# Life out there: expectations and reality

Frances Westall

## ► To cite this version:

Frances Westall. Life out there: expectations and reality. (Toward) Discovery of Life beyond Earth and its Impact, Hermione Landt, Apr 2024, Durham (GB), United Kingdom. pp.7-34, 10.1017/S1743921324002059. hal-05481360

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Submitted on 28 Jan 2026

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# Life out there: expectations and reality

Frances Westall

CNRS-Centre de Biophysique Moléculaire, Orléans, France

email: frances.westall@cnrs.fr

**Abstract.** We are in the age of the direct and indirect search for extraterrestrial life. The major question is: what are we looking for and to what extent can life on Earth provide an analogy for extraterrestrial life. We will address these questions by examining the emergence of life and its evolution through geological time, based on consideration of the Earth as an exoplanet. We conclude that, with the exception of a couple of hundred million years during the Great Oxidation Event 2.3-2.1 Ga, it would not have been possible for an extraterrestrial observer to detect traces of terrestrial life until the advent of planktonic eukaryotic algae after the Neoproterozoic Oxidation Event about 800 Ma. Thus, for most of the history of life on Earth, there would be no externally observable evidence of its existence. We address the implications of this for the search for extraterrestrial life.

**Keywords.** Exoplanets, extraterrestrial life, biosignatures, evolution of life on Earth, Earth as an exoplanet

## Introduction

The expanding possibilities brought by new space telescopes to search for habitable planets and, perhaps even, traces of life are exciting in prospect, fuelling the imagination. They are accompanied by incredible advances in technology on Earth in artificial intelligence. Mankind is endowed with intelligence and an active imagination and writers of science fiction from the 20th century are finding some of their stories coming true.

The hypothesis that life exists on other planets is predicated on a number of fundamental, Earth-centric considerations: (1) What is life? (2) How did life appear on Earth? (3) How did life evolve on Earth and how did the geological evolution of the Earth influence biological evolution? We will address these questions, using the Earth as an analogue of an exoplanet and considering at what stage signatures of life would have been detectable to extraterrestrial observers (cf. Westall et al. 2025). I will conclude with a short discussion on the consequences of these reflections for the search for extraterrestrial life.

In this contribution, my approach will be based on the geological record of the Earth and the evolution of terrestrial life, and the implications for biosignatures that could be recognised from space. In this respect, it could be considered as a minimalist approach to the consideration of life in the Universe.

## What is life?

With respect to the question, what is life, the NASA Astrobiology working definition states that life is a self-sustaining chemical system capable of Darwinian evolution. This definition has been revisited by Vitas and Dobovišek (2019) to address a thermodynamical aspect of life as a far from equilibrium system. It also includes the flow of information from the environment to the living system. Thus, the definition of Vitas and Dobovišek (2019) is the following: “Life is a far from equilibrium self-maintaining chemical system

© The Author(s), 2026. Published by Cambridge University Press on behalf of International Astronomical Union. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
capable of processing, transforming and accumulating information acquired from the environment." This is the definition that I will be using.

## How did life appear on Earth?

There are many scenarios concerning the emergence of life on Earth that have been reviewed previously (e.g. Westall et al. 2018, 2023a; Colón-Santos et al. 2024). Here, I will address the pertinent constraints for the emergence of life, i.e. the critical ingredients of life ("as we know it"), and briefly summarise the different scenarios suggested.

Life as we know it is composed of a variety of classes of organic molecules, namely amino acids, fatty acids, carbohydrates and nucleobases, that are assembled together to form the fundamental composites of a cell: proteins, phospholipids, polysaccharides, and an information transferring molecule (RNA/DNA). The principal elements of these critical molecules are the CHNOPS elements, as well as a number of transition elements used as cofactors and as important structural ingredients of the larger molecules. These cofactors include Fe, Zn, Mn, Co, Cu, Mo, Ni, W, and V (Zerkle et al. 2005). Zerkle et al. (op.cit.). However, note that Cu and Mo utilisation appeared with the oxygenation of the Earth through time and would not have been prevalent at the origin of life.

A wide variety of organic molecules was brought to the early Earth through accretion of extraterrestrial materials, such as meteorites (carbonaceous chondrites that comprise up to 3-4% organic matter, Sephton and Botta 2008; Schmitt-Kopplin et al. 2010; Martins and Pasek 2024) and micrometeorites (Maurette and Brack 2006; Dartois et al. 2013). While about 70% of extraterrestrial organic matter is insoluble (IOM), up to 30% is soluble (DOM). Meteorites and micrometeorites falling into the early seawater (or lakes) would break up and release the soluble organics into the water. The insoluble matter would have been disaggregated, particularly in shallow water environments subjected to storms and strong tidal action. Sedimented with the detrital fragments of the volcanic early Earth, the IOM would have been reprocessed by the very active hydrothermal systems at the surface and by hydrothermal fluids percolating through the upper crust (Martins and Pasek 2024).

Other sources of organic molecules to the early Earth would have included those formed in the atmosphere by lightening in CO₂-rich volcanic clouds (Cleaves et al. 2008) and those formed endogenously in the crust by Fischer Tropsch reactions during the serpentinisation of the ultramafic materials in the presence of hydrothermal fluids (Konn et al. 2009, 2015; McCollom et al. 2015).

The other critical ingredient for life is liquid water. This is because a liquid environment permits a variety of chemical interactions. On the one hand, certain molecules may be dissolved and transported while, on the other hand, others are hydrophobic and, therefore, stable. Thus, hydrophobic molecules, such as lipids, can form a micellar membrane that protects the internal machinery of a cell from a potentially hostile exterior environment while, at the same time, the porous hydrophobic membrane permits certain dissolved molecules to be transported in and out of the cell. Water is a particularly good combination with organic molecules (Brack 2002) because it is stable under a wide range of temperature and pressure conditions. Its density permits it to maintain critical concentrations of reactants, such as salts. It has high heat retention and surface tension, and a dielectric constant. Moreover, the fact that it is a polar molecule facilitates covalent bonding and therefore polymer building. Finally, water contributes to the structural organisation of molecules.

Finally, no chemistry can take place without an energy source. A large number of potential sources of energy for prebiotic reactions on the early Earth has been suggested. They range from highly energetic sources, such as UV (Pascal et al. 2013) and lightening (Pascal et al. 2013; Cleaves et al. 2008), through shock impact of meteorites (Martins et al. 2013), chemical redox reactions between water and highly reactive minerals, such as
sulphides (Wächtershäuser 1998) and ferrous oxides (Russell and Hall 1997; Martin et al. 2008), to piezoelectric energy produced in deep-seated fault systems (Schreiber et al. 2012). Certain scenarios for the emergence of life suggest a nuclear energy source for the prebiotic reactions (Adam 2007; Adam et al. 2018) but there is no evidence for such an energy source on the early Earth (Westall et al. 2023a).

Within the constraints of the ingredients of life as we know it and the potential energy sources available, many hypotheses have been expounded as to where and how life emerged (see reviews in Westall et al. 2018, 2023a; Colón-Santos et al. 2024). Very briefly, the scenarios envisaged fall into two main groups, those implicating some form of hydrothermal activity (the majority) and those predicated on other sources of energy. Hydrothermal environments are popular for a number of reasons. They provide a source of energy in the form of heat and redox reactions and may be a conduit for organic molecules in hydrothermal effluent (e.g. Kelley et al. 2001; Konn et al. 2009, 2015; McCollom et al. 2015; Ménez et al. 2018). Hydrothermal environments are characterised by porous materials, either the vent constructs themselves or the sediments that surround them. These materials comprise reactive materials, either metal sulphides or mafic (magnesium and iron rich) minerals that produce energy through redox reactions at their surfaces. The surfaces of the reactive minerals and their alteration products (phyllosilicates, for instance), facilitate the concentration and conformation of organic minerals. Moreover, gradients in temperature, concentrations, pH form within the porous hydrothermal materials and gradients mean disequilibria, an essential element for abiogenesis and prebiotic chemistry.

