Serpentinization as a source of energy at the origin of life

A methylotrophic origin of methanogenesis and early divergence of anaerobic multicarbon alkane metabolism

Methanogens are considered as one of the earliest life forms on Earth, and together with anaerobic methane-oxidizing archaea, they have crucial effects on climate stability. However, the origin and evolution of anaerobic alkane metabolism in the domain Archaea remain controversial. Here, we present evidence that methylotrophic methanogenesis was the ancestral form of this metabolism. Carbon dioxide-reducing methanogenesis developed later through the evolution of tetrahydromethanopterin S-methyltransferase, which linked methanogenesis to the Wood-Ljungdahl pathway for energy conservation. Anaerobic multicarbon alkane metabolisms in Archaea also originated early, with genes coding for the activation of short-chain or even long-chain alkanes likely evolving from an ethane-metabolizing ancestor. These genes were likely horizontally transferred to multiple archaeal clades including Candidatus (Ca) Bathyarchaeia, Ca. Lokiarchaeia, Ca. Hadarchaeia, and the methanogenic Ca. Methanoliparia.

Smart Nanocages as a Tool for Controlling Supramolecular Aggregation

An important aspect in the field of supramolecular chemistry is the control of the composition and aggregation state of supramolecular polymers and the possibility of stabilizing out-of-equilibrium states. The ability to freeze metastable systems and release them on demand, under spatiotemporal control, to allow their thermodynamic evolution toward the most stable species is a very attractive concept. Such temporal blockage could be realized using stimuli-responsive "boxes" able to trap and redirect supramolecular polymers. In this work, we report the use of a redox responsive nanocontainer, an organosilica nanocage (OSCs), for controlling the dynamic self-assembly pathway of supramolecular aggregates of a luminescent platinum compound (Pt_AC). The aggregation of the complexes leads to different photoluminescent properties that allow visualization of the different assemblies and their evolution. We discovered that the nanocontainers can encapsulate kinetically trapped species characterized by an orange emission, preventing their evolution into the thermodynamically stable aggregation state characterized by blue-emitting fibers. Interestingly, the out-of-equilibrium trapped Pt species (Pt_AC@OSCs) can be released on demand by the redox-triggered degradation of OSCs, re-establishing their self-assembly toward the thermodynamically stable state. To demonstrate that control of the self-assembly pathway occurs also in complex media, we followed the evolution of the supramolecular aggregates inside living cells, where the destruction of the cages allows the intracellular release of Pt_AC aggregates, followed by the formation of microscopic blue emitting fibers. Our approach highlights the importance of "ondemand" confinement as a tool to temporally stabilize transient species which modulate complex self-assembly pathways in supramolecular polymerization.

Diversification of methanogens into hyperalkaline serpentinizing environments through adaptations to minimize oxidant limitation

Metagenome assembled genomes (MAGs) and single amplified genomes (SAGs) affiliated with two distinct Methanobacterium lineages were recovered from subsurface fracture waters of the Samail Ophiolite, Sultanate of Oman. Lineage Type I was abundant in waters with circumneutral pH, whereas lineage Type II was abundant in hydrogen rich, hyperalkaline waters. Type I encoded proteins to couple hydrogen oxidation to CO₂ reduction, typical of hydrogenotrophic methanogens. Surprisingly, Type II, which branched from the Type I lineage, lacked homologs of two key oxidative [NiFe]-hydrogenases. These functions were presumably replaced by formate dehydrogenases that oxidize formate to yield reductant and cytoplasmic CO₂ via a pathway that was unique among characterized Methanobacteria, allowing cells to overcome CO₂/oxidant limitation in high pH waters. This prediction was supported by microcosm-based radiotracer experiments that showed significant biological methane generation from formate, but not bicarbonate, in waters where the Type II lineage was detected in highest relative abundance. Phylogenetic analyses and variability in gene content suggested that recent and ongoing diversification of the Type II lineage was enabled by gene transfer, loss, and transposition. These data indicate that selection imposed by CO₂/oxidant availability drove recent methanogen diversification into hyperalkaline waters that are heavily impacted by serpentinization.

Histidine Self-assembly and Stability on Mineral Surfaces as a Model of Prebiotic Chemical Evolution: An Experimental and Computational Approach

The abiotic synthesis of histidine under experimental prebiotic conditions has proven to be chemically promising and plausible. Within this context, the present results suggest that histidine amino acid may function as a simple prebiotic catalyst able to enhance amino acid polymerization. This work describes an experimental and computational approach to the self-assembly and stabilization of DL-histidine on mineral surfaces using antigorite ((Mg, Fe)₃Si₂O₅(OH)₄), pyrite (FeS₂), and aragonite (CaCO₃) as representative minerals of prebiotic scenarios, such as meteorites, and subaerial and submarine hydrothermal systems. Experimental results were obtained through polarized-light microscopy, IR spectroscopy (ATR-FTIR), and differential scanning calorimetry (DSC). Molecular dynamics was performed through computational simulations with the MM + method in HyperChem software. IR spectra suggest the presence of peptide bonds in the antigorite-histidine and aragonite-histidine assemblages with the presence of amide I and amide II vibration bands. The FTIR second derivative inspection supports this observation. Moreover, DSC data shows histidine stabilization in the presence of antigorite and aragonite by changes in histidine thermodynamic properties, particularly an increase in histidine decomposition temperature (272ºC in antigorite and 275ºC in aragonite). Results from molecular dynamics are consistent with DSC data, suggesting an antigorite-histidine closer interaction with decreased molecular distances (cca. 5.5 Å) between the amino acid and the crystal surface. On the whole, the experimental and computational outcomes support the role of mineral surfaces in prebiotic chemical evolution as enhancers of organic stability.

The Autotrophic Core: An Ancient Network of 404 Reactions Converts H2, CO2, and NH3 into Amino Acids, Bases, and Cofactors

The metabolism of cells contains evidence reflecting the process by which they arose. Here, we have identified the ancient core of autotrophic metabolism encompassing 404 reactions that comprise the reaction network from H₂, CO₂, and ammonia (NH₃) to amino acids, nucleic acid monomers, and the 19 cofactors required for their synthesis. Water is the most common reactant in the autotrophic core, indicating that the core arose in an aqueous environment. Seventy-seven core reactions involve the hydrolysis of high-energy phosphate bonds, furthermore suggesting the presence of a non-enzymatic and highly exergonic chemical reaction capable of continuously synthesizing activated phosphate bonds. CO₂ is the most common carbon-containing compound in the core. An abundance of NADH and NADPH-dependent redox reactions in the autotrophic core, the central role of CO₂, and the circumstance that the core’s main products are far more reduced than CO₂ indicate that the core arose in a highly reducing environment. The chemical reactions of the autotrophic core suggest that it arose from H₂, inorganic carbon, and NH₃ in an aqueous environment marked by highly reducing and continuously far from equilibrium conditions. Such conditions are very similar to those found in serpentinizing hydrothermal systems.

