Chemical Evolution and Meteorites: An Update

Carbonaceous chondrites are a primitive group of meteorites, which contain abundant organic material and provide a unique natural record of prebiotic chemical evolution. This material comprises a varied suite of soluble organic compounds that are similar, sometimes identical, to those found in the biosphere, such as amino acids, carboxylic acids, and sugar derivatives. Some amino acids of this suite also show L-enantiomeric excesses, and suggest the possibility they may have contributed to terrestrial homochirality by direct input of meteoritic material to the early Earth. This optical activity appears to be limited to the subgroup of alpha-methyl amino acids which, although not common in the extant biosphere, would have been well suited to provide the early earth with both enantiomeric excesses and means for their amplification by subsequent chemical evolution. We can also envision this exogenous delivery of carbonaceous material by meteorites and comets as having coincided with the endogenous formation of prebiotic precursors and influenced their evolution by complementary reactions or catalysis.     

The Organic Composition of Carbonaceous Meteorites: The Evolutionary Story Ahead of Biochemistry

Carbon-containing meteorites provide a natural sample of the extraterrestrial organic chemistry that occurred in the solar system ahead of life''s origin on the Earth. Analyses of 40 years have shown the organic content of these meteorites to be materials as diverse as kerogen-like macromolecules and simpler soluble compounds such as amino acids and polyols. Many meteoritic molecules have identical counterpart in the biosphere and, in a primitive group of meteorites, represent the majority of their carbon. Most of the compounds in meteorites have isotopic compositions that date their formation to presolar environments and reveal a long and active cosmochemical evolution of the biogenic elements. Whether this evolution resumed on the Earth to foster biogenesis after exogenous delivery of meteoritic and cometary materials is not known, yet, the selective abundance of biomolecule precursors evident in some cosmic environments and the unique L-asymmetry of some meteoritic amino acids are suggestive of their possible contribution to terrestrial molecular evolution.     

The origin of the biologically coded amino acids

Biology uses essentially 20 amino acids for its coded protein enzymes, representing a very small subset of the structurally possible set. Most models of the origin of life suggest organisms developed from environmentally available organic compounds. A variety of amino acids are easily produced under conditions which were believed to have existed on the primitive Earth or in the early solar nebula. The types of amino acids produced depend on the conditions which prevailed at the time of synthesis, which remain controversial. The selection of the biological set is likely due to chemical and early biological evolution acting on the environmentally available compounds based on their chemical properties. Once life arose, selection would have proceeded based on the functional utility of amino acids coupled with their accessibility by primitive metabolism and their compatibility with other biochemical processes. Some possible mechanisms by which the modern set of 20 amino acids was selected starting from prebiotic chemistry are discussed.     

Origin of organic compounds on the primitive earth and in meteorites

The role and relative contributions of different forms of energy to the synthesis of amino acids and other organic compounds on the primitive earth, in the parent bodies or carbonaceous chondrites, and in the solar nebula are examined. A single source of energy or a single process would not account for all the organic compounds synthesized in the solar system. Electric discharges appear to produce amino acids more efficiently than other sources of energy and the composition of the synthesized amino acids is qualitatively similar to those found in the Murchison meteorite. Ultraviolet light is also likely to have played a major role in prebiotic synthesis. Although the energy in the sun's spectrum that can be absorbed by the major constituents of the primitive atmosphere is not large, reactive trace components such as H2S and formaldehyde absorb at longer wavelengths where greater amounts of energy are available and produce amino acids by reactions involving hot hydrogen atoms. The thermal reaction of CO + H2 + NH3 on Fischer-Tropsch catalysts generates intermediates that lead to amino acids and other organic compounds that have been found in meteorites. However, this synthesis appears to be less efficient than electric discharges and to require a special set of reaction conditions. It should be emphasized that after the reactive organic intermediates are generated by the above processes, the subsequent reactions which produce the more complete biochemical compounds are low temperature homogenous reactions occurring in an aqueous environment.     

