Early Earth Environments Research Articles

Darwinian behavior in a cold, sporadically fed pool of ribonucleotides.

A testable, explicit origin for Darwinian behavior, feasible on a chaotic early Earth, would aid origins discussion. Here I show that a pool receiving unreliable supplies of unstable ribonucleotide precursors can recurrently fill this role. By using numerical integration, the differential equations governing a sporadically fed pool are solved, yielding quantitative constraints for the proliferation of molecules that also have a chemical phenotype. For example, templated triphosphate nucleotide joining is >10(4) too slow, suggesting that a group more reactive than pyrophosphate activated primordial nucleotides. However, measured literature rates are sufficient if the Initial Darwinian Ancestor (IDA) resembles a 5'-5' cofactor-like dinucleotide RNA, synthesized via activation with a phosphorimidazolide-like group. A sporadically fed pool offers unforeseen advantages; for example, the pool hosts a novel replicator which is predominantly unpaired, even though it replicates. Such free template is optimized for effective selection during its replication. Pool nucleotides are also subject to a broadly based selection that impels the population toward replication, effective selection, and Darwinian behavior. Such a primordial pool may have left detectable modern traces. A sporadically fed ribonucleotide pool also fits a recognizable early Earth environment, has recognizable modern descendants, and suits the early shape of the phylogenetic tree of Earthly life. Finally, analysis points to particular data now needed to refine the hypothesis. Accordingly, a kinetically explicit chemical hypothesis for a terran IDA can be justified, and informative experiments seem readily accessible.

Structural phylogenomics uncovers the early and concurrent origins of cysteine biosynthesis and iron-sulfur proteins

1Cysteine (Cys) has unique chemical properties of catalysis, metal chelation, and protein stabilization. While Cys biosynthesis is assumed to be very ancient, the actual time of origin of these metabolic pathways remains unknown. Here, we use the molecular clocks of protein folds and fold superfamilies to time the origin of Cys biosynthesis. We find that the tRNA-dependent biosynthetic pathway appeared ∼3.5 billion years ago while the tRNA-independent counterpart emerged ∼500 million years later. A deep analysis of the origins of Cys biosynthesis in the context of emerging biochemistry uncovers some intriguing features of the planetary environment of early Earth. Results suggest that iron-sulfur (Fe-S) proteins that use cysteinyl sulfur to bind iron atoms were not the first to arise in evolution. Instead, their origin coincides with the appearance of the first Cys biosynthetic pathway. It is therefore likely that Cys did not play an important role in the make up of primordial protein molecules and that Fe-S clusters were not part of active sites at the beginning of biological history. 1Current address: Biochemical Engineering Institute, Saarland University, Campus A 1.5, 66123 Saarbrücken, Germany

Microbial utilization of abiogenic carbon and hydrogen in a serpentinite-hosted system

Mantle rocks exposed on the seafloor constitute a highly reactive chemical and thermal system, in which interaction with seawater to produce serpentinite has major consequences for lithospheric cooling, global geochemical cycles, and microbial activity. Serpentinite-hosted hydrothermal activity is exemplified by the Lost City Hydrothermal Field (30°N, Mid-Atlantic Ridge) where fluid–rock reactions in the underlying ultramafic rocks result in high concentrations of abiotic hydrogen, methane, C2+ alkanes, and formate. Such systems have been proposed as possible analogs to the Early Earth environments that gave rise to the first biochemical pathways. Thus, characterizing the local microbial communities and their potential link with abiogenic compounds is of particular significance. Here we demonstrate that in active carbonate chimneys where microbial sulfate reduction is important, up to 50% of the microbial biomass is synthesized from mantle carbon. Conversely, mantle carbon contributes only ∼10% of the biomass in areas with minimal sulfate reduction. We attribute this difference to greater incorporation of formate or methane by the dominant microbial species, the Lost City Methanosarcinales, in locations where sulfate reducers are able to facilitate this assimilation. The ability of autotrophic communities at Lost City to capitalize on the steady stream of chemical products resulting from serpentinization reactions and to utilize abiogenic mantle carbon lend credence to the hypothesis that early biosynthetic pathways could have developed in similar environments.