Chemical Garden Membranes in Temperature-Controlled Microfluidic Devices

Thin-walled tubes that classically form when metal salts react with sodium silicate solution are known as chemical gardens. They share similarities with the porous, catalytic materials in hydrothermal vent chimneys, and both structures are exposed to steep pH gradients that, combined with thermal factors, might have provided the free energy for prebiotic chemistry on early Earth. We report temperature effects on the shape, composition, and opacity of chemical gardens. Tubes grown at high temperature are more opaque, indicating changes to the membrane structure or thickness. To study this dependence, we developed a temperature-controlled microfluidic device, which allows the formation of analogous membranes at the interface of two coflowing reactant solutions. For the case of Ni(OH)₂, membranes thicken according to a diffusion-controlled mechanism. In the studied range of 10-40 °C, the effective diffusion coefficient is independent of temperature. This suggests that counteracting processes are at play (including an increased solubility) and that the opacity of chemical garden tubes arises from changes in internal morphology. The latter could be linked to experimentally observed dendritic structures within the membranes.

A methylotrophic origin of methanogenesis and early divergence of anaerobic multicarbon alkane metabolism

Methanogens are considered as one of the earliest life forms on Earth, and together with anaerobic methane-oxidizing archaea, they have crucial effects on climate stability. Yet, the origin and evolution of anaerobic alkane metabolism in the domain Archaea remain controversial. Here, we show that methanogenesis was already present in the common ancestor of Euryarchaeota, TACK archaea, and Asgard archaea likely in the late Hadean or early Archean eon and that the ancestral methanogen was dependent on methylated compounds and hydrogen. Carbon dioxide-reducing methanogenesis developed later through the evolution of tetrahydromethanopterin S-methyltransferase, which linked methanogenesis to the Wood-Ljungdahl pathway for energy conservation. Multicarbon alkane metabolisms in Archaea also originated early, with genes coding for the activation of short- or even long-chain alkanes likely evolving from an ethane-metabolizing ancestor. These genes were likely horizontally transferred to multiple archaeal clades including Candidatus (Ca) Bathyarchaeota, Ca. Helarchaeota, Ca Hadesarchaeota, and the methanogenic Ca. Methanoliparia.

Six 'Must-Have' Minerals for Life's Emergence: Olivine, Pyrrhotite, Bridgmanite, Serpentine, Fougerite and Mackinawite

Life cannot emerge on a planet or moon without the appropriate electrochemical disequilibria and the minerals that mediate energy-dissipative processes. Here, it is argued that four minerals, olivine ([Mg>Fe]2SiO4), bridgmanite ([Mg,Fe]SiO3), serpentine ([Mg,Fe,]2-3Si2O5[OH)]4), and pyrrhotite (Fe(1-x)S), are an essential requirement in planetary bodies to produce such disequilibria and, thereby, life. Yet only two minerals, fougerite ([Fe2+6xFe3+6(x-1)O12H2(7-3x)]2+·[(CO2-)·3H2O]2-) and mackinawite (Fe[Ni]S), are vital-comprising precipitate membranes-as initial "free energy" conductors and converters of such disequilibria, i.e., as the initiators of a CO2-reducing metabolism. The fact that wet and rocky bodies in the solar system much smaller than Earth or Venus do not reach the internal pressure (≥23 GPa) requirements in their mantles sufficient for producing bridgmanite and, therefore, are too reduced to stabilize and emit CO2-the staple of life-may explain the apparent absence or negligible concentrations of that gas on these bodies, and thereby serves as a constraint in the search for extraterrestrial life. The astrobiological challenge then is to search for worlds that (i) are large enough to generate internal pressures such as to produce bridgmanite or (ii) boast electron acceptors, including imported CO2, from extraterrestrial sources in their hydrospheres.

The Role of Meteorite Impacts in the Origin of Life

The conditions, timing, and setting for the origin of life on Earth and whether life exists elsewhere in our solar system and beyond represent some of the most fundamental scientific questions of our time. Although the bombardment of planets and satellites by asteroids and comets has long been viewed as a destructive process that would have presented a barrier to the emergence of life and frustrated or extinguished life, we provide a comprehensive synthesis of data and observations on the beneficial role of impacts in a wide range of prebiotic and biological processes. In the context of previously proposed environments for the origin of life on Earth, we discuss how meteorite impacts can generate both subaerial and submarine hydrothermal vents, abundant hydrothermal-sedimentary settings, and impact analogues for volcanic pumice rafts and splash pools. Impact events can also deliver and/or generate many of the necessary chemical ingredients for life and catalytic substrates such as clays as well. The role that impact cratering plays in fracturing planetary crusts and its effects on deep subsurface habitats for life are also discussed. In summary, we propose that meteorite impact events are a fundamental geobiological process in planetary evolution that played an important role in the origin of life on Earth. We conclude with the recommendation that impact craters should be considered prime sites in the search for evidence of past life on Mars. Furthermore, unlike other geological processes such as volcanism or plate tectonics, impact cratering is ubiquitous on planetary bodies throughout the Universe and is independent of size, composition, and distance from the host star. Impact events thus provide a mechanism with the potential to generate habitable planets, moons, and asteroids throughout the Solar System and beyond.

Lipid Biomarker Record of the Serpentinite-Hosted Ecosystem of the Samail Ophiolite, Oman and Implications for the Search for Biosignatures on Mars

Serpentinization is a weathering process in which ultramafic rocks react with water, generating a range of products, including serpentine and other minerals, in addition to H₂ and low-molecular-weight hydrocarbons that are capable of sustaining microbial life. Lipid biomarker analyses of serpentinite-hosted ecosystems hold promise as tools for investigating microbial activity in ancient Earth environments and other terrestrial planets such as Mars because lipids have the potential for longer term preservation relative to DNA, proteins, and other more labile organic molecules. Here, we report the first lipid biomarker record of microbial activity in the mantle section of the Samail Ophiolite, in the Sultanate of Oman, a site undergoing active serpentinization. We detected isoprenoidal (archaeal) and branched (bacterial) glycerol dialkyl glycerol tetraether (GDGT) lipids, including those with 0-3 cyclopentane moieties, and crenarchaeol, an isoprenoidal GDGT containing four cyclopentane and one cyclohexane moieties, as well as monoether lipids and fatty acids indicative of sulfate-reducing bacteria. Comparison of our geochemical data and 16S rRNA data from the Samail Ophiolite with those from other serpentinite-hosted sites identifies the existence of a common core serpentinization microbiome. In light of these findings, we also discuss the preservation potential of serpentinite lipid biomarker assemblages on Earth and Mars. Continuing investigations of the Samail Ophiolite and other terrestrial analogues will enhance our understanding of microbial habitability and diversity in serpentinite-hosted environments on Earth and elsewhere in the Solar System.

The ambivalent role of water at the origins of life

Life as we know it would not exist without water. However, water molecules not only serve as a solvent and reactant but can also promote hydrolysis, which counteracts the formation of essential organic molecules. This conundrum constitutes one of the central issues in origin of life. Hydrolysis is an important part of energy metabolism for all living organisms but only because, inside cells, it is a controlled reaction. How could hydrolysis have been regulated under prebiotic settings? Lower water activities possibly provide an answer: geochemical sites with less free and more bound water can supply the necessary conditions for protometabolic reactions. Such conditions occur in serpentinising systems, hydrothermal sites that synthesise hydrogen gas via rock-water interactions. Here, we summarise the parallels between biotic and abiotic means of controlling hydrolysis in order to narrow the gap between biochemical and geochemical reactions and briefly outline how hydrolysis could even have played a constructive role at the origin of molecular self-organisation.