Hydrothermal environments on the present Earth and on the early Earth occur in a wide range of settings. They are most commonly associated with deep sea rifting and plume volcanism, the higher temperature black smoker vents occurring closer to the volcanic sources with lower temperature white smokers or diffuse venting occurring further away. However, hydrothermal activity is not only a deep-sea phenomenon but also occurs in shallow water environments. This was particularly the case on the volcanically more active and hotter early Earth, as demonstrated by evidence from Palaeoarchaean rocks (Hofmann and Harris 2008; Westall et al. 2015a, 2023a). Moreover, there is some evidence for subaerial hydrothermal activity (at the edges of exposed landmasses) on the early Earth (Djorkic et al. 2017; Van Kranendonk et al. 2021). Furthermore, hydrothermal activity at the edges of impact craters may have been of importance (Osinski et al. 2013, 2020; Sasselov et al. 2020).

Hydrothermal environments offer a variety of characteristics that would have been potentially useful for prebiotic chemical reactions (cf. Russell and Hall 1997; Martin et al. 2008; Westall et al. 2018, 2023a; Van Kranendonk et al. 2021). Their generally porous structures comprise mafic (Fe and Mg-rich) minerals either as precipitates of hydrothermal effluent (metal oxides in black smokers), as well as detrital volcanic sediments, or the products of aqueous alteration of the mineral surfaces (phyllosilicate minerals, such as clays). All of these minerals have reactive surfaces that are conducive for trapping or chelating organics and that can contribute to their structural organisation and complexification. The porous hydrothermal structures and sediments permit the circulation of fluids and the formation of gradients in pH, temperature and concentration of reactants, thus creating chemical disequilibria.

Even though no deep-sea black smoker from early Earth environments have been preserved, given our knowledge of the geology of the early Earth, they must have existed. Thus, Westall et al. (2023a) conclude that subaqueous or subaerial hydrothermal environments, some exposed to UV radiation, some not, could have hosted abiogenesis. However, there is little evidence for other environments suggested for abiogenesis, pumice rafts (Brasier 2011), nuclear placer sands (Adam 2007; Adam et al. 2018), although deep seated faults would have been present (Schreiber et al. 2012).
Many scenarios for the potential emergence of life on other planetary bodies have been hypothesised for example hydrothermal activity at the edges of impact craters on Mars (Osinski et al. 2013, 2020; Sasselov et al. 2020), as well as subaqueous hydrothermal activity (Westall et al. 2015b). In the case of an early (Pre-Noachian) land-locked planet such as Mars, environments exposed to UV radiation would have abounded. This may or may not have been the case on Venus (Westall et al. 2023b). However, such UV-bathed environments would not have been available on ocean planets. The question is, if UV energy was a prerequisite for the emergence of life on Earth, does this mean that life could not have emerged on ocean planets? To date, we do not know how life emerged on Earth. Moreover, if it did emerge in only one particular scenario, does this mean that it could not emerge in another on another planet? The finding of life on another rocky planet or an icy satellite in the Solar System will be highly significant in terms of confirming de Duve's (1995) hypothesis that, in the an appropriate environment, biology should emerge naturally from chemistry. Finding life on an icy satellite will tell us that life can appear without UV radiation, but not that this would be the only way in which it could occur. Nor will finding life on Mars constrain the environments for abiogenesis. Based on these essential requirements, life likely emerged on Earth during the Hadean Eon (4.5-4.0 Ga), after the Moon-forming impact, variously dated between 4.51 Ga (Barboni et al. 2017) and 4.36 Ga (Borg and Carlson 2023).

## How did life evolve on Earth and how did the geological evolution of the Earth influence biological evolution?

### *Eo-Palaeoarchaean eras: earliest traces of life*

Some of the most ancient rocks and minerals on Earth may contain indications that life was present. 4.1 Ga old zircon crystals contain graphitic inclusions, whose $\delta^{13}$C isotopic composition is consistent with that produced by life (Bell et al. 2015), but is also consistent with carbon isotope signatures from abiotic matter (cf. Summons et al. 2011). The oldest known sediments are preserved in 3.7-3.8 Ga rocks from the Isua Greenstone Belt in Greenland. They comprise highly metamorphosed rocks, including a banded iron formation (BIF). Again, evidence for life in these rocks was interpreted from carbon isotopes (Schidlowsky et al. 1988; Rosing et al. 1999). More recently, iron isotope measurements were used to interpret the possible presence of anaerobic phototrophs (Czaja et al. 2013), while Hassenkam et al. (2017) analysed carbonaceous inclusions in garnet crystals in the metasediments (metamorphosed sediments), concluding that the carbonaceous molecules were likely remnants of life. Metasediments of controversial age (4.3-3.7 Ga, Cates et al. 2013) from the Nuvvuagittuq terrane in northern Canada contain an apparent BIF in which large filamentous structures have been interpreted as iron oxidising microbes from hydrothermal vents (Dodd et al. 2017; Papineau et al. 2022). However, the filaments may be of abiotic origin (McMahon 2019; Lan et al. 2022). Moreover, the sediments are now believed to be Eoarchaean (4.0-.06 Ga) rather than Hadean (4.56-4.0 Ga) in age (Darling et al. 2013).

These are the oldest traces of life known to date. It could be argued that they are “ten-uous” in the sense that, normally, reliable identification of traces of life would require a number of concurring lines of evidence, for example, physical remains of cells, colonies or microbial constructions; organic, isotopic or geochemical signature, amongst others (Summons et al. 2011; Neveu et al. 2018). However, in light of the very clear evidence of life in well-preserved metasediments in the Palaeoarchaean (3.6-3.2 Ga) (reviewed in Westall and Xiao 2025), the existence of life in Eoarchaean sediments is tenable. As of the Paleaoarchean, the fossilised evidence for life becomes stronger. Importantly, the oldest traces of life in these sediments document not only its widespread distribution (to be clear,
its occurrence is limited to the few, preserved Palaeoarchaean terranes in the Pilbara Greenstone Belt, Australia and the Barberton Greenstone Belt in South Africa), but also its rapid evolution. The first cells are believed to have been chemolithotrophic autotrophs, i.e. organisms obtaining energy from oxidation of inorganic substances, such as hydrogen or sulphur, or redox reactions on mineral surfaces (e.g. (Russell and Hall 1997; Martin et al. 2008)). However, by about 3.5 Ga there is evidence of chemorganotrophs, i.e. organisms obtaining energy by oxidation of carbonaceous matter (Westall et al. 2006a, 2011, 2015a, 2025; Hickman-Lewis et al. 2020a; Clodoré et al. 2024), as well as phototrophs, i.e. organisms obtaining energy from sunlight (Noffke et al. 2008, 2013; Byerly et al. 1986; Walsh 1992; Hofmann et al. 1999; Tice and Lowe 2004; Allwood et al. 2006; Westall et al. 2006b), to name just a few of the many studies). All of these organisms were obligate anaerobes because oxygen was not present on the early Earth (although very small quantities were produced by a variety of abiotic processes, Westall et al. 2023a). Thus, the earliest, well-preserved biosignatures at 3.5 Ga document an already flourishing and evolved biome. This does not mean that life “started out with a bang”, it is simply a reflection of the fact that any earlier traces of life were not preserved.