Silicic magma petrogenesis in Iceland by remelting of hydrothermally altered crust based on oxygen isotope diversity and disequilibria between zircon and magma with implications for MORB

Terra Nova, 24, 227–232, 2012AbstractPetrogenesis of silicic magmas in Iceland has fundamental significance for understanding the relative importance of fractional crystallization of mantle‐derived basalt and partial melting of hydrothermally altered basaltic crust in formation of the earliest continental crust. First results of in situ oxygen isotope investigation of zircons in large‐volume silicic eruptive products of three volcanoes in Iceland (Askja, Torfajökull, and Hekla) demonstrate isotope diversity and disequilibria and long U–Th zircon pre‐eruptive residence of 103–104 year. This suggests that zircons did not grow from their host melts but instead were inherited from older magma batches and leftover cumulates with generally low and variable δ18O values. This study demonstrates that segregation of cubic kilometres of silicic magma is faster than mineral‐diffusive or recrystallization time‐scales (estimated at ∼103 years), and it suggests that partial melting of hydrothermally altered and oxidized oceanic crust is the mechanism that best explains silicic rocks in Iceland and early earth environments.

NATURAL HISTORY FIRST (BUT DON'T STOP THERE)

This quote from Agassiz welcomes visitors to the main entrance of the Ruthven Museums Building at the University of Michigan, beseeching them to collect their own data and draw their own conclusions. The sentiment has inspired generations of evolutionary biologists, who continue the long tradition of deductive science epitomized by Darwin and Wallace. The fundamental questions about evolution have always been borne from direct observation of, and curiosity about, nature. In the best cases, the process leads to elegant tests of hypotheses and formulations of general theories. No practitioners of this art of investigation have done it with more rigor, provided deeper insight, or better illuminated the aesthetic qualities of science, than Peter and Rosemary Grant. It is fitting, then, that the careers of the Grants (while showing no signs of ebbing toward a twilight!) should be celebrated by assembling a fine cadre of scientific naturalists to share their own observations and deductions. In Search of the Causes of Evolution is a collection of essays based on presentations at a symposium organized by Princeton University on the occasion of the Grants’ official “retirement.” The chapters are remarkably true to the theme that observation leads the way to detailed dissections of the underlying mechanisms of evolution. The author list represents some of the most significant contributors at the forefront of evolutionary research. Although readers will recognize most of the study systems (e.g., anoles, beach mice, dung beetles, sticklebacks even make two appearances), the authors generally have done an admirable job of resisting the temptation to rehash old work and have contributed new data or syntheses to the volume. The resultant book thereby serves as a low-altitude reconnaissance flight over some of the most exciting current research in evolutionary ecology. The ground covered by the essays is extensive, as it should be under a title purporting to encompass the causes of evolution. As anyone who has taught an introductory evolution course can attest, the scope of topics that we seek to explain is staggering. One person’s bread and butter might not even be included on another’s list. The Grants have at least attempted to span this space by including chapters across the spectrum from the evolution of early earth environments (Andrew Knoll and David Johnston) and macroevolutionary patterns of diversification (David Jablonski) to the search for “behavior genes” (Hopi Hoekstra) and the population genetics of island endemism (Trevor Price and colleagues). True to the tradition of the Grants’ own work, however, the majority of essays emphasizes the interplay of selection and genetics as drivers of biodiversity. This is a collection that is unapologetically adaptationist in its perspective on evolutionary mechanisms. So what do we learn about where the grander field of evolutionary biology is headed when we collect marquis names with license to write about whatever they want? First, evolutionary research is not dominated by traditional model systems. Indeed,

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 ~5h, but at higher and lower pH ranges the half-life decreased to ~1h. 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.

Evolution and possible storage of information in a magnetite system of significance for brain development

The initial evolutionary electromagnetic steps in the history of brain development are still unknown, although such knowledge might be of high relevance in understanding human degenerative diseases. All prokaryote organisms, one-celled or multicellular, must have an inherited system to process and store information activating instincts and reflexes, in order to give a quick response to external stimuli. We argue that magnetite is an obvious compound to be evaluated as an initial precursor from prebiotic Earth history in the evolution of such a system. Magnetite is a stable ferrimagnetic compound, present in organisms ranging from bacteria to humans. It occurred naturally in the early Earth environment and was later synthesized de novo in biotic organisms. We suggest that the use of magnetite has evolved to represent the main storage system for learned memory in all organisms living today.

