Chapter 4: A Geological and Chemical Context for the Origins of Life on Early Earth

4.2.2.1. Carbon

On early Earth, most carbon would have existed in oxidized forms: CO₂ in the atmosphere or carbonate (CO₃ ^2-) in the early oceans. The long-term geochemical cycling of CO₂ and carbonate (i.e., the carbonate-silicate cycle) modulates Earth's climate over geologic timescales (see Chapter 3.4.2.2). To a lesser extent, carbon monoxide (CO) and possibly (in trace amounts) methane (CH₄) could have been sourced from volcanic emissions, impact events, or photochemistry (see Table 4.2; Harman et al., 2015). However, any methane generated would have been unstable and readily destroyed by UV irradiation. While a myriad of chemical processes could have transformed gaseous CO₂, CO, and CH₄ into organic matter (Chapter 4.2.3), important prebiotic molecules may have also been delivered to early Earth via impactors (see Chapter 3.3.4).

Life on Earth is composed mostly of H and O in the form of water, which makes up ≥70% of cellular mass. Given the properties of water (see Chapter 2.2.2.1) and its abundance on Earth, H and O were likely critical for the origins of life. Although water would have been the dominant source of H and O, UV splitting of H-containing species (i.e., H₂O, CH₄, NH₃, H₂S), serpentinization reactions (see Chapter 4.1.4), volcanic outgassing (see Chapter 3.4.3.4), and impact events provided numerous sources of H₂ (Zahnle et al., 2020). Atmospheric H₂ is readily lost to space through thermal escape; if the H is sourced from H₂O, the residual O₂ remains behind (see Chapter 3.4.1.5). Gaseous oxygen (O₂) could have also been generated via photolysis of oxygen-containing compounds other than water (i.e., CO, CO₂, N₂O, NO₂, NO₃, SO₂).

Due to the understood redox state of the mantle, volcanism is not expected to have been a significant source of O₂ on early Earth (see Chapter 4.1.4). Rather than accumulating in the atmosphere, photolysis-generated O₂ would have been consumed via oxidation of reduced gases. Calculations suggest that the volcanic emission of H₂ alone was more than sufficient to quench O₂ generated via H₂O splitting (Kasting and Catling, 2003); O₂ would have also been lost due to oxidation of crustal materials. Overall, models estimate that in the Hadean O₂ surface levels were ∼10⁻¹³ present atmospheric level (PAL) (Kasting and Catling, 2003). It has been predicted that H₂ could have made up at least ∼1% of the Hadean atmosphere (Kasting and Catling, 2003; Tian et al., 2005; Liggins et al., 2020).

4.2.2.3. Nitrogen

Nitrogen was likely critical for the emergence of life, as it is a key component of nucleic acids and proteins (see Chapter 2.2.3). Like the modern atmosphere, N₂ would have been the dominant N-containing species on early Earth, with trace amounts of NH₃ also present (Chapter 4.1.4). Radical chemistry (see Chapter 4.2.3.8) and/or fixed N species (e.g., nitrate (NO₃ ^2-), nitrite (NO₂ ^2-), or ammonia/ammonium (NH₃/NH₄ ⁺)) would have been required for abiotic synthesis of N-containing biomolecules. Abiotic nitrogen fixation could have occurred via lightning, mineral redox, photolysis, or impact events (Table 4.2; Doane, 2017).

The conversion of atmospheric N₂ and NO_x ^- to ammonia (NH₃) on early Earth is highly relevant to origins-of-life chemistry given that (i) amine groups (R-NH₂) are ubiquitous in living matter, whereas those with nitroso groups (R-N = O), nitro groups (R-NO₂), and nitrite/nitrate esters (R-O-N = O/R-O-NO₂) are rarely found in life; and (ii) life uses ammonia as the precursor for N-containing molecules. While ammonia is produced via the irradiation of N₂ adsorbed onto dried titanium-oxides, continued irradiation oxidizes ammonia to N₂ and NO_x ^-, hindering it from accumulating above trace levels in Earth's atmosphere (Kasting and Catling, 2003; Doane, 2017). However, if the minerals were submerged in water, the resulting ammonia could have been readily sequestered to deeper waters, inhibiting further photolysis (Hirakawa et al., 2017; Mateo-Marti et al., 2019). Alternatively, ammonia could have formed in deeper waters via hydrothermal reduction of nitrate, nitrite, and N₂ in the presence of Fe-containing minerals (Smirnov et al., 2008; Nishizawa et al., 2021).

Green rust as well as dissolved ferrous iron (Fe²⁺) or copper (Cu²⁺) can also reduce nitrite and nitrate to ammonia under mild, alkaline conditions (pH >7, ∼25°C; Cho et al., 2010). Nitrite and nitrate are generated efficiently via lightning through an N₂-rich atmosphere (Cleaves et al., 2008; Wong et al., 2017). Models accounting for the various sources and sinks of nitrate and nitrite built by Ranjan and colleagues postulate that the dissolved NO_x ^- concentrations in the Hadean ocean would have been <1 μM (pH 6.5–8; T = 0–50°C), with significantly less nitrite due to its reactivity, compared to ∼40 μM nitrate in modern oceans (Olsen et al., 2016; Ranjan et al., 2019). Significant amounts of ammonia may have also been delivered to early Earth via comets and asteroids (Pizzarello et al., 2011; Altwegg et al., 2020).

4.2.2.4. Phosphorus

Since phosphate is rapidly sequestered into insoluble precipitates by dissolved cations that were likely common in Earth's early oceans (i.e., Ca²⁺, Fe²⁺, Fe³⁺, Al³⁺, or Mg²⁺), it remains unclear what the most prominent source of phosphorus may have been (Pasek et al., 2017). Early Earth's oceans on average contained an estimated 0.04–0.13 μM of dissolved phosphate (Jones et al., 2015), which is an order of magnitude less than that in modern oceans (typically 0.2–4 μM; Patey et al., 2008) and orders less than that in modern cells (0.5–10 mM; Bevington et al., 1986). This solubility issue is commonly dubbed the “phosphate problem.”

