The origin of life is one of the most fundamental questions of humanity. It has been and is still being addressed by a wide range of researchers from different fields, with different approaches and ideas as to how it came about. What is still incomplete is constrained information about the environment and the conditions reigning on the Hadean Earth, particularly on the inorganic ingredients available, and the stability and longevity of the various environments suggested as locations for the emergence of life, as well as on the kinetics and rates of the prebiotic steps leading to life. This contribution reviews our current understanding of the geological scene in which life originated on Earth, zooming in specifically on details regarding the environments and timescales available for prebiotic reactions, with the aim of providing experimenters with more specific constraints. Having set the scene, we evoke the still open questions about the origin of life: did life start organically or in mineralogical form? If organically, what was the origin of the organic constituents of life? What came first, metabolism or replication? What was the time-scale for the emergence of life? We conclude that the way forward for prebiotic chemistry is an approach merging geology and chemistry, i.e., far-from-equilibrium, wet-dry cycling (either subaerial exposure or dehydration through chelation to mineral surfaces) of organic reactions occurring repeatedly and iteratively at mineral surfaces under hydrothermal-like conditions.

Life is the outcome of a complex network of chemical reactions and molecular interactions that emerged on Earth once primitive chemical automata could self-assemble in such a way that enabled them to self-reproduce and evolve. Yet exactly how, where and when life first appeared on our planet remains unknown. In this chapter, we review the various lines of evidence from fossil and geochemical traces of early life preserved in the geological record, which provide fundamental, albeit often rudimentary, insight into early life. The oldest fossils record the nature of life more than half a billion years after it emerged on Earth and suggest that considerable metabolic diversity had already evolved by this time. Microfossils, microbial mats, stromatolites and organic and inorganic geochemical signatures have been interpreted as compelling evidence for Archean biodiversity. In spite of the chemical enigmas of the earliest life and its limited record, characterization of the various classes of biosignatures indicative of life in its geological context provides guidance as to the earliest co-evolution of the geosphere and biosphere.

During 20 years, the European astrobiologists collaborated within EANA, the European Astrobiology Network Association, to help European researchers developing astrobiology programmes to share their knowledge, to foster their cooperation, to attract young scientists to this quickly evolving interactive field of research, and to explain astrobiology to the public at large. The experiment of Stanley Miller in 1953 launched the ambitious hope that chemists would be able to shed light on the origins of life by recreating a simple life form in a test tube. However, the dream has not yet been accomplished, despite the great volume of effort and innovation put forward by the scientific community.

The primary aim of exobiology research is to recognize the routes leading to the initiation of life on Earth and its plausibility elsewhere in the universe. How would we recognize life if we encounter it or its remnants on an extraterrestrial body? This is the critical question of biosignature research to which astrochemical studies can contribute. Our understanding of preserved fossils and contemporary terrestrial life serves as a guide in the search for biosignatures in the universe. Of the various life-detection techniques available, carbon chemistry is particularly pertinent and perhaps the most significant biosignature (Summons et al., Astrobiology 11 (2):157-181; 2011). 'Life' as we know it is based on C, H, N, O, P, S chemistry and the organic matter derived from its remains is ubiquitous on Earth, constituting an extensive chemical and isotopic record of past life that surpasses by a huge margin what is recorded by visible (and microscopic) fossils. Biosignatures are highly subjective to the geological conditions in which they form and the subsequent diagenetic and metamorphic events that reprocess them (Sleep, Cold Spring Harb Perspect Biol. 2(6): a002527; 2010) and thus need careful assessing before coming to concrete conclusions concerning biogenicity. However, chemistry alone is inadequate to detect life and collaborative efforts from all of the relevant investigations, combined with considerations of geological and environmental factors, will likely provide the best evidence for the presence or absence of life, in localities of interest.

Chemists searching for chemical biosignatures begin to define the chemical prerequisites for the emergence of life, a process based on organized molecules capable of self-reproduction and also with the capability of evolution. It is generally accepted that these prerequisites are liquid water and organic molecules, i.e. molecules that contained carbon and hydrogen atoms associated with atoms of oxygen, nitrogen and sulphur. This is not just an anthropocentric point of view, since water and carbon chemistry have very specific peculiarities. Two different kinds of chemical biosignatures are considered: an overrepresentation of organics and a long strand of homochiral sequences.

A concise outlook on the potential role of confinement in phosphorylation and phosphate condensation pertaining to prebiotic chemistry is presented. Inorganic confinement is a relatively uncharted domain in studies concerning prebiotic chemistry, and even more so in terms of experimentation. However, molecular crowding within confined dimensions is central to the functioning of contemporary biology. There are numerous advantages to confined environments and an attempt to highlight this fact, within this article, has been undertaken, keeping in context the limitations of aqueous phase chemistry in phosphorylation and, to a certain extent, traditional approaches in prebiotic chemistry.