Single-Cell Genomics of Novel Actinobacteria With the Wood-Ljungdahl Pathway Discovered in a Serpentinizing System

Serpentinite-hosted systems represent modern-day analogs of early Earth environments. In these systems, water-rock interactions generate highly alkaline and reducing fluids that can contain hydrogen, methane, and low-molecular-weight hydrocarbons-potent reductants capable of fueling microbial metabolism. In this study, we investigated the microbiota of Hakuba Happo hot springs (∼50°C; pH∼10.5-11), located in Nagano (Japan), which are impacted by the serpentinization process. Analysis of the 16S rRNA gene amplicon sequences revealed that the bacterial community comprises Nitrospirae (47%), "Parcubacteria" (19%), Deinococcus-Thermus (16%), and Actinobacteria (9%), among others. Notably, only 57 amplicon sequence variants (ASV) were detected, and fifteen of these accounted for 90% of the amplicons. Among the abundant ASVs, an early-branching, uncultivated actinobacterial clade identified as RBG-16-55-12 in the SILVA database was detected. Ten single-cell genomes (average pairwise nucleotide identity: 0.98-1.00; estimated completeness: 33-93%; estimated genome size: ∼2.3 Mb) that affiliated with this clade were obtained. Taxonomic classification using single copy genes indicates that the genomes belong to the actinobacterial class-level clade UBA1414 in the Genome Taxonomy Database. Based on metabolic pathway predictions, these actinobacteria are anaerobes, capable of glycolysis, dissimilatory nitrate reduction and CO₂ fixation via the Wood-Ljungdahl (WL) pathway. Several other genomes within UBA1414 and two related class-level clades also encode the WL pathway, which has not yet been reported for the Actinobacteria phylum. For the Hakuba actinobacterium, the energy metabolism related to the WL pathway is likely supported by a combination of the Rnf complex, group 3b and 3d [NiFe]-hydrogenases, [FeFe]-hydrogenases, and V-type (H⁺/Na⁺ pump) ATPase. The genomes also harbor a form IV ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) complex, also known as a RubisCO-like protein, and contain signatures of interactions with viruses, including clustered regularly interspaced short palindromic repeat (CRISPR) regions and several phage integrases. This is the first report and detailed genome analysis of a bacterium within the Actinobacteria phylum capable of utilizing the WL pathway. The Hakuba actinobacterium is a member of the clade UBA1414/RBG-16-55-12, formerly within the group "OPB41." We propose to name this bacterium 'Candidatus Hakubanella thermoalkaliphilus.'

Older Than Genes: The Acetyl CoA Pathway and Origins

For decades, microbiologists have viewed the acetyl CoA pathway and organisms that use it for H2-dependent carbon and energy metabolism, acetogens and methanogens, as ancient. Classical evidence and newer evidence indicating the antiquity of the acetyl CoA pathway are summarized here. The acetyl CoA pathway requires approximately 10 enzymes, roughly as many organic cofactors, and more than 500 kDa of combined subunit molecular mass to catalyze the conversion of H2 and CO2 to formate, acetate, and pyruvate in acetogens and methanogens. However, a single hydrothermal vent alloy, awaruite (Ni3Fe), can convert H2 and CO2 to formate, acetate, and pyruvate under mild hydrothermal conditions on its own. The chemical reactions of H2 and CO2 to pyruvate thus have a natural tendency to occur without enzymes, given suitable inorganic catalysts. This suggests that the evolution of the enzymatic acetyl CoA pathway was preceded by-and patterned along-a route of naturally occurring exergonic reactions catalyzed by transition metal minerals that could activate H2 and CO2 by chemisorption. The principle of forward (autotrophic) pathway evolution from preexisting non-enzymatic reactions is generalized to the concept of patterned evolution of pathways. In acetogens, exergonic reduction of CO2 by H2 generates acyl phosphates by highly reactive carbonyl groups undergoing attack by inert inorganic phosphate. In that ancient reaction of biochemical energy conservation, the energy behind formation of the acyl phosphate bond resides in the carbonyl, not in phosphate. The antiquity of the acetyl CoA pathway is usually seen in light of CO2 fixation; its role in primordial energy coupling via acyl phosphates and substrate-level phosphorylation is emphasized here.

Probing the hydrothermal system of the Chicxulub impact crater

The ~180-km-diameter Chicxulub peak-ring crater and ~240-km multiring basin, produced by the impact that terminated the Cretaceous, is the largest remaining intact impact basin on Earth. International Ocean Discovery Program (IODP) and International Continental Scientific Drilling Program (ICDP) Expedition 364 drilled to a depth of 1335 m below the sea floor into the peak ring, providing a unique opportunity to study the thermal and chemical modification of Earth's crust caused by the impact. The recovered core shows the crater hosted a spatially extensive hydrothermal system that chemically and mineralogically modified ~1.4 × 105 km3 of Earth's crust, a volume more than nine times that of the Yellowstone Caldera system. Initially, high temperatures of 300° to 400°C and an independent geomagnetic polarity clock indicate the hydrothermal system was long lived, in excess of 106 years.

Factoring Origin of Life Hypotheses into the Search for Life in the Solar System and Beyond

Two widely-cited alternative hypotheses propose geological localities and biochemical mechanisms for life's origins. The first states that chemical energy available in submarine hydrothermal vents supported the formation of organic compounds and initiated primitive metabolic pathways which became incorporated in the earliest cells; the second proposes that protocells self-assembled from exogenous and geothermally-delivered monomers in freshwater hot springs. These alternative hypotheses are relevant to the fossil record of early life on Earth, and can be factored into the search for life elsewhere in the Solar System. This review summarizes the evidence supporting and challenging these hypotheses, and considers their implications for the search for life on various habitable worlds. It will discuss the relative probability that life could have emerged in environments on early Mars, on the icy moons of Jupiter and Saturn, and also the degree to which prebiotic chemistry could have advanced on Titan. These environments will be compared to ancient and modern terrestrial analogs to assess their habitability and biopreservation potential. Origins of life approaches can guide the biosignature detection strategies of the next generation of planetary science missions, which could in turn advance one or both of the leading alternative abiogenesis hypotheses.

Simulating Serpentinization as It Could Apply to the Emergence of Life Using the JPL Hydrothermal Reactor

The molecules feeding life's emergence are thought to have been provided through the hydrothermal interactions of convecting carbonic ocean waters with minerals comprising the early Hadean oceanic crust. Few laboratory experiments have simulated ancient hydrothermal conditions to test this conjecture. We used the JPL hydrothermal flow reactor to investigate CO₂ reduction in simulated ancient alkaline convective systems over 3 days (T = 120°C, P = 100 bar, pH = 11). H₂-rich hydrothermal simulant and CO₂-rich ocean simulant solutions were periodically driven in 4-h cycles through synthetic mafic and ultramafic substrates and Fe>Ni sulfides. The resulting reductants included micromoles of HS^- and formate accompanied possibly by micromoles of acetate and intermittent minor bursts of methane as ascertained by isotopic labeling. The formate concentrations directly correlated with the CO₂ input as well as with millimoles of Mg²⁺ ions, whereas the acetate did not. Also, tens of micromoles of methane were drawn continuously from the reactor materials during what appeared to be the onset of serpentinization. These results support the hypothesis that formate may have been delivered directly to a branch of an emerging acetyl coenzyme-A pathway, thus obviating the need for the very first hydrogenation of CO₂ to be made in a hydrothermal mound. Another feed to early metabolism could have been methane, likely mostly leached from primary CH₄ present in the original Hadean crust or emanating from the mantle. That a small volume of methane was produced sporadically from the ¹³CO₂-feed, perhaps from transient occlusions, echoes the mixed results and interpretations from other laboratories. As serpentinization and hydrothermal leaching can occur wherever an ocean convects within anhydrous olivine- and sulfide-rich crust, these results may be generalized to other wet rocky planets and moons in our solar system and beyond.