## The Eo-Palaeoarchaean Earth (4.0-3.2 Ga) as an exoplanet

At this point, considering the search for life elsewhere in the Solar System and on exoplanets, it is useful to step back and observe the early Earth from the eyes (or space telescopes) of an extraterrestrial spectator. First of all, spectral data of the atmosphere of the early Earth would reveal the presence of $N_2$, $CO_2$, and possibly traces of $CH_4$ and $H_2O$, depending upon the sensitivity of the instruments. The James Webb Space Telescope, for example, recently detected $CH_4$ and $CO_2$ (but no $NH_3$, $H_2O$ or $CO$, Madhusudhan et al. 2021, 2023) in the atmosphere of planet K2-18 b, a “Hycean” world. These gases were interpreted as a potential biosignature (op.cit.), although Wogan et al. (2024) consider the planet to be a mini Neptune with $CH_4$ and $CO_2$ produced thermochemically. “Hycean” worlds are hypothetical planets that would resemble $H_2O$-rich sub-Neptunes with habitable surface oceans (Madhusudhan et al. 2021). There have been a number of recent reviews on potential exoplanet biosignatures (e.g. Catling et al. 2018; Schwieterman et al. 2018; Schwieterman and Leung 2024; Quanz et al. 2021). Quanz et al. (2021, after Catling et al. 2018) that list a number of atmospheric gases that could potentially be detected in exoplanet atmospheres. Some of these are considered to be biosignatures if both reduced and oxidised gases occur simultaneously, e.g. $CH_4$ and $O_3$, although both, individually, can be produced abiotically. A variety of additional potential gases of biological (and sometimes abiotic) origin has been proposed (Table 1), including the phosphine that was detected in the atmosphere of Venus (Greaves et al. 2021; Bains 2024; Schwieterman and Leung 2024). Importantly, the presence of gases in disequilibria would suggest some form of permanent production, for example $O_2/O_3$ and $CH_4$, $CO_2$ and $CH_4$, $N_2$ with $O_2$ and $H_2O$, or $N_2O$ and $O_2/O_3$ (Schwieterman and Leung 2024).

Other signatures that may signify the presence of life on an exoplanet include: pigments of various kinds, a vegetation red edge, molecular chirality, fluorescence and bioluminescence, and seasonality owing to temporal changes in gas levels due to seasonal productivity (Schwieterman and Leung 2024; see Table 2). Habitability may be indicated by “ocean glint” (Sagan et al. 1993) or hydrogen exospheres (Kameda et al. 2017). Recently Triaud et al. (2024) have proposed an alternative combination of signatures that might indicate life on an exoplanet that includes liquid water: “A low carbon abundance in the atmosphere of a temperate rocky planet, relative to other planets of the same system, traces the presence of a substantial amount of liquid water, plate tectonics and/or biomass.”
<table><caption>**Table 1.** Potential biosignature and non biosignature gases in exoplanet atmospheres (after Quanz et al. 2021).</caption><thead><tr><th></th><th>Molecule</th><th>Origin</th></tr></thead><tbody><tr><td rowspan="7">Potential biosignature gases</td><td>CH<sub>4</sub></td><td>Abiotic: volcanic outgassing<br>Biogenic: methanogenesis</td></tr><tr><td>O<sub>3</sub></td><td>From O2 via photochemical reactions</td></tr><tr><td>N<sub>2</sub>O</td><td>Abiotic: lightning, photochemical reactions, chemodenitrification<br>Biogenic: denitrification</td></tr><tr><td>NH<sub>3</sub></td><td>Abiotic production in hydrothermal vents<br>Biogenic: volatilisation of organic waste</td></tr><tr><td>CH<sub>3</sub>Cl<br>(CH<sub>3</sub>)<sub>2</sub>S</td><td>Biogenic: from algae and tropical forests<br>Biogenic: from plankton</td></tr><tr><td>CH<sub>3</sub>SH</td><td>Biogenic: mercaptogenesis</td></tr><tr><td></td><td></td></tr><tr><td rowspan="10">Other possible atmospheric gases</td><td>CO<sub>2</sub></td><td>Abiogenic: photodegradation, thermal degradation, oxidation of reactive oxidative species, inorganic chemisry<br>Biogenic: microbial degradation, extracellular oxidative metabolism</td></tr><tr><td>H<sub>2</sub>O</td><td>Abiogenic: from serpentinisation in crust, free molecules and ice</td></tr><tr><td>N<sub>2</sub></td><td>Abiotic: redox reactions of Fe, Mn, photochemical and thermochemical reactions<br>Biogenic: enzymer-mediated biological processes</td></tr><tr><td>CO<sub>2</sub></td><td>Abiotic: CO2 photolysis and volcanoes<br>Biogenic: photooxidation of organic matter</td></tr><tr><td>C<sub>2</sub>H<sub>6</sub></td><td>Abiotic: photochemically produced from CH4 and other C-bearing species</td></tr><tr><td>H<sub>2</sub></td><td>Abiogenic: volcanic outgassing</td></tr><tr><td>H<sub>2</sub>S</td><td>Abiogenic: Volcanic gas<br>Biogenic: microbial degradation</td></tr><tr><td>SO<sub>2</sub></td><td>Abiogenic: volcanic gas</td></tr><tr><td>H<sub>2</sub>SO<sub>4</sub></td><td>Abiogenic: volcanic gas</td></tr><tr><td>HCN</td><td>Abiogenic: hydrothermal environments, electrical discharge</td></tr></tbody></table>

<table><caption>**Table 2.** Variety of potential biosignatures (after Schwieterman and Leung 2024).</caption><thead><tr><th colspan="3">Surface and temporal biosignatures</th></tr><tr><th>Biosignatures</th><th>Origin</th><th>False positives</th></tr></thead><tbody><tr><td>Photosynthetic pigments</td><td>Clorophyll, bacteriochlorophyll</td><td>None</td></tr><tr><td>Non-photosynthetic pigments</td><td>Microorganism reflectance from pigments</td><td>None</td></tr><tr><td>Retinal pigments</td><td>Halaeoarchaea, bacteriorhodopsin</td><td>None</td></tr><tr><td>Vegetation red edge</td><td>Plant leaves, phototrophic cells</td><td>Surface minerals</td></tr><tr><td>Chirality</td><td>Nearly all biological molecules</td><td>weak enantiomeric excess in carbonaceous chondrites</td></tr><tr><td>Fluorescence and bioluminescence</td><td>Reprocessing or direct production of light from biological organisms</td><td>Production by non biological minerals</td></tr><tr><td>Seasonality</td><td>Temproal changes due to orbitally-constrained biological productivity</td><td>None</td></tr></tbody></table>

Chirality

Another biosignature noted by Schwieterman and Leung (2024) is chirality. Chirality refers to the structural configuration of molecules that can be mirror images of each other (L-left-, R-right handed). In biology, organic molecules are characterised by a distinct chirality, either left handed or right handed. For example, most terrestrial organisms synthesise and use α-amino acids exclusively in the L-form (homochirality), although some bacteria make and use D-amino acids (Summons et al. 2008). Degradation of the
organic molecules with time tends to obliterate this effect. However, in order to determine
the chirality of any organic matter on another planetary body, it is necessary to be able
to sample the surface of the planet. We will see below that we have enough difficulty
in sampling and analysing the surfaces of other planets in the Solar System that it is
difficult to imagine a space faring life form from another stellar system being able to
sample the Earth at any period during its geological history.

*The appearance of phototrophy on Earth and its importance for the production of space-visible biosignatures*

Considering this variety of atmospheric, surface and temporal phenomena that could
indicate life on the surface of a planet, what could have been detected on an early Earth
upon which life, in the form of anaerobic chemotrophs and phototrophs, flourished? In
terms of gases, the only potential biogas might have been CH₄ (if it had been produced
in sufficient quantities) and this would have been associated CO₂, but CH₄ can also
be produced by volcanic outgassing and serpentinisation reactions, both of which would
have been prevalent on the early Earth. In terms of any surface or temporal signatures,
we could consider photosynthetic and nonphotosynthetic pigments and chirality from
the list in Table 1. We have seen that the early, anaerobic terrestrial biome comprised
both non photosynthetic microbes as well as photosynthetic ones, the former producing
a variety of pigments and the latter characterised by chlorophyll and bacteriochlorophyll,
all of which could have spectral signatures mimicked by minerals. Anaerobic chemotrophs
on the early Earth inhabited benthic environments, generally away from the photic zone
(Westall et al. 2006a, 2015a,b; Hickman-Lewis et al. 2020a; Clodoré et al. 2024; Westall
and Xiao 2025). Whether or not they had pigments, their existence would not have been
visible from space. Phototrophs, however, abounded in shallow water environments. The
question is, what biomass did they represent and would they have been visible?