Natural evidence for chemical and early biological evolution

Meteorites, particularly type II carbonaceous chondrites, provide natural, tangible evidence for chemical evolution, but they do not appear to contain any evidence for biological evolution. On the other hand, some of the oldest sedimentary rocks of the earth have yielded good evidence for early biological evolution; whatever evidence there may be for chemical evolution in these old rocks is generally obscure. Carbonaceous chondrites (types I, II, and III) have been examined for thier content of various kinds of organic compounds. Amino acids have been reported to be present in the three types, but only in type II carbonaceous chondrites (Murray and Murchison) has an indigenous suite of amino acids been found which is apparently free of most terrestrial contaminations. These indigenous compounds are thought to have resulted from extraterrestrial, abiotic, chemical syntheses, and the presence of the amino acids in meteorites provides strong support for the theory of chemical evolution. The geological record of the Swaziland Sequence and Bulawayan System of Southern Africa contains morphological and chemical fossils which indicate that early biological evolution was taking place at least 3.0 to 3.3 aeons ago. Interpretation of the significance of the chemical fossil record has proven to be difficult. At present the occurrence of simple compounds in these very ancient rocks is believed to have little or nothing to do with biochemical processes three aeons ago. The bulk of the reduced carbonaceous material in these rocks, however, probably represents the residue of three billion years old and older organic matter. Isotopic studies of this carbonaceous material may provide chemical evidence for early biological evolution.     

The chemistry that preceded life's origin: a study guide from meteorites

Carbonaceous meteorites are rare fragments of asteroids that contain organic carbon of diverse composition, various complexity, and whose lineage can in several instances be traced back to pre-solar environments. Their analyses offer a unique glimpse into the chemistry of the solar system that preceded life and may have been available to its emergence on the early Earth. While the heterogeneity of the organic materials of meteorites is indicative of random synthetic processes for their formation, some of their components have identical counterparts in the biosphere, and a group of meteoritic amino acids were found to display chiral asymmetry, a property known since the time of Pasteur to be inextricably linked to life's processes. The ability of these amino acids to act as asymmetric catalysts, as well as indications that molecular asymmetry in meteorites may not be limited to these compounds, encourage the suggestion of possible involvement of meteoritic material in the induction of selective traits in molecular evolution.     

prebiotic phosphorus chemistry reconsidered

The available evidence indicates that the origin of life on Earth certainly occurred earlier than 3.5 billion years ago and perhaps substantially earlier. The time available for the chemical evolution which must have preceded this event is more difficult to estimate. Both endogenic and exogenic contributions to chemical evolution have been considered; i.e., from chemical reactions in a primitive atmosphere, or by introduction in the interiors of comets and/or meteorites. It is argued, however, that the phosphorus chemistry of Earth's earliest hydrosphere, whether primarily exogenic or endogenic in origin, was most likely dominated by compounds less oxidized than phosphoric acid and its esters. A scenario is presented for the early production of a suite of reactive phosphonic acid derivatives, the properties of which may have foreshadowed the later appearance of biophosphates.     

Trace elements in chemical evolution

Electric discharge experiments have been performed in a plausible primitive earth atmosphere consisting of methane, nitrogen, and water over an aqueous phase of an ammonia-ammonium buffer solution. In some experiments, ions of metal elements, calcium, magnesium, zinc, iron and molybdenum were introduced. Gas phase products and amino acids in the liquid phase were analyzed by gas chromatography. With trace metal ions, less organic compounds in the gas phase and larger amounts of amino acids were obtained than without them. The results have shown the possible importance of trace elements in chemical evolution and the origin of life on the earth.     

