of a developing planet, such as Earth, Mars, Venus, or the meteorite parent body. Eventually, the terrestrial protobiosphere was eliminated by sedimentary processes, oxidation, and biogenic activities.

2.8 CHEMICAL EVOLUTION TOWARDS THE PRIMORDIAL CELL

The present biosphere is the product of a long chain of organicinorganic interactions. Life, as an autoreplicating system, is an outgrowth of its environment and thus can be described only in relation to that environment.

The individual abiotic events necessary for the creation of a primordial cell are depicted in Figure 2.4. Following synthesis of monomeric building blocks, a total of 11 reaction boxes, which we will term `black boxes', must come into operation to produce the key components of life. Arrows interconnecting individual black boxes are used to symbolize the relationships between two or more classes of compounds. To illustrate a reaction mechanism that may proceed between black boxes, we will examine one segment of the cycle more closely, i.e. the merger between metabolism and genetic control devices. This process is considered the final step in the formation of the primordial cell (Figure 2.5).

Figure 2.4 Principal events for the creation of a primordial cell. At the beginning of abiotic synthesis, there is no connection between the metabolic, enzymatic, and genetic lines. They interact and merge at some later stage of chemical evolution (Matheja and Degens, 1971. Reproduced by permission of the Gustav Fischer Verlag, Stuttgart)

Following synthesis of the key components of cellular systems, a partition boundary must be generated to contain, separate, and protect the cellular system from the environment; these boundaries are conveniently termed membranes. Examination of phospholipids reveals that they possess all structural and chemical properties required for well-ordered membrane fabrics in terms of: (i) surface lattice geometry, (ii) charge distribution, and (iii) mechanical flexibility. These combined factors control the flux of energy, ions, and molecules between a cell and the environment, or between individual compartments of a cell. The charged fabric of the boundary layer can develop pressure and potential gradients which operate as a kind of molecular pump. This is in essence the driving force of metabolism.

The question of the primordial synthesis of membranes and in particular that of a cell caspid that will house the first cell is one of the least understood problem areas in the field of chemical evolution. A seemingly unrelated observation from the field of aquatic geochemistry may, however, contain the key to the unresolved question. Here are the details:

Lake Kivu, which is situated at the highest point of the East African Rift Valley, has a maximum water depth of 500 m. The lake is surrounded by active volcanoes and geothermal springs, which seep from the lake bottom into the anoxic deep water and discharge CO₂, CH₄, H₂, and H₂S; at surface pressure about 21 of CO₂ are released from 11 of deep water. This implies that when deep waters move up by turbulent forces, effervescence will occur at some critical depth and tiny gas bubbles will form. Gas bubble surfaces are phase boundaries which in the case of Lake Kivu with its high dissolved organic matter content extract hydrophobic resinous material and generate ionic membranes that become stabilized by metal ions (Degens et al., 1973). The resulting hollow spheres are about 1 μm in size (Figure 2.6), which is about the size of a prokaryote cell capsule.

Figure 2.6 Transmission electron micrograph (replica technique) showing hollow spheres, about 1 µm in diameter, which are found as suspended particles in Lake Kivu, East Africa (Degens et al., 1973. Reproduced by permission of the Ferdinand Enke Verlag, Stuttgart)

Figure 2.7 Auto-organizing system leading to the generation of the first living cell (Matheja and Degens, 1971. Reproduced by permission of the Gustav Fischer Verlag, Stuttgart)

In many geological and geochemical aspects, Lake Kivu resembles a primordial sea. It is tentatively concluded that the formation of a primordial cell capsid was also initiated by gas bubbles. In point of fact, gas bubbles have probably produced an infinite number of micron-sized spheres, globules, micelles, or coacervates which accumulated as a thick foam at the surface of the early Precambrian ocean. Spheres surrounded by an organic membrane have only a temporal stability. They may rupture and eject their cell content; alternately, lysis of the cell wall may take place along the line of a `viral infection' (Fiddes, 1977). Recombination will proceed and a transfer of metabolic, catalytic, or genetic material from one cell to another is accomplished. This is the most vital part of chemical evolution because it relaxes evolutionary constraints required for the final assemblage of the primordial cell.

The minimum requirements for a chemical system evolving towards the first living cell are summarized in Figure 2.7. Two main categories are apparent: (i) structural organization, and (ii) functional organization. The role of physics and chemistry is reflected in (i) and that of biology and physiology in (ii). Two feedback control systems govern the functional organization, i.e. metabolism and genetic translation apparatus, both of which are well integrated.