Des Marais (2000) calculated that the ability to capture photons as a source of energy
that vastly increased the productivity of microorganisms: while chemolithotrophs obtain
about (0.2 to 2.0) × 10¹² mol C per year from inorganic (hydrothermal) energy sources,
oxygenic phototrophs produce 9000 × 10¹² mol C per year, i.e. two to three orders of
magnitude greater carbon production. However, oxygenic phototrophy appears to have
originated on Earth during the Mesoarchean (3.2-2.8 Ga), around 3.0 Ga (see review in
Westall and Xiao 2025). We noted above the evidence for phototrophic mat development
by 3.5 Ga (Noffke et al. 2013), as well as geochemical suggestions of possible (anoxygenic)
phototrophy by ca. 3.8 Ga (Czaja et al. 2013). The lack of evidence for the production of
oxygen associated with the Palaeoarchaean phototrophic mats indicates that the earliest
phototrophs were anoxygenic. The evolution of anoxygenic phototrophy predated the
more complicated but more efficient energy producing process of oxygenic phototrophy,
whereby sunlight is used to split the water molecule (in abundant supply on Earth) to
obtain hydrogen, which is then oxidised, while oxygen is liberated as a waste product. The
early appearance of anoxygenic phototrophs was confirmed by gene sequencing analysis
of the photosynthetic green sulphur bacterium *Chlorobium tepidum* and the green non-
sulphur bacterium *Chloroflexus aurantiacus* (Xiong et al. 2000), which documented that
purple bacteria were the earliest emerging photosynthetic lineage and that bacteriochloro-
phyll biosynthesis definitely evolved before chlorophyll biosynthesis. Recent estimations
of an earlier appearance of oxygenic photosynthesis combining comparative structural
biology, phylogenetic analyses and molecular clocks with experimentation and ecological
studies suggests that oxygenic photosynthesis appeared already in the Palaeoarchaean
/Cardona et al. 2019; Sánchez-Baracaldo and Cardona 2020). However, there is no direct
fossil evidence for this and the geological underpinning of these trees is erroneous (Westall
2025).
Let us now consider the environments in which the early anoxygenic phototrophs flourished. The available phototrophic habitats comprised the shallow water environments around exposed landmasses. Today, planktonic phototrophy in the oceans is dominant but, on the early Earth, tenuous evidence for planktonic phototrophs only starts appearing after about 3.2 Ga (Javaux et al. 2010). Additionally, some large (tens of microns), enigmatic fossils from 3.43 Ga sediments in the Pilbara, Australia, have been interpreted as possible planktonic microorganisms of unknown affinity (Sugitani et al. 2015). These interpretations are still debated (e.g. Kapanarthy et al. 2024). Some studies have suggested that oxygenic planktonic phototrophs existed already by 3.8 Ga on the basis of bulk carbon isotope signatures and the presence of uranium measured in the metamorphosed sediments from the Isua Greenstone Belt, Greenland (Rosing et al. 1999; Rosing and Frei 2004). This date was also suggested by the phylogenetic studies of Sánchez-Baracaldo et al. (2022). However, modern contamination of the metasediments by endolithic oxygenic phototrophs (FW, personal observation, see also Westall and Folk 2003) will influence carbon isotopes measured on the bulk sample. Moreover, small amounts of uranium, an element most easily transported in its oxidised form and, hence, a frequent proxy for aerobic conditions, were common on the early Earth. At that time, very small and local amounts of oxygen were produced by abiotic processes, such as photolysis at the surface of water or dissociation of water vapour exiting shallow hydrothermal vents at high pressure (cf. Westall et al. 2023a). We can therefore exclude the planktonic realm from the Eo-Palaeoarchaean eras.

Landmasses on the early Earth comprised exposed land around volcanoes formed above mantle hot spots (similar to the Hawai'i Islands or the Réunion archipelago), or they formed on oceanic plateaus topped by volcanoes (similar to the Kerguelen Islands) that represented the protocontinents (Westall et al. 2023a; Westall 2024). Most of these low relief volcanic island landmasses formed in relatively shallow waters, similar to semi-enclosed basins around the volcanoes (cf. Nijman et al. 2017). The photic zone around the exposed landmasses would have been rather turbid during volcanic eruptions but in between eruptions because of the volcanic airfall would have been clearer. This is documented in sediments in the Palaeoarchaean terranes of the Pilbara and Barberton Greenstone Belts, where there is widespread evidence for flourishing phototrophic microbial mats (Noffke et al. 2008, 2013; Tice and Lowe 2004; Westall et al. 2006b; Hickman-Lewis et al. 2018, 2020b), and rare stromatolites (Byerly et al. 1986; Walsh 1992; Hofmann et al. 1999; Allwood et al. 2006). However, these are just two locations of probably many more that were not preserved.

To date, it is difficult to estimate the extent of shallow water biomes that would have been available to the early phototrophs. Estimates of the rate and amount of continental crust formation do not provide information about the aerial extent of the shallow water regions, because most of the early continents were effectively submerged (Fig. 1). The semi-circular water basins (collapse basins, Nijman et al. 2017) formed on top of them were not necessarily very deep, perhaps 50 to several hundreds of metres deep. Analysis of the sediments deposited in these basins documents shoreface to foreshore environments that were periodically influenced by storms with their deep reaching waves (Lowe 1999; de Vries et al. 2010; Westall et al. 2015a; Nijman et al. 2017). These shallow water environments were the recipients of nutrients sourced from the eroded, exposed landmasses and transported by rivers into the littoral areas around the landmasses, nutrients that contributed to the nourishment of the early microbial colonies (cf. Hickman-Lewis et al. 2020b). In the quieter periods in between volcanic eruptions, phototrophic mat formation could be quite extensive. For example, Westall et al. (2015a) document the formation of microbial mats in a period of quiet sedimentation (possibly lasting for hundreds of thousands of years, maybe even up to a million years; note that anaerobic phototrophs develop
<figure><img src="image_2.png" /></figure>

**Figure 1.** 3D schematic showing the development of broad, shallow semi-circular basins at the edges of exposed volcanic edifices, formed above oceanic plateaus (the protocontinents). The inset indicates the higher tidal resonance for the Palaeoarchaean compared to today (Nijman et al. 2017).

much more slowly than aerobic ones), in a basin that was probably many kilometres in diameter. In comparison, the modern Laguna Figuero on the Pacific coast of Mexico supports a vibrant (aerobic) phototrophic microbial mat community over an aerial extent of about 8 km² that was compared to microbial mats of the Palaeoarchaean (Margulis et al. 1980). Given the degree of UV penetration through the early atmosphere because of the lack of attenuating oxygen and ozone (Sagan and Mullen 1972; Tehrany et al. 2002; for example, Cockell 2000 calculated DNA-weighted UVB and UVC values of 10 W/m², and up to 1000W/m² in a worse case scenario, compared to 1W/m² today), the early phototrophs would necessarily have had recourse to protective pigments.

In geological terms, the existence of Palaeoarchaean anaerobic phototrophic microbial mats having an aerial extent of several square kilometres and persisting over a period of several hundreds of thousands of years up to a million years is not significant. Moreover, this would be one isolated example on a planet that may have hosted several contemporaneously isolated examples. At best, the situation could be described as spatially and temporarily ephemeral and it is unlikely that any pigments could have been detected from space.