Life on Mars: chemical arguments and clues from Martian meteorites

Primitive terrestrial life – defined as a chemical system able to transfer its molecular information via self-replication and to evolve – probably originated from the evolution of reduced organic molecules in liquid water. Several sources have been proposed for the prebiotic organic molecules: terrestrial primitive atmosphere (methane or carbon dioxide), deep-sea hydrothermal systems, and extraterrestrial meteoritic and cometary dust grains. The study of carbonaceous chondrites, which contain up to 5% by weight of organic matter, has allowed close examination of the delivery of extraterrestrial organic material. Eight proteinaceous amino acids have been identified in the Murchison meteorite among more than 70 amino acids. Engel reported that l-alanine was surprisingly more abundant than d-alanine in the Murchison meteorite. Cronin also found excesses of l-enantiomers for nonprotein amino acids. A large collection of micrometeorites has been recently extracted from Antarctic old blue ice. In the 50- to 100-μm size range, carbonaceous micrometeorites represent 80% of the samples and contain 2% of carbon, on average. They might have brought more carbon than that involved in the present surficial biomass. The early histories of Mars and Earth clearly show similarities. Liquid water was once stable on the surface of Mars, attesting the presence of an atmosphere capable of deccelerating C-rich micrometeorites. Therefore, primitive life may have developed on Mars as well and fossilized microorganisms may still be present in the near subsurface. The Viking missions to Mars in 1976 did not find evidence of either contemporary or past life, but the mass spectrometer on the lander aeroshell determined the atmospheric composition, which has allowed a family of meteorites to be identified as Martian. Although these samples are essentially volcanic in origin, it has been recognized that some of them contain carbonate inclusions and even veins that have a carbon isotopic composition indicative of an origin from Martian atmospheric carbon dioxide. The oxygen isotopic composition of these carbonate deposits allows calculation of the temperature regime existing during formation from a fluid that dissolved the carbon dioxide. As the composition of the fluid is unknown, only a temperature range can be estimated, but this falls between 0° and 90°C, which would seem entirely appropriate for life processes. It was such carbonate veins that were found to host putative microfossils. Irrespective of the existence of features that could be considered to be fossils, carbonate-rich portions of Martian meteorites tend to have material, at more than 1000 ppm, that combusts at a low temperature; i.e., it is an organic form of carbon. Unfortunately, this organic matter does not have a diagnostic isotopic signature so it cannot be unambiguously said to be indigenous to the samples. However, many circumstantial arguments can be made to the effect that it is cogenetic with the carbonate and hence Martian. If it could be proved that the organic matter was preterrestrial, then the isotopic fractionation between it and the carbon is in the right sense for a biological origin. Received: January 22, 1998 / Accepted: February 16, 1998     

Formation of prebiochemical compounds in models of the primitive Earth's atmosphere

In order to understand the formation of organic compounds in the primitive atmosphere, the first steps of evolution in models of the primitive atmosphere were investigated. Mixtures containing C−H−N elements were subjected to a low pressure silent electric discharge for several seconds, and the resulting effluents were analysed mainly by gas chromatography, infrared spectrometry and chemical analysis. The formation of hydrocarbons (i.e. ethylene, acetylene, methylacetylene) and of nitrogen containing compounds (i.e. hydrogen cyanide, cyanogen, saturated nitriles, acrylonitrile, cyanoacetylene) is reported. The influence of the initial mixture composition on the amount of compounds formed was systematically studied. The nature of the nitrogen source (N₂ or NH₃) in the primitive atmosphere has a great influence on the amount and on the very nature of the synthesized products. It is shown that important precursors such as cyanogen and cyanoacetylene are formed only in very rich N₂ mediums. There results show the important role played by the nature of the primitive atmosphere in the determination of the chemical evolution pathways.     

Delivery of Complex Organic Compounds from Evolved Stars to the Solar System

Stars in the late stages of evolution are able to synthesize complex organic compounds with aromatic and aliphatic structures over very short time scales. These compounds are ejected into the interstellar medium and distributed throughout the Galaxy. The structures of these compounds are similar to the insoluble organic matter found in meteorites. In this paper, we discuss to what extent stellar organics has enriched the primordial Solar System and possibly the early Earth.     

Mineral interface in extreme habitats: a niche for primitive molecular evolution for the appearance of different forms of life on earth

Innumerable primitive membrane and protocell models in latter stages of chemical evolution are based on the properties of minerals' interfaces with primitive seawater. The ordering mechanism induced by mineral interfaces has been the basis of several prebiotic models of molecular complexification and compartmentalization towards the appearance and evolution of different forms of life. Since mineral-aqueous media interfaces have been considered as initial stages of prebiotic models dealing with the formation of energy-transducing systems, the interface formed by pyrite in the presence of artificial primitive seawater was chosen to show the functional richness of this special niche. Interfaces--especially sulphide interfaces--were proposed as suitable niches for a two-carbon extant metabolism, synthesis and polymerization of nucleotides--to form ancient RNA strands--and assembly of amino acids synthesized in its vicinity. Accumulation of precursors at sulphide interfaces could have avoided their dilution into the Hadean seas and provided a suitable geochemical environment for a variety of molecular interactions. In this essay, we present a short review of the proposed roles of mineral interfaces in chemical evolution towards the appearance of primitive membranes, which might have been relevant for the advent of cellular life before its divergent evolution and differentiation. This survey covers several previous studies on the early cycles of energy conservation and of the formation of molecules carrying genetic information.     