The phosphate problem could have been resolved in solutions with (i) anions that bind cations other than phosphate preferentially (e.g., carbonate-rich waters that sequester Ca²⁺; Toner and Catling, 2019), (ii) organic acids or other solubilizing agents that liberate precipitated phosphate (Burcar et al., 2016), or (iii) minerals that adsorb phosphate and later release it in concentrated batches or polymerize it with co-adsorbed organics (Barge et al., 2014; Flores et al., 2021). The reduced form of phosphate (i.e., phosphite, HPO₃ ^2-) is more soluble and is relatively stable under early Earth conditions (see Table 4.2). Oxidation of phosphite via Fenton chemistry generates phosphate and polyphosphates, the latter of which can phosphorylate organics in aqueous solutions (see Chapter 4.2.3.9); polyphosphates can also be generated from phosphate minerals subjected to lightning (Çalışkanoğlu et al., 2023). Alternatively, iron-rich meteorites may have delivered a significant amount of the phosphorus mineral schreibersite ([Fe,Ni]₃P) that upon corrosion can phosphorylate organics (Gull et al., 2015). Notably, organophosphate bonds break in aqueous solutions subjected to UV irradiation—this process would have been key for recycling phosphorus on prebiotic Earth (Farr et al., 2023).

4.2.2.5. Sulfur

On Hadean Earth, S existed mostly as sulfur dioxide (SO₂) and, to a lesser degree, hydrogen sulfide (H₂S) or polysulfur (S₈). In the absence of oxygen, sulfate (SO₄ ^2-), the most common form of S on modern Earth, would have been rare (Crowe et al., 2014; Fakhraee and Katsev, 2019). Upon dissolution into Earth's early oceans, SO₂ formed sulfurous acid (H₂SO₃) and existed in equilibrium with bisulfite (HSO₃ ^-) and sulfite (SO₃ ^2-). The equilibrium was pH dependent with strongly acidic solutions (pH <1.85) favoring SO₂ and neutral/alkaline (pH >7) favoring sulfite.

Unlike modern Earth, where sulfite species are rapidly oxidized to sulfate, sulfite could have accumulated on early Earth since its only major sink would have been oxidation to sulfate via UV irradiation (Ranjan et al., 2018). This oxidation mechanism can be coupled with reduction of HCN to form biologically relevant organics (Ritson et al., 2018; Xu et al., 2018). Meanwhile, sulfide from dissolved H₂S would have been consumed by organic electrophiles and metals forming organic thiols (R-SH) and metal sulfides. Mineralization of organic thiols would have regenerated dissolved sulfide (HS^-) and the production of sulfite (SO₃ ^2-) (Fakhraee and Katsev, 2019). Models predict that (bi)sulfite species were more abundant (μM at 25°C; mM at 0°C; nM at 50°C) than dissolved sulfide (nM range 0–50°C; Ranjan et al., 2018).

4.2.3.1. Serpentinization (Fig. 4.7A)

The serpentinization reaction is the hydrolysis of a mineral within a rock, generating H₂ as a by-product. Serpentinization most often occurs with ultramafic minerals that are common in Earth's mantle (see Chapter 4.1.2), and it may have also served as a source for H₂ on early Earth (Sleep et al., 2004) or on other worlds (see Chapter 7.1). The production of H₂ from ongoing serpentinization has been observed in the field including at Lost City Hydrothermal Field (Kelley et al., 2005), Rainbow Hydrothermal Field (Charlou et al., 2002), and the Oman ophiolite (Neal and Stanger, 1983). Serpentinization generates OH^- anions that would have resulted in pH gradients with early Earth's slightly acidic waters, fueling CO₂ reduction to methane and potentially ATP production for the earliest forms of life (Martin et al., 2008; Russell et al., 2010; Sojo et al., 2016; Hudson et al., 2020).

4.2.3.2. Haber-Bosch process (Fig. 4.7B)

In the Haber-Bosch reaction, nitrogen (N₂) and hydrogen-containing gases (usually H₂ or CH₄) are combined under high temperatures (400–650°C) and pressures (150–300 bar) to form ammonia (Rodriguez et al., 2012). This reaction requires dry conditions (i.e., the absence of water) and the presence of a strongly reducing metal catalyst, typically zero-valent iron (Fe⁰) or ferrous iron (Fe²⁺).

The prebiotic relevance of Haber-Bosch reactions remains uncertain given that the required conditions would have been rare on early Earth. The Haber-Bosch process still could have served as a minor source of ammonia during impact events where the requisite dry, high temperatures and high pressures could occur (Shimamura et al., 2017). Impacts by stony-iron meteorites would be particularly conducive to this chemistry, given that they are enriched in the catalysts that can drive the reaction such as reduced iron and nickel alloys and the minerals magnetite (Fe₃O₄; source of Fe²⁺), wüstite (FeO; source of Fe⁰), and schreibersite ([Fe,Ni]₃P; source of Fe⁰) (Rubin and Ma, 2017).

4.2.3.3. Strecker synthesis (Fig. 4.7C)

Strecker synthesis occurs when ammonia, cyanide, and an aldehyde combine to form α-amino acids (Fig. 4.6). It begins with condensation of the ammonia and aldehyde to form an imine, which is then attacked by the cyanide anion. The cyano group can then be hydrated, generating a carboxylic acid. The Strecker synthesis has been posited to have occurred in the primitive ocean (Miller and Van Trump, 1981) and to have been the major source for α-amino acids within meteorites (Burton et al., 2012).

OPEN IN VIEWER

FIG. 4.6.