With the evolution of oxygenic photosynthesis and oxygenic photosynthesisers during the Mesoarchaean (3.2-2.8 Ga), microbial productivity increased by orders of magnitude, as noted by Des Marais (2000). Larger, domal stromatolites started to appear in the Neoarchaean (2.8-2.4 Ga) on well-developed continental platforms, at the same time as gradual changes in the geological evolution of the Earth led to the stabilisation of continents and the availability of greater extents of shallow water environments for colonisation by benthic phototrophic microbial mats (Westall and Xiao 2025).
*The Meso-Neoarchaean Earth (3.2-2.4 Ga) and the flourishing of oxygenic photosynthesis*

We discussed above the importance of photosynthesisers and their biosignatures potentially detectable from space, namely pigments. We noted also that oxygenic photosynthesisers are able to obtain orders more energy through reduction of the water molecule, thus liberating oxygen (Des Marais 2000). One of the limiting factors for the appearance of this very effective means of obtaining energy could have been the availability of nutrients, in particular phosphorus. Indeed, as of the Mesoarchaean (3.2-2.8 Ga), there was a gradual but significant change in geological processes on the Earth, processes that resulted in the emergence of stable continents and contributed to an increase in the amount of nutrients brought into the shallow seas. At this stage, plate tectonics resulted in the amalgamation of the protocontinents whose granitic roots were eroded, thus permitting phosphorus to be transported by rivers to the sea.

Fossil evidence of oxygenic phototrophy appears in the Mesoarchaean. Stromatolites and associated geochemical signatures documenting the presence of oxygen are preserved in the 2.9 Ga Pongola Supergroup (Eickmann et al. 2018), while there is abundant evidence for the development of phototrophic mats on quartzitic sediments, the quartz reflecting erosion of granitic cores and therefore the presence of evolved continents (Noffke et al. 2008; Heubeck 2009; Homann et al. 2015). Cavity-dwelling microbes in the 3.2 Ga Moodies Group sediments from the Barberton Greenstone Belt are hypothesised to have been oxygenic phototrophs (Homann et al. 2015). Stromatolites become increasingly more important in the late Mesoarchaean and the Neoarchaean (2.8-2.4 Ga), occurring on terranes as far flung as North America, Australia and Southern Africa.

*The Great Oxidation Event and further evolution during the Proterozoic*

The oxygen produced by the flourishing oxygenic microorganisms (cyanobacteria) of the Neoarchaean did not immediately appear in the Earth's atmosphere. The large amounts of reduced terrestrial rocks and minerals in the oceans, especially the shallow seas, acted as sinks for oxygen. The reductants, H₂, C, S, Fe²⁺, that were oxidised were those previously utilised as energy sources for anoxygenic microorganisms. Their removal was a stimulus for the exploitation of another reductant, i.e. water, through oxygenic photosynthesis.

When most of the surficial, reduced materials had been oxidised, there is very strong evidence for a sudden rise in atmospheric oxygen at the end of the Neoarchaean/beginning of the Palaeoproterozoic, after 2.4 Ga, termed the Great Oxidation Event (GOE, Holland 2002; Lyons et al. 2014). The GOE coincided with major mantle overturn, the production of large igneous provinces, and the amalgamation of supercontinents and their subsequent erosion and breakup, as illustrated in Figure 2 (Gumsley et al. 2017; Chen et al. 2022). The change from a reducing atmosphere to an oxidising one is strongly marked in the mass dependent/mass independent sulphur isotope record (Fig. 3) (Farquhar et al. 2007). The significant rise in atmospheric oxygen and the consequent massive increase in biomass development led to the strongest known carbon isotope excursion in the geological record, the Lomagundi Excursion event that lasted from 2.3 to 2.1 Gyr ago (Karhu and Holland 1996; Bekker et al. 2006). These events brought the oxygen content of the atmosphere up to present day levels and lasted about 200 million years. Such an event would have been recognisable from space, but for a relatively short period of time (from the geological point of view). At this point, an extraterrestrial observer would have identified the presence of H₂O, O₂, O₃, and CO₂ in the atmosphere. Depending upon the observational capacities, perhaps land distribution would have been visible as well. Thus, evidence of life would have been detectable for the short window of time between 2.3-2.1 Ga.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Co-evolution of atmospheric oxygen with crustal processes, tectonics and LIPs through geological time. (Chen et al. 2022).</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Sulphur and carbon (carbonate) isotope variations throughout geological time (Lyons et al. 2014).</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 4.** Precambrian evolution of life on Earth (Agič 2021).</figcaption></figure>

However, the GOE was followed by a dramatic fall in productivity owing to an “overkill” in biomass activity. The excessive, frenetic activity of the cyanobacteria exhausted available nutrients and carbon sources and led to a crash in their life style. Nevertheless, they continued to flourish and dominated the Proterozoic biome until about 800 Ma. Oxygen levels in the atmosphere remained modest (Fig. 2) and, while the upper oceans were oxidised, the deeper oceans were reduced (Canfield, 1998), except for occasional, local oxidation events (e.g. Xu et al. 2023). From a geological point of view, the Proterozoic, especially the Mesoproterozoic from about 1.8-0.8 Ga, was a relatively quiet era and is often referred to as the “boring billion” (e.g. Lyons et al. 2014).

The Proterozoic Eon (2.4-0.56 Ga) was dominated by very large expanses of stromatolitic bioconstructions covered well-developed carbonate platforms around the margins of the stabilised continents (Westall and Xiao 2025). We note that throughout most of the Proterozoic, the phototrophs were concentrated on shallow water platforms around landmasses. Although the platform areas covered a wider area than those of earlier eras (Meso-Neoproterozoic), the distribution of the phototrophs was still relatively limited. Oxygen levels in the atmosphere remained relatively low (Fig. 2) and the pigments associated with the phototrophs would have had a limited distribution. Detection of atmospheric or a pigment biosignature from space during much of the Proterozoic may have been difficult.

Life continued to evolve through the Proterozoic with the appearance of eukaryotes, organisms comprising membrane-bound organelles (Fig. 4, Agič 2021). It is difficult to determine when eukaryotes first appeared. The fossil remains of single-celled eukaryotes are readily confused with those of the single cell prokaryotes. Molecular clocks provide some indication but are not definitive. For example, estimates of the divergence of crown-group eukaryotes (i.e., the age of the last eukaryotic common ancestor, or LECA) range from >2,300 Ma to ~1,000 Ma (Agič 2021; Westall and Xiao 2025).

What is important for our discussion is the emergence of planktonic algae. As noted above, stromatolites and their cyanobacterial-dominated communities were responsible for the production of large amounts of oxygen in the Proterozoic, but these oxygenic phototrophic microorganisms develop best under conditions where there is a significant
amount of CO2. A decrease in atmospheric CO2 would have inhibited their development and contributed to the flourishing of eukaryotic animals, such as green and red algae. Moreover, as Monty (1974) noted, blooms of algae produce toxins that have negative effects on potential eukaryotic grazers. This may have had an inhibiting effect on the rise of the latter in the Neoproterozoic. There is convincing evidence for multicellularity (and eukaryotes) in the Meso-Neoproterozoic, 1.8-0.56 Ga Agič 2021; Westall and Xiao 2025) and recently potentially multicellular microfossils (Qingshania magnifica) have been described from the ~1635- million-year-old Chuanlinggou Formation, North China (Miao et al. 2024).