Amphiphilic components of the murchison carbonaceous chondrite: Surface properties and membrane formation

We have investigated physicochemical properties of amphiphilic compounds in carbonaceous meteorites. The primary aim was to determine whether such materials represent plausible sources of lipid-like compounds that could have been involved as membrane components in primitive cells. Samples of the Murchison CM2 chondrite were extracted with chloroform-methanol, and the chloroform-soluble material was separated by two-dimensional thin layer chromatography. Fluorescnece, iodine stains and charring were used to identify major components on the plates. These were than scraped and eluted as specific fractions which were investigated by fluorescence and absorption spectra, surface chemical methods, gas chromatography-mass spectrometry, and electron microscopy. Fraction 5 was strongly fluorescent, and contained pyrene and fluoranthene, the major polycyclic aromatic hydrocarbons of the Murchison chondrite. This fraction was also present in extracts from the Murray and Mighei CM2 chondrites. Fraction 3 was surface active, forming apparent monomolecular films at air-water interfaces. Surface force measurements suggested that fraction 3 contained acidic groups. Fraction 1 was also surface active, and certain components could self-assemble into membranous vesicles which encapsulated polar solutes. The observations reported here demonstrate that organic compounds plausibly available on the primitive Earth through meteoritic infall are surface active, and have the ability to self-assemble into membranes.     

Pyrolysis experiment of simulated exogenous complex organics synthesized from the gas mixtures of CO, NH3, and H2O by 3 MeV proton irradiation.

High molecular weight organic matter synthesized from mixtures of carbon monoxide, ammonia and water gases similar to those found in the interstellar medium were irradiated with a 3 MeV proton beam and analyzed by Curie point pyrolysis with detection by gas chromatograph and mass spectrometer (Pyr-GC-MS). A wide variety of organic compounds, not only a number of amide compounds, but also heterocyclic and polycyclic aromatic hydrocarbons (PAHs), were detected among the products of the pyrolysis. The present data shows that primary and primitive organic matter serving as precursors to bioorganic compounds such as amino acids, nucleic acid bases and sugar might have been formed in a gaseous mixture of similar composition to that of the interstellar dust environment.     

早期地球的环境变化和生命的化学进化

生命起源是当代最大的科学疑谜之一,也是历来人类普遍关注的一个焦点。在地球上最早的生物出现之前,有机物经历了漫长而复杂的化学进化过程,称为生命的化学进化。地球上生命的化学进化与非生物部分的早期演化过程,是密切地相互关联、相互作用并相互制约的。文章着重阐述与生命的化学关系最为密切的冥古宙和太古宙的地球演化历史,指出这两个阶段所形成的还原性原始大气和古海洋条件在生命的化学进行中起了极其重要的作用,并且从宇宙形成、太阳系演化和地球环境早期演化的角度,探讨地球生命的化学进化历程;以地球形成初期发生了一系列复杂的有机化学反应过程,由无机分子生成生物小分子,再进一步生成生物大分子,直至最后产生原始细胞。此外,文章评述当前国际上最流行的生命化学进化学说,对早期地球的化学进化是发生在地球表面的原始海洋、粘土矿物、火山喷发等,或是来源于地球之外的宇宙空间进行了综合的阐述。     

Primary Sources of Phosphorus and Phosphates in Chemical Evolution

In this work we consider the role of phosphorus in chemical evolution from an interdisciplinary approach. First we briefly review the presence of this element in different cosmic sites, such as massive stellar cores, circumstellar and interstellar clouds, meteorites, lunar and Martian samples, interplanetary dust particles, cometary dust and planetary atmospheres. Thus we illustrate the fact that phosphorus seems to be, at the same time, scarce and ubiquitous in the solar system. Afterwards, by comparing the phosphorus content of our planet's main reservoirs with the amount of cometary and meteoritic matter captured by the primitive Earth, we conclude that comets may have provided a primary source for phosphorus compounds of prebiotic interest. Finally, we make a number of proposals aimed to gain observational supporting evidence to the above conclusion and other suggestions made in the article.     