Habitability of the marine serpentinite subsurface: a case study of the Lost City hydrothermal field

The Lost City hydrothermal field is a dramatic example of the biological potential of serpentinization. Microbial life is prevalent throughout the Lost City chimneys, powered by the hydrogen gas and organic molecules produced by serpentinization and its associated geochemical reactions. Microbial life in the serpentinite subsurface below the Lost City chimneys, however, is unlikely to be as dense or active. The marine serpentinite subsurface poses serious challenges for microbial activity, including low porosities, the combination of stressors of elevated temperature, high pH and a lack of bioavailable ∑CO₂. A better understanding of the biological opportunities and challenges in serpentinizing systems would provide important insights into the total habitable volume of Earth's crust and for the potential of the origin and persistence of life in Earth's subsurface environments. Furthermore, the limitations to life in serpentinizing subsurface environments on Earth have significant implications for the habitability of subsurface environments on ocean worlds such as Europa and Enceladus. Here, we review the requirements and limitations of life in serpentinizing systems, informed by our research at the Lost City and the underwater mountain on which it resides, the Atlantis Massif. This article is part of a discussion meeting issue 'Serpentinite in the Earth System'.

Mariana serpentinite mud volcanism exhumes subducted seamount materials: implications for the origin of life

The subduction of seamounts and ridge features at convergent plate boundaries plays an important role in the deformation of the overriding plate and influences geochemical cycling and associated biological processes. Active serpentinization of forearc mantle and serpentinite mud volcanism on the Mariana forearc (between the trench and active volcanic arc) provides windows on subduction processes.  Here, we present (1) the first observation of an extensive exposure of an undeformed Cretaceous seamount currently being subducted at the Mariana Trench inner slope; (2) vertical deformation of the forearc region related to subduction of Pacific Plate seamounts and thickened crust; (3) recovered Ocean Drilling Program and International Ocean Discovery Program cores of serpentinite mudflows that confirm exhumation of various Pacific Plate lithologies, including subducted reef limestone; (4) petrologic, geochemical and paleontological data from the cores that show that Pacific Plate seamount exhumation covers greater spatial and temporal extents; (5) the inference that microbial communities associated with serpentinite mud volcanism may also be exhumed from the subducted plate seafloor and/or seamounts; and (6) the implications for effects of these processes with regard to evolution of life. This article is part of a discussion meeting issue 'Serpentine in the Earth system'.

A synthesis and meta-analysis of the Fe chemistry of serpentinites and serpentine minerals

The iron chemistry of serpentinites and serpentine group minerals is often invoked as a record of the setting and conditions of serpentinization because Fe behaviour is influenced by reaction conditions. Iron can be partitioned into a variety of secondary mineral phases and undergo variable extents of oxidation and/or reduction during serpentinization. This behaviour influences geophysical, geochemical and biological aspects of serpentinizing systems and, more broadly, earth systems. Iron chemistry of serpentinites and serpentines is frequently analysed and reported for single systems. Interpretations of the controls on, and the implications of, Fe behaviour drawn from a single system are often widely extrapolated. There is a wealth of serpentinite/serpentine chemical composition data available in the literature. Consequently, compilation of a database including potential predictors of Fe behaviour and measures of Fe chemistry enables systematic investigation of trends in Fe behaviour across a variety of systems and conditions. The database presented here contains approximately 2000 individual data points including both bulk rock and serpentine mineral geochemical data which are paired whenever possible. Measures of total Fe and Fe oxidation state, which are more limited, are compiled with characteristics of the systems from which they were sampled. Observations of trends in Fe chemistry in serpentinites and serpentines across the variety of geologic systems and parameters will aid in verifying and strengthening interpretations made on the basis of Fe chemistry. This article is part of a discussion meeting issue 'Serpentinite in the Earth system'.

Bioenergetic constraints on the origin of autotrophic metabolism

Autotrophs form the base of all complex food webs and seemingly have done so since early in Earth history. Phylogenetic evidence suggests that early autotrophs were anaerobic, used CO₂ as both an oxidant and carbon source, were dependent on H₂ as an electron donor, and used iron-sulfur proteins (termed ferredoxins) as a primary electron carrier. However, the reduction potential of H₂ is not typically low enough to efficiently reduce ferredoxin. Instead, in modern strictly anaerobic and H₂-dependent autotrophs, ferredoxin reduction is accomplished using one of several recently evolved enzymatic mechanisms, including electron bifurcating and coupled ion translocating mechanisms. These observations raise the intriguing question of why anaerobic autotrophs adopted ferredoxins as central electron carriers only to have to evolve complex machinery to reduce them. Here, we report calculated reduction potentials for H₂ as a function of observed environmental H₂ concentration, pH and temperature. Results suggest that a combination of alkaline pH and high H₂ concentration yield H₂ reduction potentials low enough to efficiently reduce ferredoxins. Hyperalkaline, H₂ rich environments have existed in discrete locations throughout Earth history where ultramafic minerals are undergoing hydration through the process of serpentinization. These results suggest that serpentinizing systems, which would have been common on early Earth, naturally produced conditions conducive to the emergence of H₂-dependent autotrophic life. The primitive process of hydrogenotrophic methanogenesis is used to examine potential changes in methanogenesis and Fd reduction pathways as these organisms diversified away from serpentinizing environments. This article is part of a discussion meeting issue 'Serpentinite in the earth system'.

Getting Beyond the Toy Domain. Meditations on David Deamer's "Assembling Life"

David Deamer has written another book, Assembling Life, on the origin of life. It is unapologetically polemic, presenting Deamer's view that life originated in fresh water hydrothermal fields on volcanic islands on early Earth, arguing that this provided a unique environment not just for organic chemistry but for the self-assembling structure that drive that chemistry and form the basis of structure in life. It is worth reading, it is an advance in the field, but is it convincing? I argue that the Origin of Life field as a whole is unconvincing, generating results in Toy Domains that cannot be scaled to any real world scenario. I suggest that, by analogy with the history of artificial intelligence and solar astronomy, we need much more scale, and fundamentally new ideas, to take the field forward.