Although planktonic algae and cyanobacteria became widespread during the Neoproterozoic, molecular clock analyses indicate that phototrophic algae appeared earlier in the Mesoproterozoic (Sánchez-Baracaldo et al. 2022). The Neoproterozoic was characterised by the breakup of the supercontinent Rodinia and global glaciation, after which the spike in nutrient delivery to the oceans stimulated further evolution of life with the appearance of planktonic algae. Planktonic phototrophs, therefore, spread over the world's oceans and contributed to the further rise in atmospheric oxygen. This is called the Neoproterozoic Oxygenation Event (NEO). It is the appearance of the planktonic algae and the rise in oxygen that really placed the Neoproterozoic world on the galactic map of habitable planets. O2 and O3, together with smaller amounts of CO2 and CH4, would have been detectable in the atmosphere, as well as pigments from the ubiquitous algae (red and green) at the surface of the oceans (Schwieterman and Leung 2024). It is, therefore, at this point in time, about 3.7 billion years after the emergence of life, assuming that life emerged after 4.36 Ga (Westall et al. 2023b), that life would have been detectable from space – at least detectable with the kind of space telescopes operating today. Another, technologically capable civilisation may well have (had) more sensitive instrumentation and, thus, been able to detect life on Earth at an earlier stage.

At this point, we come back to the search for extraterrestrial life using the knowledge and instrumentation that we have at our disposal today. Apart from a brief moment in geological time, the GOE between about 2.3 and 2.1 Ga, when oxygen levels rose significantly in the atmosphere, it would be impossible to detect from orbit any forms of life "as we know" it on another planet until the extraterrestrial equivalent of phototrophic life forms would have become widespread on the surface of the planet. If we take the analogy of the Earth, this means significant co-evolution of that planet and its life. The evolution of life on Earth may have been retarded during the Proterozoic because of the absence of significant geological events that could have influenced it. An exoplanet hosting life may not have had to wait 3.5 billion years before being able to manifest its existence to outside observers. Nevertheless, evolution to the extraterrestrial "algal" stage may have required, at a minimum, more than 2 billion years.

## What are the consequences of these reflections for the search for extraterrestrial life?

Our ability to detect traces of extraterrestrial life on exoplanets from orbit is limited to forms of life having signatures equivalent to, at a minimum, phototrophic planktonic algae, organisms that appeared on Earth only about 800 Ma. For 3.5 billion years, life flourished on Earth without demonstrating its existence. To external observers, that is an extremely long period of time. Moreover, any evolution of life during the first 3.5 billion years of its existence was predicated on co-evolution of the geology of the Earth. There are thus many constraining factors to take into account.

Let us go back to the beginning, the emergence of life. Life emerged on Earth in an, as yet, unknown location by an, as yet, unknown process after the Moon-forming impact (about 4.36 Ga; Borg and Carlson 2023). Nevertheless, we are able to constrain the
general characteristics of the early Earth and find them to be similar to our concept of habitable rocky planets, i.e. a rocky planet with liquid water at moderate temperatures conducive to prebiotic chemistry (< ca. 120°C), an anaerobic atmosphere comprising CO₂, a source of organic molecules, and sources of energy. Rocky planets and, especially, rocky planets with liquid water, are the holy grail of exoplanet research (e.g. Foley 2024; Kaltenegger 2017). Providing that the parameters of the planetary systems around the star(s) permit the existence of liquid water at their surfaces (e.g. Kaltenegger 2017), we expect such planets to exist. Organic molecules abound in space (Guélin and Cernicharo 2022); they are found in the planetary building blocks on early Earth, and continue to bombard the surface of the Earth. Carbon being one of the most common elements in the Universe, organic molecules will be commonplace on any physical body in the Universe.

Water is also a common element in the Universe and the presence of liquid water on a planet is one of the constraints for defining the habitable zone around a star (Kasting et al. 1993; Seager et al. 2013). Water on Earth was likely sourced from a number of complementary sources, all formed in the outer, cold zones of the protoplanetary disk, including carbonaceous chondrites, comets, and asteroids, as could be common in other stellar systems (Genda 2016 and references therein; Triaud et al. 2024). Water is essential for the emergence of life, as we know it. It is also essential for the maintenance of an equable climate – but only in association with exposed landmasses where a complex feedback system of terrestrial weathering depletes the atmosphere of carbon (which is then replaced by volcanic effusions). Thus, according to Triaud et al. (2024) depletion of CO₂ in the atmosphere of a temperate rocky planet is considered to be a tracer of water oceans (a habsignature). The sources of energy evoked above for prebiotic chemistry on Earth would be among those available for the emergence of life on habitable exoplanets. These energy sources include UV radiation, redox chemistry, heat, and piezoelectric energy (in faults).

Thus, following the considerations of De Duve (1995), life should be a common occurrence on habitable rocky planets. The earliest life on Earth is believed to have used a chemolithotrophic metabolism (Russell and Hall 1997; Martin et al. 2008). Although life may have appeared as early as 4.2-4.3 Ga (or even earlier, depending upon when the surface of the early Earth became habitable; Westall et al. 2023a), the oldest, clear fossil evidence of anoxygenic phototrophy occurs at 3.5 Ga (Noffke et al. 2013) (although there may be geochemical evidence for its existence already at 3.7 Ga, Czaja et al. 2013). At a minimum, this is about 500-600 million years after the emergence of life. Moreover, we have seen that the early phototrophs would not have provided a signature visible from space, at least using present day space telescopes. In fact, although phototrophs continued to develop significantly during the Archaean, it is only for a very brief period during the GOE between 2.3-2.1 Ga that oxygen appeared in the atmosphere in sufficient quantities to be visible. All of this took place on an Earth that was continuously habitable and continued to evolve geologically with the initiation of horizontal plate tectonics, continent amalgamation, an increase in nutrient flux from the eroded continents to the seas, and the formation and breakup of supercontinents. Permanent indications of life in the form of a melange of oxidised and reduced atmospheric gases and pigments only arrived with the advent of planktonic algae about 800 Ma. Evidence from space of life on Earth depended upon many complementary contingencies and took a long time, even though we noted above that the very slow period of evolution during the Proterozoic was probably linked to a slowdown in mantle to surface tectonic processes.

The implications of this are that, in order to be detectable, life on another planet would need to have reached a relatively elevated degree of evolution, at least up to the equivalent of the terrestrial planktonic photosynthesis. Thus, for about 80% of its history
(bar the GOE), it would not have been possible to detect evidence of life of Earth (with the equivalent of present day technology).

*Life in the Solar System?*

Let us consider the evolution of life on Earth with respect to life on other planetary bodies in the Solar System. Mars and Venus are rocky planets and the icy satellites may be considered as rocky planets covered with frozen oceans. The lack of data from Venus makes analogies difficult but we have much more data from Mars. Mars did not experience a Moon-forming impact that reset timing for the appearance of a volatile envelope having temperatures conducive to prebiotic chemistry. However, it lost its early reducing atmosphere due to the very high EUV radiation from the early Sun (Lammer et al. 2013). As the Sun's activity decreased, a secondary CO₂-rich atmosphere was outgassed and the volatile envelope, including water, was complemented by exogenous meteorites and micrometeorites. This occurred towards the end of the Pre-Noachian period, ca. 4.2 Ga (Lammer et al. 2013; Wordsworth 2016). Compiling geological evidence with that obtained from measurements on martian meteorites and the *in situ* measurements from the MAVEN orbiter, Jakosky (2021) concludes that Mars may have lost as much as 80-99 % of its water inventory to space through geological time. However, although much of the planet (the Southern Highlands) apparently retains its ancient morphology dating back to the Noachian/Hesperion period, the history of water on the surface of the planet is complex. There was overall loss of surface volatiles to space or sequestered underground (in the form of ice or, potentially, carbonates). However, the strong obliquity experienced by the planet contributed to the transfer of water from the poles to the equator at various times in the history of the planet (Figs. 5 and 6, Wordsworth 2016; Jakosky 2021).