Chemical markers of prebiotic chemistry in hydrothermal systems.

The goal of this chapter is to suggest some organic compounds which may be indicative of prebiotic processes in hydrothermal systems or laboratory simulations of them. While the exact processes which led to the origins of life are not known, studies of life's origins of the past forty years have uncovered a plethora of potential precursor molecules. Some of these same molecules were probably present in hydrothermal systems if chemical processes there had a role in the origins of life. The types of molecules formed in primitive Earth simulation experiments and observed in the interstellar medium, on comets and meteorites will be reviewed in Section 2 of this chapter. Some reactions involving these molecules which may have been important in prebiotic syntheses will be outlined. Since near- to supercritical water is found in hydrothermal systems, its properties and aspects of organic chemistry in supercritical water at high temperature and pressure will be discussed in Section 3. Fischer-Tropsch type (FTT) reactions, which are a potential source of the building blocks of biological molecules in hydrothermal systems, are discussed in Section 4. In the concluding section, Section 5, the possible formation in hydrothermal systems of organic molecules that are believed to have been important for the origins of life is discussed.     

生命起源的历程始于何处?--从化学进化到生物进化的探索

生命起源开始于从化学进化进入生物进化．然而,在原始细胞出现之前,已存在一个有蛋白质、核酸等生命要素的过渡期．那么,生命起源的历程究竞从何处开始?对这个未解之迹,进行了讨论．首先,从化学进化到生物进化要实现3个相变,即：从随机的有机化学反应相变为定向代谢途径;从消旋体环境相变为生物手性环境：从化学混沌状态相变为生命耗散结构．通过比较分析发现,由于前生命化学时期出现了丙酮酸．它的独特性质导致了这3种相变同步发生,其中起驱动作用的是定向代谢途径．它起源于以丙酮酸为基质的逆向糖酵解(糖生成途径)．     

The origin of life on Earth

Terrestrial life can be schematically described as organic molecules organized in liquid water. According to Oparin's hypothesis, organic building blocks required for early life were produced from simple organic molecules formed in a primitive reducing atmosphere. Precursors of lipids, nucleic acids and enzymes obtained in the laboratory under simulating conditions are reviewed. Geochemists favor now a less reducing atmosphere dominated by carbon dioxide. In such an atmosphere, very few building blocks are formed under prebiotic conditions. Import of extraterrestrial organic molecules may represent an alternative supply. Experimental support for such an alternative scenario is examined in comets, cosmic dust, meteorites and micrometeorites. Even the prebiotic broth receives today severe criticism for being implausible. In contrast to the classical scenario, a chemoautotrophic origin of life is discussed. Finally, interesting information related to early terrestrial life may be gained from Mars exploration.     

Catalytic effects of Murchison Material: Prebiotic Synthesis and Degradation of RNA Precursors

Mineral components of the Murchison meteorite were investigated in terms of potential catalytic effects on synthetic and hydrolytic reactions related to ribonucleic acid. We found that the mineral surfaces catalyzed condensation reactions of formamide to form carboxylic acids, amino acids, nucleobases and sugar precursors. These results suggest that formamide condensation reactions in the parent bodies of carbonaceous meteorites could give rise to multiple organic compounds thought to be required for the emergence of life. Previous studies have demonstrated similar catalytic effects for mineral assemblies likely to have been present in the early Earth environment. The minerals had little or no effect in promoting hydrolysis of RNA (24mer of polyadenylic acid) at 80°C over a pH range from 4.2 to 9.3. RNA was most stable in the neutral pH range, with a half-life ~5 h, but at higher and lower pH ranges the half-life decreased to ~1 h. These results suggest that if RNA was somehow incorporated into a primitive form of RNA-based thermophilic life, either it must be protected from random hydrolytic events, or the rate of synthesis must exceed the rate of hydrolysis.