Possible molecular building blocks relevant to the origins of life, including molecular building blocks: nucleobases, amino acids, hydroxy acids, fatty acids, and sugars. N-het is short for N-heterocycle. The reaction with the N-heterocycle and ribofuranose show how the reaction can produce two nucleoside isomers. *Proteinaceous amino acids are those used by life to make proteins.

Reductive amination is a process by which substituted amines such as amino acids can be generated via α-keto acids (Muchowska et al., 2017; Barge et al., 2020). This reaction can occur at relatively low temperatures (i.e., 23–70°C) and is initialized with the condensation of an amine or ammonia (NH₃) onto an aldehyde/ketone (R₂C = O) to form an imine intermediate (i.e., R₂C = NH) that is then further reduced to the amine product (i.e., HR₂C-NH₂). Iron is typically used as the reductant (i.e., electron source) and water as a proton source.

4.2.3.5. Fischer–Tropsch type (FTT) synthesis (Fig. 4.7E)

Fischer–Tropsch reactions can efficiently generate liquid hydrocarbons (e.g., alkanes, alkenes, alcohols) from hydrogen and carbon monoxide in the gas phase. This process is performed with metal catalysts at high temperatures and pressures (typically 200–350°C; 5–275 bar; Speight, 2015). When conducted in aqueous solutions and/or in the presence of other gases such as NH₃ or H₂S, the reactions are known as Fischer–Tropsch type (FTT). Such reactions have been proposed as potential abiotic sources of fatty acids (with up to 18-carbon-long side chains; McCollom et al., 1999), amino acids, nucleobases (Fig. 4.6), and hydroxy acids, among other organics in various planetary environments, including within the deep subsurface of the crust, hydrothermal vents, meteorites, and the solar nebula (Kress and Tielens, 2001; Simoneit, 2004). FTT chemistry may have also been the source of organic matter found in the oldest Archean rocks, making it difficult to discern the biogenicity of potential microfossils (Lindsay et al., 2005).

4.2.3.6. Formose reaction (Fig. 4.7F)

The formose reaction (Butlerov, 1861) describes the polymerization of formaldehyde (CH₂O) and is the leading mechanism proposed for the prebiotic origin of sugars, including ribose (the sugar in RNA), glucose (critical for modern metabolism), and other sugar-like compounds, namely, sugar alcohols, sugar acids, and sugar polymers (i.e., polysaccharides). Degradation of these molecules leads to the formation of a complex mixture which includes hydroxy acids relevant to metabolism (i.e., metabolites; Weber, 1992; Omran et al., 2020). The initial (and slowest) step of the mechanism is the condensation of two formaldehydes to form glycolaldehyde (Fig. 4.7F). This induction period is sped up when trace amounts of glycolaldehyde or higher-weight sugars are present in solution (Kitadai and Maruyama, 2018).

OPEN IN VIEWER

FIG. 4.7.

Chemical mechanisms relevant for transforming simple reactive molecules to biologically relevant compounds.

The formose reaction requires higher temperatures (60–80°C), basic conditions (usually pH 10–11), and a catalyst—commonly Ca²⁺, Mg²⁺, however other divalent (e.g., Ba²⁺, Sr²⁺, Pb²⁺), monovalent (e.g., Na⁺, K⁺) and trivalent cations (e.g., Fe³⁺), as well as organic bases (e.g., pyridine) (Weber, 1992; Kopetzki and Antonietti, 2011) can also catalyze the reaction. The relevance of the formose reaction for abiogenesis has been questioned given that, if not stopped quickly, it results in the degradation and polymerization of the sugars, forming insoluble tar (Shapiro, 1988). Moreover, ribose is generated in relatively low yields among a large assortment of other side products (including ribose isomers, see Fig. 4.6). Notably, the presence of inorganic reagents, namely borates and, to a much lesser extent, silicates, has been shown to stabilize cis-diols including pentoses like ribose (Lambert et al., 2010; Nitta et al., 2016). Alternatively, specific reagents present within the solution could have sequestered ribose (Springsteen and Joyce, 2004). While the formose reaction has been invoked as a potential source for sugars and sugar derivatives found in meteorites and irradiated interstellar ice analogs (Cooper and Rios, 2016; Meinert et al., 2016; Furukawa et al., 2019), it should be noted that other (lesser known) mechanisms may be at play (Nuevo et al., 2018). It also has been proposed that a “glyoxylate scenario,” rather than formose reactions, may have produced the first sugars, given it can yield ribose directly as a product (Eschenmoser et al., 2007; Fialho et al., 2018). This mechanism is in stark contrast with the formose reaction wherein a plethora of products are generated.

Hydrogen cyanide (HCN) is a weak acid (pK_a 9.2) and in aqueous solutions will exist in equilibrium with the cyanide anion: HCN ↔ CN^- + H⁺. HCN has a boiling point of only 25.6°C at low pH, but under more basic conditions (pH >8) the proportion of cyanide anion (CN^-) increases, and at pH >9.2 it is the dominant form. At high-enough concentrations, CN^- readily undergoes polymerization even at mild conditions (i.e., 1 bar, 25°C) to form a range of compounds including hydroxy acids, amino acids, N-heterocycles (e.g., purines, pyrimidines, and pteridines; Fig. 4.6), and polypeptides (Voet and Schwartz, 1982; Marín-Yaseli et al., 2015). Although a catalyst is not needed for this reaction, NH₄ ⁺ and O₂ can facilitate it (Marín-Yaseli et al., 2018). The ability for HCN polymerization to generate biochemical building blocks has led some researchers to suggest that HCN could facilitate abiogenesis on planetary bodies (Matthews, 2004; Matthews and Minard, 2006; Sasselov et al., 2020).