Mars never experienced the geological evolution that characterised the Earth. Being smaller (a dwarf planet), the internal dynamo producing a protective magnetic field faded away about 4.2 Ga (Acuna et al. 1998) leaving the secondary atmosphere exposed to erosion by the solar wind and, thus, the volatile envelope unprotected (Wordsworth 2016; Jakosky 2021). Thereafter, periodic warming influenced by volcanism and meteorite impacts (Wordsworth 2016), and possibly by changes in obliquity led to water activity, left permanent traces in the form of valley networks and fluvial features (Carr and Head 2015). Although it had previously been suggested that Mars had had a northern ocean (Clifford and Parker 2001), this possibility seems increasingly unlikely for geochemical and climatological reasons (Wordsworth 2016). However, recent geomorphological studies have, again, suggested a northern ocean during the Hesperion to early Amazonian period (3.6-2.5 Ga; Wang and Huang 2024).

Given the (very vague) geological history of Mars, can we consider the planet to have been habitable from the point of view of abiogenesis? Did Mars have the necessary ingredients of life: water, organic molecules, and energy? We have seen from the above that the volatiles, water and organic molecules would have been imported by meteorites and micrometeorites (N.B. exogenous organics have been detected in the Gale Crater sediments (Freissinet et al. 2015; Eigenbrode et al. 2018; Szopa et al. 2020). They would also have been degassed from the mantle and organic molecules could have been formed in volcanic clouds (cf. Cleaves et al. 2008). The same energy sources as those on the early Earth would have been available, namely UV radiation, redox reactions, heat, and piezoelectric energy in fault systems.

The variety of environmental scenarios available for the emergence of life would have differed, however. With no ocean and no plate tectonics, no deep ocean hydrothermal vents could be invoked. Standing water lakes existed but would have been fed by ground-water seepage (Kite et al. 2021) or rain or snow melt. However, the numerous impacts, filled by lakes, would have created associated hydrothermal environments (Osinski et al.
<figure><img src="image_6.png" /><figcaption>**Figure 5.** Summary of processes that have supplied water to the atmosphere or surface or have removed water from them (Jakosky 2021).</figcaption></figure>

2013; Sasselov et al. 2020), a favoured environment for prebiotic chemistry. The abundance of exposed land on Mars would have also favoured reactions involving UV radiation (cf. Pascal et al. 2013).

While water was a permanent phenomenon on the early Earth, this was not the case on Mars. The aqueous environments in which life may have appeared on Mars would have been widely distributed in time and in space; they were not connected (Cockell et al. 2012; Westall et al. 2015b). We have no idea how much time it took for life to appear on Earth. For subsea scenarios, Westall et al. (2023a) argued that abiogenesis had to have taken place on a shorter rather than longer time scale because of the obturation of the porous materials hosting the prebiotic reactions caused by chemical precipitation of silica at the rock/water interface. But how short a time? We have, as yet, no answers from prebiotic chemistry. The time scale, however, is more likely to have been of the order of some 10⁴ up to 10⁶ years. If suitable environments existed on Mars, life may have appeared in more than one location – and may have disappeared, either extinguished or retreated through fracture systems into the subsurface (Friedmann and Koriem 1989; Michalski et al. 2018)).

We assume that primitive cellular life on Mars would have resembled that on the early Earth, but it would not have been genetically identical. Given the starting ingredients, reduced rocks and minerals enriched in Fe and Mg, as well as transition metals, and hydrothermal sources of energy, early martian life forms were likely to be based on a chemotrophic metabolism, i.e. obtaining their energy from inorganic sources in
<figure><img src="image_7.png" /><figcaption>**Figure 6.** Schematic of the major climate processes on Mars in the Noachian and early Hesperian periods. This cartoon assumes an episodically warm scenario for the early climate with long-term transport of snow to the southern highlands interrupted by episodic melting events. Wordsworth 2016.</figcaption></figure>

the case of chemolithotrophy, and from organic sources in the case of chemoorganotrophy (Westall et al. 2015b, 2025). As we noted above, organisms using these kinds of metabolisms do not produce traces of life that could be detected in an atmosphere, although CH₄ would have been produced by organisms similar to methanogens, albeit in very small quantities that would have been rapidly lost in the very localised biomes.

Could the chemotrophs have evolved some form of photosynthesis on Mars? The abundant shallow water habitats would have been conducive to this form of metabolism although we do not know exactly how long it took for phototrophs to evolve on the early Earth. As noted above, from the fossil record, we estimate that several hundreds of millions of years may have been needed. Could martian biomes have been stable sufficiently long for such a mutation to have taken place? The geological record indicates that the groundwater- or rain/melt water-fed lakes would have been transient rather than long-lasting. It is not possible to exclude the possibility of photosynthetic evolution on Mars, but the probability is low, based on our present understanding of the planet (Westall et al. 2015b). Again, on the global scale, detection of pigments associated with anoxygenic phototrophs on Mars would be arduous, given their very localised distribution, if they had evolved.

Having appeared in one, isolated location, we may envision the transport of living cells to another habitat via impact ejection, and possibly through subsurface aquifers, although these are likely to be frozen. Cockell et al. (2012) envisage the situation on Mars whereby potentially habitable habitats remain uninhabited, while Westall et al. (2015b) consider the spatial and temporal distribution of habitats, inhabited or uninhabited, as representing punctuated habitability. This punctuated habitability would have been a limiting factor for the evolution of life on Mars.
Work on Mars-analogue terrestrial sediments from the Palaeoarchaean has documented the kinds of signatures produced by chemotrophic microorganisms (Westall et al. 2025). Given the fact that these are anaerobic microbes whose growth is slow, the quantities of organic matter produced are generally low. However, Westall et al. (2015a,b) noted that chemoorganotrophs growing in hydrothermal effluent produce more biomass than the more elusive lithotrophs, although their biomass is still low. Dead microorganisms are subjected to diagenesis, or taphonomy, i.e. degradation by various processes. In order to preserve physical features, such as cell remains, they need to be encased rapidly in a mineralised cement order to be preserved (Westall et al. 2018). Summons et al. (2011) note the following minerals capable of preserving biosignatures in order of increasing lifetime: ice, halite, sulphates, carbonates, phyllosilicates, silica, and phosphates. These biosignatures range from the physical attributes of the microorganisms, such as cells, colonies, biofilms, microbial mats and bioconstructions, such as stromatolites; the degraded organic components of the cells and microbial communities (enantiomeric excess or homochirality, diastereoisomeric preference, structural isomer preference, repeating constitutional sub-units or atomic ratios, systematic isotopic ordering at molecular and intramolecular levels, and uneven distribution patterns or clusters (e.g., C number, concentration, $\delta^{13}C$) of structurally related compounds (cf. Summons et al. 2011); to the elemental and isotopic composition of the organic molecules; and related evidence of metabolic activity, such as biprecipitated or biomediated minerals (Westall and Cavalazzi 2011; Summons et al. 2011; Neveu et al. 2018). On Earth, a range of biosignatures is generally required in order to identify the remains of microorganisms in the oldest rocks with any degree of certainty (Westall and Cavalazzi 2011) and this is the case also for Mars (Summons et al. 2011; Neveu et al. 2018). The reliability of a biogenic interpretation is increased by the complementarity of the biosignature assemblage, together with occurrence in a geologically plausible environment.