Microfossils

Defining biosignatures, i.e. features that are indicative of past or present life, has been one of the major strategies developed over the last few years for the search of life on the early Earth and in the solar system. Current knowledge about microscopic remnants of fossil organisms, namely microfossils are reviewed, focusing on: (i) studies of recent environments used as analogues for the early Earth or extraterrestrial environments; (ii) examination of Precambrian rocks; and (iii) laboratory experiments simulating biotic and abiotic processes and resulting in the formation of genuine or pseudomicrofossils. Fossils’ preservation depends on environment and chemical composition of the primary structure, although they might undergo taphonomic processes that alter their morphology and/or composition. Altogether, these examples illustrate what can be potentially preserved during the very first stages of fossilization and what can be left in the geological record after diagenesis and metamorphism. Finally, this provides a rationale to tentatively define diagnosis criteria for microfossils or ways to look for life on Earth or in extraterrestrial environments.

Organics Produced by Irradiation of Frozen and Liquid HCN Solutions: Implications for Chemical Evolution Studies

Hydrogen cyanide (HCN), an important precursor of organic compounds, is widely present in extraterrestrial environments. HCN is also readily synthesized in prebiotic simulation experiments. To gain insight into the radiation chemistry of one of the most important and highly versatile constituents of cometary ices, we examined the behavior of over-irradiated frozen and liquid HCN solutions under ionizing radiation. The samples were exposed to gamma radiation at a dose range from 0 up to 419 kGy. Ultraviolet spectroscopy and gas chromatography were used to follow the process. The analyses confirmed that gamma-ray irradiation of liquid HCN solutions generates several organic products. Many of them are essential to life; we verified the presence of carboxylic acids (some of them members of the Krebs cycle) as well as free amino acids and urea. These are the first studies to reveal the presence of these compounds in experiments performed at low temperatures and bulk irradiation. Organic material was produced even at low temperatures and low radiation doses. This work strongly supports the presumption that, as a parent molecule, HCN played a central essential role in the process of chemical evolution on early Earth, comets, and other extraterrestrial environments.

Electrochemical studies of iron meteorites: phosphorus redox chemistry on the early Earth

AbstractThe mineral schreibersite, (Fe,Ni)3P, a ubiquitous component of iron meteorites, is known to undergo anoxic hydrolytic modification to afford a range of phosphorus oxyacids. H-phosphonic acid (H3PO3) is the principal hydrolytic product under hydrothermal conditions, as confirmed here by 31P-NMR spectroscopic studies on shavings of the Seymchan pallasite (Magadan, Russia, 1967), but in the presence of photochemical irradiation a more reduced derivative, H-phosphinic (H3PO2) acid, dominates. The significance of such lower oxidation state oxyacids of phosphorus to prebiotic chemistry upon the early Earth lies with the facts that such forms of phosphorus are considerably more soluble and chemically reactive than orthophosphate, the commonly found form of phosphorus on Earth, thus allowing nature a mechanism to circumvent the so-called Phosphate Problem.This paper describes the Galvanic corrosion of Fe3P, a hydrolytic modification pathway for schreibersite, leading again to H-phosphinic acid as the key P-containing product. We envisage this pathway to be highly significant within a meteoritic context as iron meteorites are polymetallic composites in which dissimilar metals, with different electrochemical potentials, are connected by an electrically conducting matrix. In the presence of a suitable electrolyte medium, i.e., salt water, galvanic corrosion can take place. In addition to model electrochemical studies, we also report the first application of the Kelvin technique to map surface potentials of a meteorite sample that allows the electrochemical differentiation of schreibersite inclusions within an Fe:Ni matrix. Such experiments, coupled with thermodynamic calculations, may allow us to better understand the chemical redox behaviour of meteoritic components with early Earth environments.