Sufficiently high HCN concentrations (>0.01 M) would be difficult to achieve on early Earth, especially considering the high reactivity of the CN^- anion with aldehydes. Solutions containing lower concentrations undergo hydrolysis to form formamide (HCONH₂) and formic acid (HCOOH) (Sanchez et al., 1967). Mechanisms by which CN^- may have been concentrated include freezing of dilute CN^- solutions (Menor-Salván and Marín-Yaseli, 2012) or precipitation over time with dissolved metals and subsequent release (Ruiz-Bermejo et al., 2013). HCN, or products of HCN polymerization, may have also been delivered by comets (Biver et al., 2002) and meteorites (Callahan et al., 2011; Smith et al., 2019).

4.2.3.8. Free-radical chain reactions (Fig. 4.7H)

Reactions involving free radicals typically have low to no activation energy. Consequently, radicals can generate a variety of products, including polymers, from otherwise stable, less reactive molecules (e.g., CO₂, N₂, etc.). Free-radical chain reactions are characterized by three phases. First, the chain is initiated by the formation of a radical species, a process that requires significant energy. Common sources for the initiation phase include thermal energy (e.g., induced by impact events), ionizing radiation (e.g., UV light, X-rays, γ-rays, α-particles, β-particles), and electric currents (e.g., lightning, redox reactions). Second, propagation occurs due to the interaction of radicals with other atoms, molecules, or ions to form more radicals either via hydrogen abstraction (removal of •H) or addition of the radical to the double/triple bond of another compound. Finally, termination occurs when radicals react with other radicals to form nonradicals either via radical recombination or radical disproportionation (see Fig. 4.7H).

Radicals, being short-lived, exist in significantly lower concentrations than nonradicals. Consequently, the termination phase only occurs after many propagation cycles and explains how a wide range of organic compounds can be derived from radical chain reactions involving simple volatiles (e.g., CO₂, CH₃OH, CO, or CH₄). If other molecules are present (e.g., N₂, H₂O), an even wider array of products can be generated. For example, UV irradiated ices containing volatiles form a diverse array of organic compounds including nucleobases, sugars, hydroxy acids, and amino acids (Fig. 4.6; see Chapter 3.3). In addition, radicals can also efficiently transform stable molecules (e.g., benzene, naphthalene) into more reactive compounds. For a review on the organics that can be generated from irradiated ices see (Sandford et al., 2020).

4.2.3.9. Fenton reaction (Fig. 4.7I)

The classic Fenton reaction describes the generation of hydroxyl radicals (•OH) during the oxidation of ferrous iron (Fe²⁺) by hydrogen peroxide (H₂O₂). Once formed, the hydroxyl radicals oxidize inorganic ions and organic molecules. Similar processes could have occurred on prebiotic Earth given that its oceans were enriched in ferrous iron. Potential prebiotic sources for peroxide include (i) oxidation of water by pyrite and nickel disulfides (Borda et al., 2001); (ii) irradiation of atmospheric gases (Kasting et al., 1989); and (iii) water oxidation at water-rock interfaces subjected to stress (Balk et al., 2009).

Fenton reactions can occur at room temperature and proceed most efficiently within acidic solutions (pH 2–4) which promote the dissolution of minerals. Fenton-like reactions under alternative conditions have been carried out (i.e., using Cu²⁺, Mn²⁺, Co²⁺, Ni²⁺, V²⁺, Ti²⁺, Cr²⁺), suggesting this mechanism may have been prevalent at acidic black smoker vents (pH 2–4) where these metals can accumulate alongside peroxide (Borda et al., 2001; Kelley et al., 2002; Balk et al., 2009; Tivey, 2015). Notably, Fenton reactions may have been key to resolving the phosphate problem (see Chapter 4.2.2.4), as they could have oxidized soluble phosphorus species, phosphite (HPO₃ ^2-) and hypophosphite (H₂PO₂ ^-) to polyphosphates whose high-energy bonds could phosphorylate organics (Fig 4.7I; Pasek et al., 2008, 2017; Gull et al., 2015) or served as energetic precursors to ATP (Lipmann, 1984; Holm and Baltscheffsky, 2011).

4.2.3.10. Miller–Urey spark discharge reactions

In 1953, Stanley L. Miller and Harold Urey revolutionized origins-of-life research when they demonstrated that biologically relevant organic compounds could be generated from electric discharges through a reducing atmosphere (e.g., CH₄-NH₃-H₂-H₂O) over water—simulating the chemistry that may occur on Earth during a thunderstorm or volcanic eruption (Miller, 1953). Miller–Urey reactions generate a complex mixture of organics that includes hydroxy nitriles/amides/acids, amino nitriles/amides/acids, aldehydes, HCN, HCN polymers, nucleobases, and more (McCollom, 2013; Parker et al., 2016; Ferus et al., 2017). Although initial experiments showed that a N₂-CO₂ atmosphere did not facilitate the formation of organics or any detectable amino acids (Schlesinger and Miller, 1983b), subsequent work showed that organics are generated when the reaction occurs over a pH buffered solution (Cleaves et al., 2008; Rodriguez et al., 2019). When the starting gases interact with a spark discharge, radicals form and recombine into new molecules. Dissolution of these organics into an aqueous solution can lead to subsequent reactions such as HCN polymerization and the Strecker synthesis.

Researchers have noted that the products generated from Miller–Urey chemistry are similar to those formed from UV irradiation through a reducing atmosphere simulating both the formation of Titan and Triton tholins (Cleaves et al., 2014). Furthermore, the distribution of organics detected in meteorites and via FTT synthesis has also been shown to be similar to Miller–Urey mixtures (McCollom, 2013). These studies demonstrate that different energy sources may not have a significant impact on the resulting organics generated via these complex processes.