Mineral self-organization on a lifeless planet

It has been experimentally demonstrated that, under alkaline conditions, silica is able to induce the formation of mineral self-assembled inorganic-inorganic composite materials similar in morphology, texture and nanostructure to the hybrid biomineral structures that, millions of years later, life was able to self-organize. These mineral self-organized structures (MISOS) have been also shown to work as effective catalysts for prebiotic chemical reactions and to easily create compartmentalization within the solutions where they form. We reason that, during the very earliest history of this planet, there was a geochemical scenario that inevitably led to the existence of a large-scale factory of simple and complex organic compounds, many of which were relevant to prebiotic chemistry. The factory was built on a silica-rich high-pH ocean and powered by two main factors: a) a quasi-infinite source of simple carbon molecules synthesized abiotically from reactions associated with serpentinization, or transported from meteorites and produced from their impact on that alkaline ocean, and b) the formation of self-organized silica-metal mineral composites that catalyze the condensation of simple molecules in a methane-rich reduced atmosphere. We discuss the plausibility of this geochemical scenario, review the details of the formation of MISOS and its catalytic properties and the transition towards a slightly alkaline to neutral ocean.

Prospecting for life

Books with titles like 'The Call of the Wild' seemed to set a path for a life. Thus, I would be an explorer-a plan that did not work out so well, at least at first. On leaving school I got a job as a 'Works Chemist Improver', testing Ni catalysts for the hydrogenation of phenol to cyclohexanol. Taking night classes I passed enough exams to study geology at Queen Mary College, London. Armed thus I travelled to the Solomon Islands where geology is a 'happening'! Next was Canada to visit a mine sunk into a 1.5 billion year old Pb-Zn orebody precipitated from submarine hot springs. At last I reached the Yukon to prospect for silver. Thence to Ireland researching what I also took to be 'exhalative' (i.e. hot spring-related) Pb-Zn orebodies. While there in 1979, the discovery of 350°C metal-bearing acidic waters issuing from submarine Black Smoker chimneys in the Pacific sent us searching for fossil examples in the Irish mines. However, the chimneys we found were more like chemical gardens than Black Smokers, a finding that made us think about the emergence of life. After all, what better for life's emergence than to have a membrane comprising Fe minerals dosed with Ni in our chimneys to mediate the 'hydrogenation' of CO2-life's job anyway. Indeed, such a membrane would keep redox and pH disequilibria at bay, just like biological membranes. At the same time, my field research among Alpine ophiolites-ocean floor mafic rocks obducted to the Alps-indicated that alkaline waters bearing H2 and CH4 were a result of serpentinization, a process that must have operated in all ocean floors over all time. Thus it was that we could predict the Lost City hydrothermal field 10 years before its discovery in the North Atlantic in the year 2000. Lost City comprises a number of alkaline springs at up to 90°C that produce carbonate and brucite (Mg[OH]2) chimneys. We had surmised that Ni-enriched FeS chimneys would have precipitated at comparable alkaline springs issuing into a metal-rich carbonic ocean on the very early Earth (inducing membrane potentials comparable to those capable of succouring all life, and presumably, sufficient to drive life into being). However, our laboratory precipitates also revealed green rust, thought to be the precursor to the magnetite now comprising the Archaean Banded Iron Formations. We now look upon green rust, also known as fougèrite, as the tangible, base fractal of life.

On the beneficent thickness of water

In the 1930s, Lars Onsager published his famous 'reciprocal relations' describing free energy conversion processes. Importantly, these relations were derived on the assumption that the fluxes of the processes involved in the conversion were proportional to the forces (free energy gradients) driving them. For chemical reactions, however, this condition holds only for systems operating close to equilibrium-indeed very close; nominally requiring driving forces to be smaller than k B T. Fairly soon thereafter, however, it was quite inexplicably observed that in at least some biological conversions both the reciprocal relations and linear flux-force dependency appeared to be obeyed no matter how far from equilibrium the system was being driven. No successful explanation of how this 'paradoxical' behaviour could occur has emerged and it has remained a mystery. We here argue, however, that this anomalous behaviour is simply a gift of water, of its viscosity in particular; a gift, moreover, without which life almost certainly could not have emerged. And a gift whose appreciation we primarily owe to recent work by Prof. R. Dean Astumian who, as providence has kindly seen to it, was led to the relevant insights by the later work of Onsager himself.

Environmental boundary conditions for the origin of life converge to an organo-sulfur metabolism

It has been suggested that a deep memory of early life is hidden in the architecture of metabolic networks, whose reactions could have been catalyzed by small molecules or minerals before genetically encoded enzymes. A major challenge in unravelling these early steps is assessing the plausibility of a connected, thermodynamically consistent proto-metabolism under different geochemical conditions, which are still surrounded by high uncertainty. Here we combine network-based algorithms with physico-chemical constraints on chemical reaction networks to systematically show how different combinations of parameters (temperature, pH, redox potential and availability of molecular precursors) could have affected the evolution of a proto-metabolism. Our analysis of possible trajectories indicates that a subset of boundary conditions converges to an organo-sulfur-based proto-metabolic network fuelled by a thioester- and redox-driven variant of the reductive tricarboxylic acid cycle that is capable of producing lipids and keto acids. Surprisingly, environmental sources of fixed nitrogen and low-potential electron donors are not necessary for the earliest phases of biochemical evolution. We use one of these networks to build a steady-state dynamical metabolic model of a protocell, and find that different combinations of carbon sources and electron donors can support the continuous production of a minimal ancient 'biomass' composed of putative early biopolymers and fatty acids.

Thermodynamic Impact of Mineral Surfaces on Amino Acid Polymerization: Aspartate Dimerization on Goethite

This article presents a thermodynamic predictive scheme for amino acid polymerization in the presence of minerals as a function of various environmental parameters (pH, ionic strength, amino acid concentration, and the solid/water ratio) using l-aspartate (Asp) and goethite as a model combination. This prediction is enabled by the combination of the surface adsorption constants of amino acid and its polymer, determined from the extended triple layer model characterization of the corresponding experimental results, with the thermodynamic data of these organic compounds in water reported in the literature. Calculations for the Asp-goethite system showed that the goethite surface drastically shifts the Asp monomer-dipeptide equilibrium toward the dipeptide side; when the dimerization of 0.1 mM Asp was considered in the presence of 10 m² L^-1 of goethite, an Asp dipeptide concentration around 10⁵ times larger was computed to be thermodynamically attainable compared with that in the absence of goethite at acidic pH (4-5) and low ionic strength (0.1 mM NaCl). Under this condition, the dipeptide-to-monomer molecular ratio in the adsorbed state reached 20%. In contrast, no significant enhancement by goethite was predicted at alkaline pH (>8), where the electrostatic interactions of the goethite surface with Asp and Asp dipeptide are weak. Thus, mineral surfaces should have had a significant impact on the thermodynamics of prebiotic peptide bond formation on the early Earth, although the influences likely depended largely on the environmental conditions. Future experimental studies for various amino acid-mineral interactions using our proposed methodology will provide a quantitative constraint on favorable geochemical settings for the chemical evolution on Earth. This approach can also offer important clues for future exploration of extraterrestrial life.