地球の海水組成と生命の進化４６億年

The redox state of the surface environment of early Earth is still controversial (e.g. Ohmoto, 1997). Many previous papers suggest that oxygen was free before 2.7 Ga, and then gradually increased due to oxygen-producing photosynthesis (e.g. Holland, 1999; Farquhar et al., 2000). But, a detailed and quantitative estimate is still lacking. It is well known that deposited carbonate minerals are equilibrated with ambient seawater in a microbial or abiotic environment. The composition and mineralogy allow us to estimate the physical and chemical properties of pale minerals with primary sedimentary structures in shallow and deep-sea deposits, in order to eliminate secondary carbonate and contamination of detrital materials, and to estimate the redox condition of seawater over time.We estimated the depositional environments from the field occurrence of coexisting basaltic lava and sedimentary rocks and the fabric of the carbonates themselves. The shallow marine deposits have included sedimentary carbonates with a stromatolite structure and elastic layers, and amygdaloidal and matrix carbonates of hot-spot basaltic lava since 3.5 Ga. Deep-sea carbonates have included interstitial carbonate minerals in a matrix of hyaloclastite and amygdaloidal carbonate minerals accompanied by MORB-type basalts since 3.5 Ga. In addition, we excavated at three localities in South China, and obtained the complete sequence from the Marinoan tillite to early Cambrian rocks. The carbonate rocks belong to shallow marine deposits.Deep-sea carbonates have only faint Ce and Eu anomalies between 3.5 and 1.9 Ga. The negative Ce anomaly of shallow carbonates has frequently deviated from those of deep-sea carbonate since 2.78 Ga. It fluctuated greatly, and was very large at 2.5 and 2.3 Ga. We calculated the oxygen activity of shallow and deep seawater respectively, based on Ce content and anomalies of carbonate minerals at given parameters of atmospheric carbon dioxide content (pCO2) and Ca. content of seawater. The results show that the oxygen content of the deep sea was low and constant until at least 1.9 Ga. The oxygen content of shallow seawater increased from 2.7 Ga, but fluctuated. In particular, it was at a minimum during and after Snowball Earth events. It became quite high at 2.5 and 2.3 Ga, but eventually increased after the Phanerozoic. We also calculated it under another condition of high pCO2 to show that the seawater was more oxic even in the Archean than at present. The calculation suggests a relatively low pCO2 through geologic time.

Clues from Fe Isotope Variations on the Origin of Early Archean BIFs from Greenland

Archean rocks may provide a record of early Earth environments. However, such rocks have often been metamorphosed by high pressure and temperature, which can overprint the signatures of their original formation. Here, we show that the early Archean banded rocks from Isua, Akilia, and Innersuartuut, Greenland, are enriched in heavy iron isotopes by 0.1 to 0.5 per mil per atomic mass unit relative to igneous rocks worldwide. The observed enrichments are compatible with the transport, oxidation, and subsequent precipitation of ferrous iron emanating from hydrothermal vents and thus suggest that the original rocks were banded iron formations (BIFs). These variations therefore support a sedimentary origin for the Akilia banded rocks, which represent one of the oldest known occurrences of water-laid deposits on Earth.

Low marine sulphate and protracted oxygenation of the Proterozoic biosphere.

Progressive oxygenation of the Earth's early biosphere is thought to have resulted in increased sulphide oxidation during continental weathering, leading to a corresponding increase in marine sulphate concentration. Accurate reconstruction of marine sulphate reservoir size is therefore important for interpreting the oxygenation history of early Earth environments. Few data, however, specifically constrain how sulphate concentrations may have changed during the Proterozoic era (2.5-0.54 Gyr ago). Prior to 2.2 Gyr ago, when oxygen began to accumulate in the Earth's atmosphere, sulphate concentrations are inferred to have been <1 mM and possibly <200 microM, on the basis of limited isotopic variability preserved in sedimentary sulphides and experimental data showing suppressed isotopic fractionation at extremely low sulphate concentrations. By 0.8 Gyr ago, oxygen and thus sulphate levels may have risen significantly. Here we report large stratigraphic variations in the sulphur isotope composition of marine carbonate-associated sulphate, and use a rate-dependent model for sulphur isotope change that allows us to track changes in marine sulphate concentrations throughout the Proterozoic. Our calculations indicate sulphate levels between 1.5 and 4.5 mM, or 5-15 per cent of modern values, for more than 1 Gyr after initial oxygenation of the Earth's biosphere. Persistence of low oceanic sulphate demonstrates the protracted nature of Earth's oxygenation. It links biospheric evolution to temporal patterns in the depositional behaviour of marine iron- and sulphur-bearing minerals, biological cycling of redox-sensitive elements and availability of trace metals essential to eukaryotic development.