4.2.4.1. Informational molecules: nucleic acids and their precursors

The informational polymers used by extant life are the nucleic acids DNA and RNA (see Chapter 2.2.3.1). It is hypothesized that RNA may have emerged first, serving as a dual genetic-enzymatic biomolecule (i.e., the RNA World hypothesis; Gilbert, 1986) given (i) a wide array of ribonucleotides (RNAs) function as universal metabolic cofactors (Fig. 4.8C) and (ii) catalytic RNAs carry out various critical roles in the cell (Saad, 2018). Alternatively, RNA could have preceded DNA but still have been the product of evolution stemming from a different informational molecule (i.e., pre-RNA/proto-RNA; Robertson and Joyce, 2012; Hud et al., 2013; Cleaves et al., 2019). However, it is not certain whether (proto)life transitioned from one genetic system to another (i.e., a genetic takeover) or whether various genetic systems (i.e., pre-RNAs, RNA, DNA) could have coexisted and coevolved (Bhowmik and Krishnamurthy, 2019; Xu et al., 2020).

OPEN IN VIEWER

FIG. 4.8.

A primer to nucleic acids. (A) Diagram of a nucleotide, adenosine monophosphate. (B) Base pairing by hydrogen bonds. The biological (i.e., canonical) nucleobases used in RNA and DNA store information via Watson–Crick base pairing. The blue numbers indicate how the atoms are numbered (and thus referenced) in the pyrimidines (uracil, cytosine, thymine) and purines (adenine and guanine). The * indicates nucleophilic positions on the nucleobases where ribose, a ribose precursor, or another connector in a pre-RNA monomer could add, forming a structural isomer (same molecular weight and chemical formula) as the (often desired) product. In RNA/DNA, the sugar (R in the figure) is attached to the N1 and N9 atoms of pyrimidines and purines, respectively. In non-encoding RNA molecules, such as transfer-RNA, the way in which the molecule folds (which is critical to its function) is partially stabilized by non-Watson-Crick base pairing between the various nucleobases (i.e., Hoogsteen base pairing). (C) A myriad of ribonucleotides (RNA monomers) are used as metabolic cofactors in all life-forms; the deoxyribonucleotide equivalent (i.e., DNA monomers) are never used. The ubiquity of these molecules, together with the catalytic functionality of RNAs, is considered strong evidence for an RNA world.

Figure 4.9 showcases how pre-RNAs could have used alternative molecules for each of the three units (the nucleobase, sugar, and phosphate) within nucleic acids. Researchers have suggested a variety of prebiotic mechanisms for the formation of (non)canonical nucleobases, including cyanide polymerization (see Chapter 4.2.3.7), and others (Menor-Salván, 2009; Ferus et al., 2017). Ribose/deoxyribose sugars may have been derived from the formose reaction (see Chapter 4.2.3.6) or alternative pathways (Eschenmoser, 2007; Roche et al., 2022).

OPEN IN VIEWER

FIG. 4.9

Alternative or nonstandard biomolecules with prebiotic significance. Sets of molecules that could have had prebiotic significance as either facilitators or precursors of canonical, or modern, biomolecules. (A) Genetic polymers could have been composed of different ionized linkers, trifunctional connectors, and recognition units, and may have even comprised backbones that combine several of these roles into one molecule. (B) Peptides could have been composed of hydroxy acids as well as amino acids, the combination of which (i.e., the depsipeptide backbone) has potentially beneficial chemical properties. (C) Prebiotic compartmentalization could have involved membranes composed of fatty acids that resemble the phospholipids in life today, or they may have been made up of membraneless aggregates and polymers of specific hydroxy acids like those discussed in Chapter 4.2.4.4. The search for connections between extant life and prebiotic materials continues to discover plausible chemical systems that show preliminary functionality similar to life. Figure adapted from Hud et al. (2013), Jia et al. (2019), and Frenkel-Pinter et al. (2020).

The prebiotic synthesis of nucleotides (a combination of the nucleobase, sugar, and phosphate units) can be accomplished by one of two different strategies (Fig. 4.10). In the first, the three units are produced separately and then combined via condensation reactions that are typically thermodynamically unfavorable in water (see Chapter 4.2.4.3; Fuller et al., 1972), though it may be more favorable for noncanonical nucleobases (Kolb et al., 1994; Fialho et al., 2020). This could involve sugar condensation onto nucleobases followed by phosphorylation. Alternatively, cyclic phosphorylated sugars in solution could have reacted with purine nucleobases, generating the corresponding nucleotides (Kim and Benner, 2017). In the second strategy (i.e., the indirect approach), the molecular units are formed simultaneously on a single scaffold via low molecular weight compounds (Powner et al., 2009; Becker et al., 2019; Kruse et al., 2020). Alternatively, solutions containing primed pyrimidines such as those with a formyl (R-CHO) or vinyl (R-CH = CH) group can readily undergo aldol reactions with surrounding sugars forming purine ribonucleosides (Becker et al., 2019) and pyrimidine deoxyribonucleosides (Teichert et al., 2019).

OPEN IN VIEWER

FIG. 4.10

Prebiotic synthesis pathways of nucleotides and RNA via the direct (left) and indirect (right) approaches. In the direct approach, sugar, phosphate, and nucleobase components are formed separately and react with each other directly to form nucleotide monomers, which can then polymerize to form RNA. In the indirect approach, atoms destined for the final nucleotide are built onto chemical intermediates in a series of reactions in one-pot (i.e., all together in one solution) before undergoing phosphorylation to form the requisite nucleotides. In either case, polymerization of the resulting nucleotides remains a separate stage in chemical evolution. Figure adapted from Becker et al. (2019).

Although phosphorylation of nucleosides is thermodynamically unfavorable in aqueous solutions (see Chapter 4.2.2), the reaction has been shown to occur (i) in the presence of activated phosphates, namely, diamidophosphate (Gibard et al., 2019), trimetaphosphate (Kolb and Orgel, 1996), or phosphate radicals generated from schreibersite (a meteoritic mineral, [Fe,Ni]₃P) corrosion (Gull et al., 2015); and (ii) in solutions with sufficiently low water activity (i.e., formamide, urea-rich solvents, or solutions subjected to wet-dry cycles) (Gull et al., 2015).