This scheme summarizes the concepts which we regard as essential for the origin and maintenance of life (Matheja and Degens, 1971; Degens, 1977). The phenomenon of `aperiodic crystals' and their significance in the structure of macromolecules have been pointed out by Schrödinger (1945). The importance of hydrogen bonds in biopolymers has been shown by Pauling et al. (1951). The significance of phosphate/metal ion coordination for structure and function in cellular systems has been treated comprehensively by Matheja and Degens (1971). The three aforementioned factors combine to form the molecular-structural basis on which life is built.

2.9 SYNOPSIS

Three models one cosmic, one atmospheric, and one of Earth are presented on the chemical evolution of organic matter. They are tested for their rationale using three criteria: (i) thermodynamic feasibility, (ii) reaction yield, and (iii) potential to generate cellular systems.

Cosmic synthesis will allow for a large-scale generation of the principal monomeric building blocks of life. However, due to the general conditions of the space environment, for instance, temperature, lack of fluid water, etc., the formation of cellular systems is impossible.

Synthesis of organic matter in the primordial Earth atmosphere is thermodynamically feasible only if the system incorporates the ocean as a sink. Conditions (ii) and (iii) are not met.

Figure 2.8 Composite evolutionary tree (schematic) summarizing the principal steps in chemical and biological evolution. The sequence of events depicted for chemical evolution follows from the discussion in the text; the biological scheme is based on data by Schwartz and Dayhoff (1978). The upward progression from anaerobic to facultative to aerobic forms is indicated in the shading pattern. Mitochondrial and chloroplast invasions are roughly drawn between points of suggested origin and uptake, respectively 

Differentiation processes associated with the formation of the planet Earth fulfil all three criteria needed for the synthesis of the first living cell. Organic compounds generated during the cosmic state in large interstellar clouds were mobilized as temperature increased within the protoplanet. All gaseous carbon compounds escaped together with hydrogen and helium as the first atmosphere left the protoplanet. Further degassing of the solid earth generated a new atmosphere and hydrosphere which were retained.

In the course of differentiation, carbon associated with iron and nickel droplets moved towards the Earth's centre where it either remained in the melt as dissolved carbon, or as graphite in the solid Fe-Ni core. In the upper, cooler regions of the protoplanet, carbon compounds moved upward and were released to the surface environment. In the course of this rise, minerals could have acted as templates and catalysts and polymeric building blocks such as peptides, polysaccharides, lipids, and the nucleic acids came into existence. Following discharge into the primordial hydrosphere and atmosphere, the gaseous compounds, principally CO, CO₂, and CH₄, accumulated in the atmosphere, while the rest split into water-soluble and water-insoluble materials. A protobiosphere was generated, which may have served as a substrate for the generation of the first living cell.

The most decisive step in the generation of life is the synthesis of a capsid to house the first cell. Gas bubbles, by virtue of an active phase boundary to the water can concentrate and structure a series of organic molecules, metal ions, and even virus particles (Baylor et al., 1977). When bubbles burst, these compounds are jetted into the air (aerosol), where they are picked up by the wind and carried for enormous distances as sea spray. Other organic-coated bubbles can either merge, or become stabilized by metal ion coordination, or upon rupture release the adsorbed material into the sea. This scenario offers infinite possibilities for the transfer of physiologically meaningful `bits and pieces' without invoking deus ex machina solutions. Yet, major impacts during terminal cataclysm, about 3.9 Ga (Giga anni= 10<sup>9</sup> years) ago (Wasserburg et al., 1977), are an attractive `machina' for the generation of catalysts such as Ca(OH)₂, bubbles, and organics (Degens, 1974).

The protobiosphere became gradually depleted through: (i) biogenic consumption of organic matter, (ii) incorporation into sediment, and (iii) general oxidation processes. The resulting CO₂ was picked up in the atmosphere, ocean and limestones, and the reduced C-H-N-O-P-S compounds entered the rock formation principally as dispersed organic matter. This implies that in the absence of a biosphere and hydrosphere most of the protobiosphere would have ended up as volatile carbon compounds in an atmosphere resembling that of Venus.

To illustrate the direction of chemical and biological evolution, data of the present work is combined with a perspective view derived from protein and nucleic acid sequence data (Schwartz and Dayhoff, 1978) in the form of a `composite evolutionary tree' (Figure 2.8). Most significant are the findings that: (i) anaerobic bacterial photosynthesis is quite ancestral, and (ii) aerobic respiration preceded oxygen-releasing photosynthesis, which implies that photodissociated oxygen was a critical molecule in early biological evolution (Towe, 1978).

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