A postulate to assess ‘habitability’

One principal challenge in biology is defining a postulate by which the habitability of other planets can be assessed. Current assessments suffer from two potential weaknesses. With respect to other planets, either assumptions are made about the physical and chemical conditions of environments that err on the side of biological optimism without empirical constraint by spacecraft observations or novel physiologies of microorganisms are invented to fit extraterrestrial environmental conditions with no demonstrated microbiological counterparts on Earth. Attempts to assess the habitability of the early Earth suffer from similar problems. We discuss the following postulate: ‘the proposition that a planet is or was habitable requires that the physiological requirements of microorganisms on Earth known at the time of assessment match the empirically determined combined physical and chemical conditions in the extraterrestrial or early Earth environment being assessed’ as a means of evaluating ‘habitability’. We use as tests for our postulate the early Earth and the cloud deck of Venus (a habitat that has been a source of optimistic debate for forty years). We conclude that, although the early Earth was habitable, Venus is a dead world.

Formation, Chemistry and Fertility of Extraterrestrial Soils: Cohesion, Water Adsorption and Surface Area of Carbonaceous Chondrite. Prebiotic and Space Resource Applications

Following microbial and plant responses to Murchison CM2 meteorite nutrients, further soil fertility parameters are examined. Cohesion of the matrix is tested by dissolving in acidic disaggregation agents, 0.4 M CH3COOH, 10% HNO3, H2SO4(pH 3), 50%H2O2+H2SO4(pH 3), and saturated CO2solution. The responses suggest that carbonates and the organic polymer contribute as cementing agents, and that enhanced disaggregation by H2O2and diminished disaggregation in saturated CO2solutions may contribute to the weathering of carbonaceous meteorites in martian or early Earth environments, respectively. The cation exchange capacities (CEC) of the solid (7.2±0.8 meq/100 g) and powdered Murchison (7.8±1.5 meq/100 g) are comparable. The cation exchange capacity is not reduced by oxidation or acetylation, suggesting that the binding sites of exchangeable cations are mostly inorganic. The CEC correlates with water vapor adsorption similar to that for terrestrial soils. For example, at 20°C andP(H2O)=10.8 mbars, 17 mg/g H2O is adsorbed on Murchison (CEC=7.2 meq/100 g) and only 0.6 mg/g on Allende (CEC=0.4 meq/100 g). Under these conditions 13 H2O molecules are adsorbed per surface cation in Murchison and 8 H2O molecules/cation in Allende, similar to 9 H2O molecules/cation in terrestrial soils and in montmorillonite. Adsorption isotherms on the Murchison are convex at low pressures, indicating strong bonding of the first water molecules, and concave at high pressures, indicating a broad pore size distribution. Isotherms at 278, 293, and 311 K yield isosteric heat of adsorption of 56.5±2.5 kJ/mol at 4–8 H2Oadsorbedmolecules/cation, similar to that for montmorillonite. The isotherms yield a specific surface area of Sw=37×103m2/kg for Murchison, larger than the 19×103m2/kg of pure serpentine, suggesting contributions, in addition to the main serpentine-like phyllosilicates, by components equivalent to a 9–12% smectic clay-like content. Water evaporation curves from meteorite surfaces display the multistage behavior typical of soils, with slow rates for the evaporation of surface-adsorbed water. The gas adsorption data allow an assessment of gas-adsorbed water equilibrium in the solar nebula and suggest possible self-catalytic effects in adsorption/aqueous alteration processes. The observed fertility indicators and biological effects support the potential of carbonaceous chondrites to support early biological processes and as soils for space-based agriculture.