In situ detection of biosignatures on Mars is all the more arduous, given the limitations of instrumentation. This situation is underlined by the difficulties encountered in reliably identifying biosignatures in the oldest, well-preserved sediments on Earth. One study of fossilised (silicified) chemolithotrophs in Palaeoarchaean, coastal, volcanic sediments from the Pilbara, the 3.45 Ga Kitty's Gap Chert, documented features exhibiting the characteristics of cells, colonies and biofilms on the surfaces of volcanic particles and in the fine, volcanic dusty matrix of the sediments (Fig. 7a) (Westall et al. 2006a; Westall and Cavalazzi 2011). These would be the oldest fossil cells known to date. However, despite the combined physical organo-chemical and isotopic biosignatures presented, the features described were dismissed as abiotic artefacts (Wacey 2009) and ignored by the scientific community. At the time of this study, the polemic surrounding the possible identification of traces of life in the martian meteorite ALH84001 (McKay et al. 1996) rendered the scientific psychological environment hostile to ancient microfossil identification, especially that related to very small microfossils (individual cells in the Kitty's Gap Chert were 0.5 and 0.8 µm in diameter, too small to be visible with a light microscope). Large structures, such as stromatolites, were more readily identifiable (Fig. 7b) (e.g. Allwood et al. 2006). Recently, Clodoré et al. (2024) made an in depth investigation of the carbonaceous matter in the Kitty's Gap Chert using a wide variety of techniques, including synchrotron radiation, that demonstrated the presence of organic matter of mixed biogenic and abiotic origin in the sediments. This very broad investigation was complemented by a state of the art study (Westall et al. 2025) using a novel Cluster SIMS technique that definitively demonstrated the biogenicity of the cellular structures observed by Westall et al. (2006b). Such studies are crucial for demonstrating the level of investigation that will be necessary to determine the potential biogenicity of organic features in samples returned from Mars (cf. Bosak et al. 2024).
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Fossil structures from the Palaeoarchaean. (A) Colony of small chemolithotrophs from the 3.45 Ga Kitty's Gap Chert, Pilbara (@ Westall). (B) stromatolites from the 3.4 Ga Trendall locality, Pilbara (@Marchesini).</figcaption></figure>

The holy grail of the missions to Mars and the search for traces of martian life remains the larger scale biosignatures, microbial mats and stromatolites (e.g. Fig. 7b) (Noffke 2015; Hickman-Lewis et al. 2023). However, as we noted above, phototrophic microorganisms, if they ever evolved on Mars, are likely to be extremely rare.

The search for life in the Solar System is not only limited to Mars, the icy satellites of Saturn (Enceladus) and Jupiter (Europa) are objects of interest for space missions, such as the Europa Clipper mission (Pappalardo et al. 2024). If there is life on these icy worlds, that life will be similar to the terrestrial chemotrophs and, consequently, very difficult to identify. Recently, phosphine was identified in the atmosphere of Venus (Greaves et al. 2021). Phosphine is produced by a variety of biogenic and abiotic processes (Bains 2024). Its discovery has led to the Venus Life Finder Mission project (Seager et al. 2022).

The finding of traces of extraterrestrial life will lead to enormous polemic, as demonstrated by the furore following the NASA announcement of the finding of life in the martian meteorite ALH84001 (McKay et al. 1996). The NASA team made, for the time, a very careful study, documenting a number of features that contributed to their interpretation of traces of life: the presence of carbonaceous molecules in the meteorite (polycyclic aromatic hydrocarbons); the presence of structures that, to the team, had the appearance of fossilised microbial cells; the presence of tiny magnetite crystals that are very similar to those formed by magnetotactic microorganisms on Earth; and the presence of calcareous deposits thought to be biomediated precipitations, all formed in cracks in the meteorite through which water had flowed. At that time, understanding of the fossilised signatures of life was limited. We now know that carbonaceous matter of abiotic origin — from the mantle and from the infall of meteorites and micrometeorites — is common in martian materials (Steele et al. 2012; Freissinet et al. 2015; Eigenbrode et al. 2018). The fossilised cells, originally suggested to be “nanobacteria”, were simply corrosion features of the carbonate deposits. The magnetite crystals are similar to those produced by one particular kind of magnetotactic bacterium, but other types of magnetotactic bacteria produce crystals of different shapes and sizes, and experiments show that they can also be formed by abiotic reactions (Golden et al. 2004). The calcareous deposits do appear to have been formed from low temperature water but can also be deposited by abiotic means. Thus, the balance of evidence is against a biogenic interpretation for the individual features described by McKay et al. (1996). However, as noted by Westall et al. (2021), the data are correct, although the interpretation erroneous but not surprising given the state of knowledge at that time.
A similar polemic followed the publication of the discovery of phosphine in the atmosphere of Mars (Greaves et al. 2021), but later investigations has confirmed its presence (Bains 2024; Schwieterman and Leung 2024).

The examples listed above, ranging from the oldest fossil cells on Earth, through traces of life in the meteorite ALH84001, and the phosphine signal in the Venusian clouds illustrate the heightened emotions surrounding such findings. It is not just a question of scientific veracity, but there is also communal and personal element to reactions that include simple denial and professional jealousy. This will happen repeatedly when future discoveries of extraterrestrial life are made, whether it be of simple organisms on Mars, or a biosignature gas on an exoplanet indicating that some kind of photosynthetic life form exists. Humans are fallible creatures, governed not just by the brain but also by emotions. Moreover, the brain does not like a vacuum and has a tendency to over-interpret data, especially the less the data there are. This does not help the scientific endeavour. Fortunately, we learn from our mistakes: the furore surrounding the ALH84001 meteorite stimulated the field of astrobiology and a plethora of original research is helping us to understand life and its traces better.

## Conclusion

The evolution of life on Earth was not serendipitous. The composition of planet Earth and its position in the Solar System disk already placed it in an advantageous position for the creation of habitable conditions and the emergence of life. The continuous link between mantle processes, the surface of the Earth and the atmosphere has been maintained throughout geological history, albeit with variations. Life appeared sometime, somehow, somewhere in the Hadean, but by at least 3.5 Ga, if not before, it was widespread and relatively evolved, having diversified into chemotrophs and phototrophs. Certainly, the initiation of horizontal plate tectonics and creation of amalgamated continents with increased runoff of nutrients promoted the flourishing of oxygenic phototrophs, once they had appeared in the Mesoarchaean. Their rapid rise in the Neoarchaean resulted in the Great Oxygenation Event (GOE) in the Palaeoproterozoic, a geologically brief period of about a couple of hundred million years during which atmospheric oxygen levels rose to present day levels and would have been briefly visible to a space observer.

However, although oxygenic phototrophs dominated the biome since the Neoarchaean and throughout most of the Proterozoic, it was only with the evolution of eukaryotes, and the rapid rise of planktonic red and green algae during the Neoproterozoic, that an atmospheric signature of life would have become permanently visible to a space observer. Both the GOE and the Neoproterozoic Oxygenation Event (NOE) were the result of plate tectonic activity: the breakup of supercontinents, global glaciation, and an increase in nutrient supply. Without these stimuli to evolution, the rapid diversification of life that occurred at the cusp of the Neoproterozoic/beginning of the Phanerozoic would not have occurred.

Thus, for much of Earth's history, life flourished but would have been invisible from space, if it were observed using the kind of instrumentation we have today. However, more advanced technologies may have been able to detect the weak traces of biogases in the terrestrial atmosphere earlier – but this point is purely hypothetical. Moreover, the evolution of life on Earth was intimately interlinked with tectonic processes – and Earth has the right size for maintaining mantle overturn and a magnetic dynamo and maintaining habitable conditions.

Considering Christian de Duve's philosophy (1996) that biology should naturally evolve from chemistry, habitable rocky planets should be inhabited, at a minimum by organisms resembling terrestrial chemotrophs. These would be the kinds of organisms whose remains could be found on Mars or the icy satellites of the Solar System. Unfortunately for the red
planet, its disconnected habitats and punctuated habitability argue strongly against the
appearance of phototrophs, although we cannot conclude that such a metabolic strategy
never evolved.

We have arrived at the era of sample return from asteroidal and planetary bodies in
the Solar System. A range of instrumentation including highly sophisticated technology
will need to be adopted to investigate the returned samples. As demonstrated by various
polemical discussions concerning the oldest traces of life or of extraterrestrial life, very
strong evidence of diverse nature is necessary to arrive at a degree of probability – weak,
moderate, or strong. Unless the extraterrestrial organisms are living and can be analysed
using the terrestrial tools at hand, we may only be able to arrive at the conclusion of
“very strong probability” rather than absolute certainty.

In particular, we need to be prepared for a polemical reaction from scientists (with
very human reactions) and from the general public, many of whom may fear that the
foundations of their philosophy of life are threatened.

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