Hydrogen sulfide, reactive sulfur species and coping with reactive oxygen species

Life began in a ferruginous (anoxic and Fe²⁺ dominated) world around 3.8 billion years ago (bya). Hydrogen sulfide (H₂S) and other sulfur molecules from hydrothermal vents and other fissures provided many key necessities for life's origin including catalytic platforms (primordial enzymes) that also served as primitive boundaries (cell walls), substrates for organic synthesis and a continuous source of energy in the form of reducing equivalents. Anoxigenic photosynthesis oxidizing H₂S followed within a few hundred million years and laid the metabolic groundwork for oxidative photosynthesis some half-billion years later that slightly and episodically increased atmospheric oxygen around 2.3 bya. This oxidized terrestrial sulfur to sulfate which was washed to the sea where it was reduced creating vast euxinic (anoxic and sulfidic) areas. It was in this environment that eukaryotic cells appeared around 1.5 bya and where they evolved for nearly 1 billion additional years. Oxidative photosynthesis finally oxidized the oceans and around 0.6 bya oxygen levels in the atmosphere and oceans began to rise toward present day levels. This is purported to have been a life-threatening event due to the prevalence of reactive oxygen species (ROS) and thus necessitated the elaboration of chemical and enzymatic antioxidant mechanisms. However, these antioxidants initially appeared around the time of anoxigenic photosynthesis suggesting a commitment to metabolism of reactive sulfur species (RSS). This review examines these events and suggests that many of the biological attributes assigned to ROS may, in fact, be due to RSS. This is underscored by observations that ROS and RSS are chemically similar, often indistinguishable by analytical methods and the fact that the bulk of biochemical and physiological experiments are performed in unphysiologically oxic environments where ROS are artifactually favored over RSS.

Metamorphism and the evolution of plate tectonics

Earth's mantle convection, which facilitates planetary heat loss, is manifested at the surface as present-day plate tectonics¹. When plate tectonics emerged and how it has evolved through time are two of the most fundamental and challenging questions in Earth science^1-4. Metamorphic rocks-rocks that have experienced solid-state mineral transformations due to changes in pressure (P) and temperature (T)-record periods of burial, heating, exhumation and cooling that reflect the tectonic environments in which they formed^5,6. Changes in the global distribution of metamorphic (P, T) conditions in the continental crust through time might therefore reflect the secular evolution of Earth's tectonic processes. On modern Earth, convergent plate margins are characterized by metamorphic rocks that show a bimodal distribution of apparent thermal gradients (temperature change with depth; parameterized here as metamorphic T/P) in the form of paired metamorphic belts⁵, which is attributed to metamorphism near (low T/P) and away from (high T/P) subduction zones^5,6. Here we show that Earth's modern plate tectonic regime has developed gradually with secular cooling of the mantle since the Neoarchaean era, 2.5 billion years ago. We evaluate the emergence of bimodal metamorphism (as a proxy for secular change in plate tectonics) using a statistical evaluation of the distributions of metamorphic T/P through time. We find that the distribution of metamorphic T/P has gradually become wider and more distinctly bimodal from the Neoarchaean era to the present day, and the average metamorphic T/P has decreased since the Palaeoproterozoic era. Our results contrast with studies that inferred an abrupt transition in tectonic style in the Neoproterozoic era (about 0.7 billion years ago^1,7,8) or that suggested that modern plate tectonics has operated since the Palaeoproterozoic era (about two billion years ago^9-12) at the latest.

Oral Ingestion of Deep Ocean Minerals Increases High-Intensity Intermittent Running Capacity in Soccer Players after Short-Term Post-Exercise Recovery: A Double-Blind, Placebo-Controlled Crossover Trial

This study examined whether deep ocean mineral (DOM) supplementation improved high-intensity intermittent running capacity after short-term recovery from an initial bout of prolonged high-intensity running in thermoneutral environmental conditions. Nine healthy recreational male soccer players (age: 22 ± 1 y; stature: 181 ± 5 cm; and body mass 80 ± 11 kg) completed a graded incremental test to ascertain peak oxygen uptake (V^·O_2PEAK), two familiarisation trials, and two experimental trials following a double-blind, repeated measures, crossover and counterbalanced design. All trials were separated by seven days and at ambient room temperature (i.e., 20 °C). During the 2 h recovery period after the initial ~60 min running at 75% V^·O_2PEAK, participants were provided with 1.38 ± 0.51 L of either deep ocean mineral water (DOM) or a taste-matched placebo (PLA), both mixed with 6% sucrose. DOM increased high-intensity running capacity by ~25% compared to PLA. There were no differences between DOM and PLA for blood lactate concentration, blood glucose concentration, or urine osmolality. The minerals and trace elements within DOM, either individually or synergistically, appear to have augmented high-intensity running capacity in healthy, recreationally active male soccer players after short-term recovery from an initial bout of prolonged, high-intensity running in thermoneutral environmental conditions.

Self-Assembling Ice Membranes on Europa: Brinicle Properties, Field Examples, and Possible Energetic Systems in Icy Ocean Worlds

Brinicles are self-assembling tubular ice membrane structures, centimeters to meters in length, found beneath sea ice in the polar regions of Earth. We discuss how the properties of brinicles make them of possible importance for chemistry in cold environments-including that of life's emergence-and we consider their formation in icy ocean worlds. We argue that the non-ice composition of the ice on Europa and Enceladus will vary spatially due to thermodynamic and mechanical properties that serve to separate and fractionate brines and solid materials. The specifics of the composition and dynamics of both the ice and the ocean in these worlds remain poorly constrained. We demonstrate through calculations using FREZCHEM that sulfate likely fractionates out of accreting ice in Europa and Enceladus, and thus that an exogenous origin of sulfate observed on Europa's surface need not preclude additional endogenous sulfate in Europa's ocean. We suggest that, like hydrothermal vents on Earth, brinicles in icy ocean worlds constitute ideal places where ecosystems of organisms might be found.

Living at the Extremes: Extremophiles and the Limits of Life in a Planetary Context

Prokaryotic life has dominated most of the evolutionary history of our planet, evolving to occupy virtually all available environmental niches. Extremophiles, especially those thriving under multiple extremes, represent a key area of research for multiple disciplines, spanning from the study of adaptations to harsh conditions, to the biogeochemical cycling of elements. Extremophile research also has implications for origin of life studies and the search for life on other planetary and celestial bodies. In this article, we will review the current state of knowledge for the biospace in which life operates on Earth and will discuss it in a planetary context, highlighting knowledge gaps and areas of opportunity.

The Dallol Geothermal Area, Northern Afar (Ethiopia)-An Exceptional Planetary Field Analog on Earth

The Dallol volcano and its associated hydrothermal field are located in a remote area of the northern Danakil Depression in Ethiopia, a region only recently appraised after decades of inaccessibility due to severe political instability and the absence of infrastructure. The region is notable for hosting environments at the very edge of natural physical-chemical extremities. It is surrounded by a wide, hyperarid salt plain and is one of the hottest (average annual temperature_Dallol: 36-38°C) and most acidic natural systems (pH_Dallol ≈0) on Earth. Spectacular geomorphologies and mineral deposits produced by supersaturated hydrothermal waters and brines are the result of complex interactions between active and inactive hydrothermal alteration of the bedrock, sulfuric hot springs and pools, fumaroles and geysers, and recrystallization processes driven by hydrothermal waters, degassing, and rapid evaporation. The study of planetary field analog environments plays a crucial role in characterizing the physical and chemical boundaries within which life can exist on Earth and other planets. It is essential for the definition and assessment of the conditions of habitability on other planets, including the possibility for biosignature preservation and in situ testing of technologies for life detection. The Dallol area represents an excellent Mars analog environment given that the active volcanic environment, the associated diffuse hydrothermalism and hydrothermal alteration, and the vast acidic sulfate deposits are reminiscent of past hydrothermal activity on Mars. The work presented in this paper is an overview of the Dallol volcanic area and its hydrothermal field that integrates previous literature with observations and results obtained from field surveys and monitoring coupled with sample characterization. In so doing, we highlight its exceptional potential as a planetary field analog as well as a site for future astrobiological and exploration programs.