Possible complex organic compounds on Mars

It is suggested that primitive Mars had somehow similar environments as primitive Earth. If life was born on the primitive earth using organic compounds which were produced from the early Earth environment, the same types of organic compounds were also formed on primitive Mars. Such organic compounds might have been preserved on Mars still now. We are studying possible organic formation on primitive and present Mars. A gaseous mixture of CO 2, CO, N 2 and H 2O with various mixing ratios were irradiated with high energy protons (major components of cosmic rays). Hydrogen cyanide and formaldehyde were detected among volatile products, and yellow-brown-colored water-soluble non-volatile substances were produced, which gave amino acids after acid-hydrolysis. Major part of “amino acid precursors” were not simple molecules like aminonitriles, but complex compounds which eluted earlier than free amino acids in cation-exchange HPLC. These organic compounds should be major targets in the future Mars mission. Strategy for the detection of the complex organics on Mars will be discussed.

The seeding of life by comets

The evidence that living organisms were already extant on the earth almost 4 Gyr ago and that early bombardment by comets and asteroids created a hostile environment up to about this time has revived the question of how it was possible for prebiotic chemical evolution to have provided the necessary ingredients for life to have developed in the short intervening time. The actual bracketed available temporal space is no more than 0.5 Gyr and probably much less. Was this sufficient time for an earth-based source of the first simple organic precursor molecules to have led to the level of the prokaryotic cell? If not, then the difficulty would be resolved if the ancient earth was impregnated by organic molecular seed from outer space. Curiously, it seems that the most likely source of such seeds was the same a one of the sources of the hostile enviroment, namely the comets which bombarded the earth. With the knowledge of comets gained by the space missions it has become clear that a very large fraction of the chemical composition of comet nuclei consists of quite complex organic molecules. Furthermore it has been demonstrated that comets consist of very fluffy aggregates of interstellar dust whose chemistry derives from photoprocessing of simple ice mixtures in space. Thus, the ultimate source of organics in comets comes from the chemical evolution of interstellar dust. An important and critical justification for assuming that interstellar dust is the ultimate source of prebiotic molecular insertion on the earth is the proof that comets are extremely fluffy aggregates, which have the possibility of breaking up into finely divided fragments when the comet impacts the earth's atmosphere. In the following we will summarize the properties of interstellar dust and the chemical and morphological structure of comets indicated by the most recent interpretations of comet observations. It will be shown that the suitable condition for comets having provided abundant prebiotic molecules as well as the water in which they could have further evolved are consistent with theories of the early earth environment.

Origin of life-facing up to the physical setting

The purpose of this article is to review what can be inferred about the environment in which life arose. Lines of interfence stem from two types of records. A physical record of geological and astronomical data provides information on the character of Earth's surface during the epoch in which life arose. A biological record of quantitative evolutionnary relationships among modern organisms documents early Earth's environment because it points to some properties of the first cells.

Habitability of the early earth: Clues from the physiology of nitrogen fixation and photosynthesis

In the absence of direct evidence concerning the nature of the early Earth environments, it is acceptable under the uniformitarian principle to attempt to define primitive habitats from modern procaryotic physiology. Combining the rock and fossil record with present phylogenetic reconstuctions, application of this paleoecological approach to the evolutionary biochemistry and physiology of nitrogen fixation and photosynthesis leads to several inferences about the nature of Archean environments: 1. To stimulate nitrogenase evolution and avoid its repression, the activity of the NH 4 + ion was less than 10−3, and probably lower. 2. To be consistent with a moderately protective ozone screen, while not also repressing nitrogenase activity, incursions of abiotic dissolved oxygen at levels in the range 10−1.2−10−3.5 PAL would have been acceptable. 3. To induce the formation and activity of RuBP carboxylase, the pCO2 was less than 100 PAL. 4. To support Photosystem I activity, sulfide concentrations of at least 10−4 M were present in the photic zone. 5. To avoid a too-rapid oxidation of sulfide, the pH was probably between 6–7, where H2S exceeds HS−.