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A substantially complete skull of a small crocodyliform recently found on the foreshore near Yaverland on the south-east coast of the Isle of Wight

In the context of the origin of life, phyllosilicate surfaces might favor the adsorption, concentration and reactivity of otherwise diluted prebiotic molecules. The primitive oceanic seafloor was certainly rich in Fe-Mg-rich phyllosilicates. The salinity of the primitive seawater remains largely unknown. Values ranging from 1 to 15 times modern salinity have been proposed and the salt composition of the primitive ocean also remains elusive although it may have played a role in the interactions between nucleotides and mineral surfaces. Therefore we studied the adsorption of 5'-monophosphate deoxyguanosine (dGMP) as a model nucleotide onto a Fe-rich swelling clay, i.e. nontronite, and an Al-rich phyllosilicate, i.e. pyrophyllite, for comparison. Experiments were carried out at atmospheric pressure, 25 °C and natural pH, with a series of salts NaCl, MgCl, CaCl, MgSO, NaHPO and LaCl in order to evaluate the effect of cations and anions on dGMP adsorption. The present study shows that nucleotides are adsorbed on both phyllosilicates via a ligand exchange mechanism. The phosphate group of the nucleotide is adsorbed on the lateral metal hydroxyls of the broken edges of phyllosilicates. The presence of divalent cations or molecular anions, such as phosphate or sulfate, tends to inhibit this interaction on mineral surfaces. However, in the presence of divalent cations, cationic bridging on the basal surfaces of the swelling clay also occurs and could induce a higher retention capacity of the swelling clays compared to non-swelling phyllosilicates in primitive and modern natural environments.

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.

The chemical evolution of early life requires the concentration of monomers to polymerize from the diluted primordial ocean. Transition metals such as Fe, Mn, and Zn, could have reached considerable levels in the early seawater and/or hydrothermal fluid but their influences on the adsorption of biomolecules have not been clearly addressed yet. In this study, we conducted batch adsorption experiments to explore effects of various metal cations (Li, Mg, Ca, Zn, Ni, and Mn) on the adsorption of selected nucleotides (dGMP, dAMP, and AMP) and adenosine onto nontronite and montmorillonite. We also varied the concentration of the cations and pH of the solutions to evaluate their effects. Our results show that Zn and to some extent Ni increase the adsorption of nucleotides and adenosine compared with Na, Mg, and Ca which are major salts in modern seawater. This increased adsorption is primarily attributed to the mediating role of transition metals between the clays and nucleotides and adenosine. The enhancing effect depends little on salt concentration, but strongly varies as the pH of the solution changes. Presence of transition metals reverses the declining trend of the adsorption of dGMP as the elevation of pH and strongly favors adsorption of dGMP at alkaline pH presumably through precipitation of metal-hydroxides on the clay surface. Enhanced adsorption amount of biomolecules mediated by transition metals would potentially ease the origin of life in two aspects: concentration of simple organics for polymerization and protection of early biomolecules against UV radiation and heating in early seawater.

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The abiotic polymerization of nucleotides and amino acids is a prerequisite for the emergence of life. It has been proposed that hydrothermal conditions might favor the polymerization of amino acids. In the present study, we analyzed by in situ Raman spectroscopy in a diamond anvil cell the fate of the simplest and most abundant amino acid, glycine, under hydrothermal conditions at 200 °C and pressures ranging between 50 and 3500 MPa. We also tested the effect of magnetite on the reactivity of glycine. The polymerization of glycine is highly favored under pressure and in the presence of magnetite. Linear dimers are more abundant than the cyclic ones up to a threshold pressure of 500 MPa. Above 800 MPa, amino acids stop reacting and the system is “frozen”. Our findings suggest that pressure and mineral–water interface strongly favor the formation of linear peptides. The optimum conditions for polymerization obtained in the present study suggest that the prebiotic chemical evolution of amino acids was not restricted to hydrothermal vents at oceanic ridges but might also occur much deeper in the first 15–30 km of the crust, widely expanding the prebiotic reactive zone.