Essay on Biomembrane Structure

Of all the macromolecular assemblies of life, the least understood is the biomembrane. This is especially true in regard to its atomic structure. Ideas on biomembranes, developed in the last 200 years, culminated in the fluid mosaic model of the membrane. In this essay, I provide a historical outline of how we arrived at our current understanding of biomembranes and the models we use to describe them. A selection of direct experimental findings on the nano-scale structure of biomembranes is taken up to discuss their physical nature, and special emphasis is put on the surprising insights that arise from atomic scale descriptions.

Subsurface Microbial Hydrogen Cycling: Natural Occurrence and Implications for Industry

Hydrogen is a key energy source for subsurface microbial processes, particularly in subsurface environments with limited alternative electron donors, and environments that are not well connected to the surface. In addition to consumption of hydrogen, microbial processes such as fermentation and nitrogen fixation produce hydrogen. Hydrogen is also produced by a number of abiotic processes including radiolysis, serpentinization, graphitization, and cataclasis of silicate minerals. Both biotic and abiotically generated hydrogen may become available for consumption by microorganisms, but biotic production and consumption are usually tightly coupled. Understanding the microbiology of hydrogen cycling is relevant to subsurface engineered environments where hydrogen-cycling microorganisms are implicated in gas consumption and production and corrosion in a number of industries including carbon capture and storage, energy gas storage, and radioactive waste disposal. The same hydrogen-cycling microorganisms and processes are important in natural sites with elevated hydrogen and can provide insights into early life on Earth and life on other planets. This review draws together what is known about microbiology in natural environments with elevated hydrogen, and highlights where similar microbial populations could be of relevance to subsurface industry.

Origin of Life's Building Blocks in Carbon- and Nitrogen-Rich Surface Hydrothermal Vents

There are two dominant and contrasting classes of origin of life scenarios: those predicting that life emerged in submarine hydrothermal systems, where chemical disequilibrium can provide an energy source for nascent life; and those predicting that life emerged within subaerial environments, where UV catalysis of reactions may occur to form the building blocks of life. Here, we describe a prebiotically plausible environment that draws on the strengths of both scenarios: surface hydrothermal vents. We show how key feedstock molecules for prebiotic chemistry can be produced in abundance in shallow and surficial hydrothermal systems. We calculate the chemistry of volcanic gases feeding these vents over a range of pressures and basalt C/N/O contents. If ultra-reducing carbon-rich nitrogen-rich gases interact with subsurface water at a volcanic vent they result in 10 - 3 ⁻ 1 M concentrations of diacetylene (C₄H₂), acetylene (C₂H₂), cyanoacetylene (HC₃N), hydrogen cyanide (HCN), bisulfite (likely in the form of salts containing HSO₃-), hydrogen sulfide (HS-) and soluble iron in vent water. One key feedstock molecule, cyanamide (CH₂N₂), is not formed in significant quantities within this scenario, suggesting that it may need to be delivered exogenously, or formed from hydrogen cyanide either via organometallic compounds, or by some as yet-unknown chemical synthesis. Given the likely ubiquity of surface hydrothermal vents on young, hot, terrestrial planets, these results identify a prebiotically plausible local geochemical environment, which is also amenable to future lab-based simulation.

Reduction of CO2 with H2S in a simulated deep-sea hydrothermal vent system

H2S is considered to be an important reductant in abiotic CO2 reduction to organics, however, almost no experimental support has been reported. Herein, the first observation of CO2 reduction to formate with H2S under alkaline hydrothermal conditions is reported, and water is found to act as a hydrogen donor.

How do Nucleotides Adsorb Onto Clays?

Adsorption of prebiotic building blocks is proposed to have played a role in the emergence of life on Earth. The experimental and theoretical study of this phenomenon should be guided by our knowledge of the geochemistry of the habitable early Earth environments, which could have spanned a large range of settings. Adsorption being an interfacial phenomenon, experiments can be built around the minerals that probably exhibited the largest specific surface areas and were the most abundant, i.e., phyllosilicates. Our current work aims at understanding how nucleotides, the building blocks of RNA and DNA, might have interacted with phyllosilicates under various physico-chemical conditions. We carried out and refined batch adsorption studies to explore parameters such as temperature, pH, salinity, etc. We built a comprehensive, generalized model of the adsorption mechanisms of nucleotides onto phyllosilicate particles, mainly governed by phosphate reactivity. More recently, we used surface chemistry and geochemistry techniques, such as vibrational spectroscopy, low pressure gas adsorption, X-ray microscopy, and theoretical simulations, in order to acquire direct data on the adsorption configurations and localization of nucleotides on mineral surfaces. Although some of these techniques proved to be challenging, questioning our ability to easily detect biosignatures, they confirmed and complemented our pre-established model.

Serpentinization: Connecting Geochemistry, Ancient Metabolism and Industrial Hydrogenation

Rock⁻water⁻carbon interactions germane to serpentinization in hydrothermal vents have occurred for over 4 billion years, ever since there was liquid water on Earth. Serpentinization converts iron(II) containing minerals and water to magnetite (Fe₃O₄) plus H₂. The hydrogen can generate native metals such as awaruite (Ni₃Fe), a common serpentinization product. Awaruite catalyzes the synthesis of methane from H₂ and CO₂ under hydrothermal conditions. Native iron and nickel catalyze the synthesis of formate, methanol, acetate, and pyruvate-intermediates of the acetyl-CoA pathway, the most ancient pathway of CO₂ fixation. Carbon monoxide dehydrogenase (CODH) is central to the pathway and employs Ni⁰ in its catalytic mechanism. CODH has been conserved during 4 billion years of evolution as a relic of the natural CO₂-reducing catalyst at the onset of biochemistry. The carbide-containing active site of nitrogenase-the only enzyme on Earth that reduces N₂-is probably also a relic, a biological reconstruction of the naturally occurring inorganic catalyst that generated primordial organic nitrogen. Serpentinization generates Fe₃O₄ and H₂, the catalyst and reductant for industrial CO₂ hydrogenation and for N₂ reduction via the Haber⁻Bosch process. In both industrial processes, an Fe₃O₄ catalyst is matured via H₂-dependent reduction to generate Fe₅C₂ and Fe₂N respectively. Whether serpentinization entails similar catalyst maturation is not known. We suggest that at the onset of life, essential reactions leading to reduced carbon and reduced nitrogen occurred with catalysts that were synthesized during the serpentinization process, connecting the chemistry of life and Earth to industrial chemistry in unexpected ways.

Is pre-Darwinian evolution plausible?

Background

This essay highlights critical aspects of the plausibility of pre-Darwinian evolution. It is based on a critical review of some better-known open, far-from-equilibrium system-based scenarios supposed to explain processes that took place before Darwinian evolution had emerged and that resulted in the origin of the first systems capable of Darwinian evolution. The researchers’ responses to eight crucial questions are reviewed. The majority of the researchers claim that there would have been an evolutionary continuity between chemistry and “biology”. A key question is how did this evolution begin before Darwinian evolution had begun? In other words the question is whether pre-Darwinian evolution is plausible.