Mapping of Mars by orbiters revealed significant geologic evidence of past water activity, including enormous outflow channels carved by floods, ancient river valley networks, and lake beds. Further clues of water activity have come from the identification of aqueously altered rocks at the surface and from Martian meteorites. Three possible sources for organics are considered: a primitive atmosphere, hydrothermal systems, and space delivery of meteorites and micrometeorites. So far, the inventory of organic molecules found on Mars is rather poor, mostly in Martian meteorites. The early histories of Mars and Earth clearly show similarities, having in common liquid water and organic material. Chemistry being reproducible, processing identical ingredients should have produced the same effects. There are, however, some limitations: the complexity of the origin of terrestrial life and the three specific features of the Earth (generalized plate tectonics, global magnetic field, and a large moon generating oceanic tides). The discovery of a second independent genesis of life would demonstrate that life is not a magic one-shot process, but probably a rather common phenomenon.

Liquid water is essential for life as we know it, i.e. carbon-based life. Although other compound-solvent pairs that could exist in very specific physical environments could be envisaged, the elements essential to carbon and water-based life are among the most common in the universe. Carbon molecules and liquid water have physical and chemical properties that make them optimised compound-solvent pairs. Liquid water is essential for important prebiotic reactions. But equally important for the emergence of life is the contact of carbon molecules in liquid water with hot rocks and minerals. We here review the environmental conditions of the early Earth, as soon as it had liquid water at its surface and was habitable. Basing our approach to life as a "cosmic phenomenon" (de Duve 1995), i.e. a chemical continuum, we briefly address the various hypotheses for the origin of life, noting their relevance with respect to early environmental conditions. It appears that hydrothermal environments were important in this respect. We continue with the record of early life noting that, by 3.5 Ga, when the sedimentary environment started being well-preserved, anaerobic life forms had colonised all habitable microenvironments from the sea floor to exposed beach environments and, possibly, in the photic planktonic zone of the sea. Life on Earth had also evolved to the relatively sophisticated stage of anoxygenic photosynthesis. We conclude with an evaluation of the potential for habitability and colonisation of other planets and satellites in the Solar System, noting that the most common life forms in the Solar System and probably in the Universe would be similar to terrestrial chemotrophs whose carbon source is either reduced carbon or CO2 dissolved in water and whose energy would be sourced from oxidized carbon, H-2, or other transition elements.

Abstract Stanley Miller demonstrated in 1953 that it was possible to form amino acids from methane, ammonia, and hydrogen in water, thus launching the ambitious hope that chemists would be able to shed light on the origins of life by recreating a simple life form in a test tube. However, it must be acknowledged that the dream has not yet been accomplished, despite the great volume of effort and innovation put forward by the scientific community. A minima, primitive life can be defined as an open chemical system, fed with matter and energy, capable of self-reproduction (i.e., making more of itself by itself), and also capable of evolving. The concept of evolution implies that chemical systems would transfer their information fairly faithfully but make some random errors. If we compared the components of primitive life to parts of a chemical automaton, we could conceive that, by chance, some parts self-assembled to generate an automaton capable of assembling other parts to produce a true copy. Sometimes, minor errors in the building generated a more efficient automaton, which then became the dominant species. Quite different scenarios and routes have been followed and tested in the laboratory to explain the origin of life. There are two schools of thought in proposing the prebiotic supply of organics. The proponents of a metabolism-first call for the spontaneous formation of simple molecules from carbon dioxide and water to rapidly generate life. In a second hypothesis, the primeval soup scenario, it is proposed that rather complex organic molecules accumulated in a warm little pond prior to the emergence of life. The proponents of the primeval soup or replication first approach are by far the more active. They succeeded in reconstructing small-scale versions of proteins, membranes, and RNA. Quite different scenarios have been proposed for the inception of life: the RNA world, an origin within droplets, self-organization counteracting entropy, or a stochastic approach merging chemistry and geology. Understanding the emergence of a critical feature of life, its one-handedness, is a shared preoccupation in all these approaches.

The premise of this talk is that, apart from liquid water and carbon molecules, specific environmental components and conditions were essential for the origin of life, i.e. phosphate, reactive rock...

In this study, we attempt to illustrate the competition that constitutes the main challenge of astrobiology, namely the competition between the probability of extraterrestrial life and its detectability. To illustrate this fact, we propose a simple statistical approach based on our knowledge of the Universe and the Milky Way, the Solar System, and the evolution of life on Earth permitting us to obtain the order of magnitude of the distance between Earth and bodies inhabited by more or less evolved past or present life forms, and the consequences of this probability for the detection of associated biosignatures. We thus show that the probability of the existence of evolved extraterrestrial forms of life increases with distance from the Earth while, at the same time, the number of detectable biosignatures decreases due to technical and physical limitations. This approach allows us to easily explain to the general public why it is very improbable to detect a signal of extraterrestrial intelligence while it is justified to launch space probes dedicated to the search for microbial life in the Solar System.

This review introduces its readers to a 'stochastic approach' to origins of life research, from the viewpoints of both prebiotic chemistry and geology. The idea of a "primordial soup" has been subject to extensive criticism from thermodynamic, biochemical and geochemical perspectives, yet recent advancements have made clearer the plausibility of this theory. Herein, we review the theoretical and experimental approaches which have previously been explored, among these modelling, laboratory- confined and geologically motivated experimentation. Of these, we consider organo-mineral interactions, uniting aspects of prebiotic chemistry and geology, to be an especially promising way forward. However, we aim here to advance current approaches by advocating a methodology involving chemical systems and their stochastic reactivity on heterogeneous geological surfaces. This models the origins of life as a continuity of chemical reactions in an analogue to the early Earth (Hadean) environment.