Researchers have also proposed informational systems based on alternative polymers. Metabolic cycles and other forms of complex chemical systems could have allowed for information storage, transfer, and replication (Segré et al., 2000). For example, amyloids, which are aggregates of polypeptides arranged in multiple β-sheets held together via hydrogen bonds, can store information, catalyze reactions, self-propagate, and (unlike RNA) are relatively stable in harsh aqueous solutions (Frenkel-Pinter et al., 2020).

Polypeptides (Fig. 4.9B), or proteins, serve as the catalytic polymers in extant life and are composed of individual monomers called amino acids (see Chapter 2.2.3.2). For proteins, modern life utilizes exclusively 20 α-amino acids in ribosomal synthesis, although non-proteinaceous amino acids (e.g., β-, γ-, and α-amino acids) are used to carry out a myriad of functions. Abiotic reactions can form β-, γ-, and other non-proteinaceous α-amino acids; thus the chemical reasoning behind the selection for the 20 encoded amino acids is an open question. One possibility is that the proteinaceous α-amino acids have a relatively easier time oligomerizing compared to their similar, non-proteinaceous counterparts (Frenkel-Pinter et al., 2019).

Amino acids are hypothesized to have formed prebiotically through several different pathways, including Miller–Urey chemistry, HCN polymerization, the Strecker synthesis, FTT reactions, and reductive amination (see Chapter 4.2.3); they may have also been delivered to early Earth by meteorites (Burton et al., 2012). The difficulty in polymerizing amino acids in aqueous solutions has led some researchers to suggest that proteins are the product of chemical evolution and that the first catalysts may have been alternative compounds such as Fe/Fe-S minerals or organic clusters (see Chapter 4.2.6 for details). Other suggestions include polymers of hydroxy acids (i.e., polyesters), hydroxy and amino acids (i.e., depsipeptides, Fig. 4.9B), or mercaptoacids and amino acids (i.e., thiodepsipeptides) (Forsythe et al., 2015; Mamajanov and Cody, 2017; Frenkel-Pinter et al., 2022).

4.2.4.3. Polymerization of organic molecules

Terran biopolymers are formed through condensation linkages, which are thermodynamically unfavorable in water. Polymerization could have been promoted by low water activity induced by wet-dry cycling, freeze-thaw cycling, hydration of inorganic salts, or the presence of brines (Da Silva et al., 2015; Forsythe et al., 2015; Campbell et al., 2019). Alternatively, polymerization could have been facilitated via deprotonation of the amine via mineral adsorption (Kitadai et al., 2017), with the help of radicals (Pasek et al., 2008) or condensing agents such as cyanamide (Ibanez et al., 1971), or by activating the monomers via phosphorylation or thiolation (adding a thiol group forming a thioester) (Weber and Orgel, 1979; Burcar et al., 2015). Polypeptide polymers may have also been generated via replacement of hydroxy acids in polyesters with amino acids during wet-dry cycling (Forsythe et al., 2015) or due to polymerization of amino amides (NH₂-CHR-CONH₂) (Brack, 2007).

Studies into how prebiotic oligomers/polymers fold, especially their tertiary and quaternary structures, and their catalytic capabilities remains understudied (Frenkel-Pinter et al., 2020). However, an understanding of the overall structures that prebiotic polymers can take on—and the stability of these structures under fluctuating environmental conditions—could inform the selective pressures which resulted in life's specificity for its building blocks (i.e., the biological nucleobases and D-ribofuranose isomer; see Fig. 4.6) for nucleic acids and the proteinaceous amino acids for proteins.

4.2.4.4. Role and formation of compartments

Modern life is composed of cells whose metabolic reactions and materials are enclosed within membranes. Encapsulation was important for chemical evolution of self-replicators as it prevents their diffusion into the environment and minimizes the possibility of a takeover by parasitic molecules (i.e., molecules with no catalytic activity; Bansho et al., 2016). The cells of modern life rely on a bilayer membrane of phospholipids (Fig. 4.9C; see Chapter 2.2.3.3), molecules containing two long carbon-chain fatty acids, glycerol, and phosphate. The prebiotic synthesis of modern phospholipids containing two hydrocarbon chains has not yet been shown, prompting some to propose that the earliest membranes used simpler molecules such as single-chain fatty acids to form vesicles (Apel et al., 2002; Hanczyc et al., 2003; Deamer, 2017). Single-chain fatty acids would require divalent cation chelators or mixed lipids to enable heat and pH stability (Adamala and Szostak, 2013; Sarkar, et al., 2020).

Hydroxy acids—a critical component of fatty acids, polyesters, and phospholipids—would have been delivered by meteorites (Lai et al., 2019) or generated via mineral-facilitated reduction of α-ketoacids (Barge et al., 2020), FTT synthesis (see Chapter 4.2.3.5), the formose reaction (see Chapter 4.2.3.6), or Miller-Urey chemistry (see Chapter 4.2.3.10). Glycerol, a key linker between the fatty acid and phosphate group, could have been formed by the formose reaction, meteoritic delivery, impact events, or synthesis in interstellar ice (Kaiser et al., 2015; Zellner et al., 2020). Phosphorylation of glycerol to form potential phospholipid precursors has also been demonstrated in eutectic or alternative solvents (Dalai and Sahai, 2019).

Liquid-liquid phase separation may be an important aspect of primitive compartmentalization given that it is critical for membraneless organelles and is an important process in the regulation and function of modern cells (e.g., chromatin; Yoshizawa et al., 2020). A variety of membraneless compartments generated from phase separation have been investigated including coacervates (i.e., aqueous phase colloids enriched in molecules/ions; Frankel et al., 2016), aqueous two-phase systems (Cakmak and Keating, 2017), polyester (polymers of hydroxy acids) microdroplets (Jia et al., 2019), and supercritical carbon dioxide (Mayer et al., 2017). Geological structures on early Earth could also have served as precursor membranes. For example, porous cavities in rocks at hydrothermal sites could facilitate thermophoresis (i.e., transportation of molecules driven by a temperature gradient), or synthesis and accumulation of genetic materials (Kreysing et al., 2015). Alternatively, iron-sulfide bubbles precipitated at submarine vents or mineral sheets could have concentrated organics and served as a source of electrical (redox) or mechanical energy for driving prebiotic reactions, respectively (Russell and Hall, 1997; Hansma, 2013).