Results

Strengths and weaknesses of the reviewed scenarios are presented. They are distinguished between metabolism-first, replicator-first and combined metabolism-replicator models. The metabolism-first scenarios show major issues, the worst concerns heredity and chirality. Although the replicator-first scenarios answer the heredity question they have their own problems, notably chirality. Among the reviewed combined metabolism-replicator models, one shows the fewest issues. In particular, it seems to answer the chiral question, and eventually implies Darwinian evolution from the very beginning. Its main hypothesis needs to be validated with experimental data.

Conclusion

From this critical review it is that the concept of “pre-Darwinian evolution” appears questionable, in particular because it is unlikely if not impossible that any evolution in complexity over time may work without multiplication and heritability allowing the emergence of genetically and ecologically diverse lineages on which natural selection may operate. Only Darwinian evolution could have led to such an evolution. Thus, Pre-Darwinian evolution is not plausible according to the author. Surely, the answer to the question posed in the title is a prerequisite to the understanding of the origin of Darwinian evolution.

Reviewers

This article was reviewed by Purificacion Lopez-Garcia, Anthony Poole, Doron Lancet, and Thomas Dandekar.

Geochemistry and the Origin of Life: From Extraterrestrial Processes, Chemical Evolution on Earth, Fossilized Life's Records, to Natures of the Extant Life

In 2001, the first author (S.N.) led the publication of a book entitled "Geochemistry and the origin of life" in collaboration with Dr. Andre Brack aiming to figure out geo- and astro-chemical processes essential for the emergence of life. Since then, a great number of research progress has been achieved in the relevant topics from our group and others, ranging from the extraterrestrial inputs of life's building blocks, the chemical evolution on Earth with the aid of mineral catalysts, to the fossilized records of ancient microorganisms. Here, in addition to summarizing these findings for the origin and early evolution of life, we propose a new hypothesis for the generation and co-evolution of photosynthesis with the redox and photochemical conditions on the Earth's surface. Besides these bottom-up approaches, we introduce an experimental study on the role of water molecules in the life's function, focusing on the transition from live, dormant, and dead states through dehydration/hydration. Further spectroscopic studies on the hydrogen bonding behaviors of water molecules in living cells will provide important clues to solve the complex nature of life.

Geological and Geochemical Constraints on the Origin and Evolution of Life

The traditional tree of life from molecular biology with last universal common ancestor (LUCA) branching into bacteria and archaea (though fuzzy) is likely formally valid enough to be a basis for discussion of geological processes on the early Earth. Biologists infer likely properties of nodal organisms within the tree and, hence, the environment they inhabited. Geologists both vet tenuous trees and putative origin of life scenarios for geological and ecological reasonability and conversely infer geological information from trees. The latter approach is valuable as geologists have only weakly constrained the time when the Earth became habitable and the later time when life actually existed to the long interval between ∼4.5 and ∼3.85 Ga where no intact surface rocks are known. With regard to vetting, origin and early evolution hypotheses from molecular biology have recently centered on serpentinite settings in marine and alternatively land settings that are exposed to ultraviolet sunlight. The existence of these niches on the Hadean Earth is virtually certain. With regard to inferring geological environment from genomics, nodes on the tree of life can arise from true bottlenecks implied by the marine serpentinite origin scenario and by asteroid impact. Innovation of a very useful trait through a threshold allows the successful organism to quickly become very abundant and later root a large clade. The origin of life itself, that is, the initial Darwinian ancestor, the bacterial and archaeal roots as free-living cellular organisms that independently escaped hydrothermal chimneys above marine serpentinite or alternatively from shallow pore-water environments on land, the Selabacteria root with anoxygenic photosynthesis, and the Terrabacteria root colonizing land are attractive examples that predate the geological record. Conversely, geological reasoning presents likely events for appraisal by biologists. Asteroid impacts may have produced bottlenecks by decimating life. Thermophile roots of bacteria and archaea as well as a thermophile LUCA are attractive.

Green Rust: The Simple Organizing 'Seed' of All Life?

Korenaga and coworkers presented evidence to suggest that the Earth's mantle was dry and water filled the ocean to twice its present volume 4.3 billion years ago. Carbon dioxide was constantly exhaled during the mafic to ultramafic volcanic activity associated with magmatic plumes that produced the thick, dense, and relatively stable oceanic crust. In that setting, two distinct and major types of sub-marine hydrothermal vents were active: ~400 °C acidic springs, whose effluents bore vast quantities of iron into the ocean, and ~120 °C, highly alkaline, and reduced vents exhaling from the cooler, serpentinizing crust some distance from the heads of the plumes. When encountering the alkaline effluents, the iron from the plume head vents precipitated out, forming mounds likely surrounded by voluminous exhalative deposits similar to the banded iron formations known from the Archean. These mounds and the surrounding sediments, comprised micro or nano-crysts of the variable valence FeII/FeIII oxyhydroxide known as green rust. The precipitation of green rust, along with subsidiary iron sulfides and minor concentrations of nickel, cobalt, and molybdenum in the environment at the alkaline springs, may have established both the key bio-syntonic disequilibria and the means to properly make use of them-the elements needed to effect the essential inanimate-to-animate transitions that launched life. Specifically, in the submarine alkaline vent model for the emergence of life, it is first suggested that the redox-flexible green rust micro- and nano-crysts spontaneously precipitated to form barriers to the complete mixing of carbonic ocean and alkaline hydrothermal fluids. These barriers created and maintained steep ionic disequilibria. Second, the hydrous interlayers of green rust acted as engines that were powered by those ionic disequilibria and drove essential endergonic reactions. There, aided by sulfides and trace elements acting as catalytic promoters and electron transfer agents, nitrate could be reduced to ammonia and carbon dioxide to formate, while methane may have been oxidized to methyl and formyl groups. Acetate and higher carboxylic acids could then have been produced from these C1 molecules and aminated to amino acids, and thence oligomerized to offer peptide nests to phosphate and iron sulfides, and secreted to form primitive amyloid-bounded structures, leading conceivably to protocells.

Widespread abiotic methane in chromitites

Recurring discoveries of abiotic methane in gas seeps and springs in ophiolites and peridotite massifs worldwide raised the question of where, in which rocks, methane was generated. Answers will impact the theories on life origin related to serpentinization of ultramafic rocks, and the origin of methane on rocky planets. Here we document, through molecular and isotopic analyses of gas liberated by rock crushing, that among the several mafic and ultramafic rocks composing classic ophiolites in Greece, i.e., serpentinite, peridotite, chromitite, gabbro, rodingite and basalt, only chromitites, characterized by high concentrations of chromium and ruthenium, host considerable amounts of ¹³C-enriched methane, hydrogen and heavier hydrocarbons with inverse isotopic trend, which is typical of abiotic gas origin. Raman analyses are consistent with methane being occluded in widespread microfractures and porous serpentine- or chlorite-filled veins. Chromium and ruthenium may be key metal catalysts for methane production via Sabatier reaction. Chromitites may represent source rocks of abiotic methane on Earth and, potentially, on Mars.