4.2.6.1. Pre-protein catalysts

Pre-protein catalysts are molecules that are critical to the function of modern-day proteins that are hypothesized to have been capable of catalyzing reactions alone, without their protein envelope, on prebiotic Earth. If true, pre-protein catalysts were encased within a protein only later in the evolution of life, perhaps to ensure the catalyst functioned despite environmental fluctuations (White, 1976). The most favored pre-protein catalysts are cofactors (i.e., a molecule that binds to and activates proteins) such as nicotinamide adenine dinucleotide (NAD⁺; Fig. 4.8C) or metallic clusters (such as the Fe-S clusters in metalloproteins; Bonfio et al., 2018; Sanden et al., 2020). Researchers have postulated that enzymatic Fe-S clusters are an ancient remnant of the earliest life-forms that relied on Fe-S minerals to catalyze reactions (i.e., the Iron-Sulfur World hypothesis; Wächtershäuser, 1990; Goldford et al., 2017).

4.2.6.2. Non-protein catalysts

Non-protein catalysts are catalysts unrelated to the current protein/enzyme used (e.g., a mineral or a metal; Lazcano and Miller, 1999). Metals and minerals have been shown to facilitate various metabolically relevant reactions including CO₂ reduction, reductive amination, and steps of the reverse citric acid cycle (Muchowska et al., 2017; Barge et al., 2020; Weber et al., 2022). Per the Iron-Sulfur World hypothesis, iron-sulfur minerals are predicted to have been critical for abiogenesis given their abundance on early Earth and structural similarity to Fe-S clusters, which are ubiquitous in life (Wächtershäuser, 1990; Nitschke et al., 2013; Bonfio et al., 2018). Researchers have also reported the use of metal catalysis for protometabolic reactions under conditions simulating alkaline hydrothermal vents (Kitadai et al., 2019).

4.2.6.3. Protometabolic cycles and networks

Chemical reactions in isolation would not have been sufficient to facilitate the jump from chemistry to life. Indeed, life sustains itself via a series of linked chemical reactions where the product of the first reaction is the starting material for the second, and so on until the desired product (i.e., ATP) is generated (Fig. 4.11). On early Earth, a series of linked reactions could have been considered a protometabolic cycle. Research has demonstrated that large metabolic networks can be generated from 10 enzymatic functions, suggesting that the formation of a few unspecific catalysts would have been sufficient for producing protometabolic cycles/networks, consisting of many linked reactions (Goldman et al., 2012).

OPEN IN VIEWER

FIG. 4.11.

An example of a hypothetical protometabolic network composed of three protometabolic cycles (green, blue, purple). A protometabolic cycle consists of two or more reactions whose products are designated by letters and which feed into the subsequent reaction. The cycles form a network when their end products (i.e., the red letters) feed forward to a different metabolic cycle.

While protometabolic cycles could have facilitated the formation of organics needed to sustain/initiate life, such prebiotic reactions were probably inefficient with low product yields. Low yields are prohibitive toward forming more complex, larger organic molecules and sustaining protometabolic cycles, as both rely on the products generated via multiple inefficient reactions. One proposed solution is autocatalytic cycles (Preiner et al., 2019). Autocatalysis is the catalysis of a reaction by one of its products, creating a positive feedback loop. Some examples of autocatalytic reactions include the Soai reaction (which leads to nearly homochiral products; Soai et al., 1995) as well as FTT and formose reactions.

Like protometabolic cycles, autocatalytic cycles can be combined to form complex networks. If the network can be sustained via a set of ambient “food,” the network can be referred to as a “reflexively autocatalytic food-generated” (RAF) set. To fit this definition, the system must be catalytically closed (i.e., all of the reactions are able to react within the system) and self-sustaining (Hordjick et al., 2019).

4.3.1.1. The presence of salts

Salts can form complexes with organics, facilitating reactions and even promoting selectivity in complex reactions (e.g., formation of cis-diols like ribose in the formose reaction; Chapter 4.2.3.6). By coordinating with water, salts also lower the water activity of an aqueous solution, promoting polymerization (see Chapter 4.2.4.3). However, dissolved ions could also have introduced new hurdles to the origins of life: cations could inhibit phosphorylation (see Chapter 4.2.2.2) and vesicle formation by chelating with phosphates [HPO₄ ^2-] and forming soaps/crystal aggregates with carboxylates [R-COO^-], respectively (Milshteyn et al., 2018). Although cations disrupt the formation of some membrane precursors (Apel and Deamer, 2005), high salt concentrations do not disrupt vesicle formation from mixtures of amphiphiles (e.g., fatty acids, hydroxy acids; Jordan et al., 2019).

Suggestions that the ionic concentration of modern cells reflects the chemical composition of the environment in which life first evolved argue against marine environments for abiogenesis (Mulkidjanian et al., 2012). This argument is based on two related observations: (i) regulating intracellular salinity (i.e., osmoregulation) requires complex biomolecules, and (ii) intracellular trends of K⁺ (100 mM) and Na⁺ (10 mM) are opposite to that of marine waters, which have about 40 times as much Na⁺ (460 mM) than K⁺ (10 mM). If protocells had similar Na⁺ and K⁺ as modern-day cells, protocells in a marine environment would have had to constantly expend energy to maintain their Na⁺ and K⁺ concentrations from equalizing with surrounding waters. Moreover, if salinity was not tightly regulated, fluctuations in the environment could have caused cells to rapidly shrink or expand and rupture. Environments more closely aligned with cellular composition include surficial hot springs where K⁺ concentrations can exceed that of Na⁺ (Mulkidjanian et al., 2012).

4.3.1.2. High versus low temperatures

Elevated temperatures can increase reaction yields and rates of prebiotic reactions (e.g., formose and FTT reactions; see Chapter 4.2.3). Early Earth's surface was relatively hot following primary accretion and continued bombardment from impactors (see Chapter 3.4.3.1 and Chapter 4.1.1), and genomic evidence suggests some of the earliest organisms may have been thermophiles, although this claim remains controversial (Forterre, 2015).

Higher temperatures can also promote organic degradation and induce nonproductive condensation reactions, which together could have driven the formation of insoluble, unreactive tar. Lower temperatures better preserve organic molecules and promote condensation reactions and the catalytic activity of proto-enzymes (Attwater et al., 2010). As ice crystals grow, they tend to exclude dissolved impurities (e.g., organics or salts) via a process known as eutectic freezing. The residual eutectic fluids are significantly enriched in dissolved compounds that can facilitate organic synthesis (Menor-Salván and Marín-Yaseli, 2012).

4.3.1.3. Acidic versus basic pH

Mildly acidic conditions could have been beneficial for the origins of life given that under such conditions (i) the RNA phosphodiester bond is most stable at pH 4–5, (ii) peptide bonds are most stable at pH 4–7.5, with pH 6 being optimum, (iii) a plethora of isolated ribozymes are more active, (iv) RNA polymerization via wet/dry cycles is more efficient, and (v) phosphate minerals slowly dissolve, releasing phosphate (Bernhardt and Tate, 2012). Alternatively, alkaline conditions may have been important for other aspects of abiogenesis, as many other prebiotically important reactions are favored in alkaline solutions (e.g., the formose reaction and HCN polymerization; see Chapter 4.2.3).

4.3.1.4. Importance of minerals

Water on Earth has been exposed to a rocky surface throughout geologic history. Thus, minerals were present for prebiotic reactions and may have even been critical for abiogenesis. Indeed, minerals can promote organic preservation, facilitate reactions and polymerization (e.g., Ferris 2005; Kitadai et al., 2017), enhance enantiomeric excess via their inherent chirality (see Chapter 4.2.5), and concentrate otherwise dilute reactants onto a catalytic surface (Salman et al., 2012; Alshameri et al., 2018; Flores et al., 2021).

While adsorption can promote reactions, it limits the mobility of organics, which inhibits their reactivity in solution. For example, when organic material was added to a hot spring pool, it quickly adsorbed onto clays and never became encapsulated into the dissolved lipid vesicles (Deamer et al., 2006). Importantly, in a fluctuating environment, geochemical changes could lead to the rapid release of adsorbed species (Ruiz-Bermejo et al., 2013).

4.3.2.1. Submarine hydrothermal vents

Several types of hydrothermal vents could have existed on early Earth, including black smoker vents (high temperatures of 350–400°C, acidic with pH 2–5, metal and sulfide rich), white smoker vents (similar temperatures and pH as black smokers, Zn-enriched, Fe and sulfide depleted), diffuse vents (moderate temperatures of 50–200°C, acidic to alkaline, metal depleted), and alkaline vents (mild temperatures of 40–75°C, alkaline with pH 10–12, sulfide/metal depleted). Acidic vents can form wherever a magma chamber is near oceanic sediments. Typically, this formation can occur at mid-ocean ridges where seafloor spreading or subduction zones develop, but they can also form at volcanic hot spots (e.g., at Lōʻihi Seamount). Alkaline vents can form wherever ongoing serpentinization of ultramafic rock or (to a lesser extent) mafic rock takes place (see Chapter 4.2.3.1). While most hydrothermal vents form in deeper waters (typically thousands of meters), they have also been observed in shallow-sea settings (5–212 m depth), where there is sufficient sunlight for photosynthesis (Gilhooly et al., 2014; Price and Giovannelli, 2017). For a review on hydrothermal vent chemistry, see Kelley et al. (2002) and Tivey (2015).

As the interior of Hadean Earth was hotter, the young planet is thought to have been more geologically active, thus promoting the formation of acidic hydrothermal vents at the seafloor. Alkaline vents were also likely widespread during the Hadean since the crust and mantle would have been less differentiated, resulting in an ultramafic Hadean seafloor that could more readily undergo serpentinization (Sleep et al., 2004). Although acidic hydrothermal vents have several useful properties for abiogenesis, the cooler alkaline vents have since gained favor in hypotheses concerning the origins of life on Earth as they have (i) longer lifetimes (up to 30,000 years; Früh-Green et al., 2003), (ii) higher concentrations of H₂ (Russell et al., 2014), (iii) alkaline pH and high concentrations of Ca²⁺ and Mg²⁺, which can facilitate the formose reaction (Kopetzki and Antonietti, 2011), and (iv) cooler temperatures, normally within the range of 40–75°C (Kelley et al., 2002).

Hydrothermal vents display several characteristics that could have been conducive for the emergence of life because they have (i) porous structures, which provided containment of reactants in a manner analogous to a protocell, (ii) pH gradients (Δ pH = 3–5) that can drive reactions and are key to metabolism (Sojo et al., 2016), (iii) thermal gradients which have been shown to promote the replication of DNA (Mast and Braun, 2010), and (iv) proliferation of catalytic metals/minerals that can drive organic synthesis (Barge et al., 2020). However, there are several limitations to submarine hydrothermal vents as settings for the origins of life such as the following: (i) wet/dry cycling (which promotes condensation reactions) is not possible, although this may not have been an issue for shallow vents (Barge and Price, 2022) or if condensation reactions proceeded via different mechanisms (see Chapter 4.2.4.3); (ii) for alkaline vents, a combination of high pH and elevated Ca²⁺ and Mg²⁺ concentrations favor the formation of insoluble phosphate minerals and destabilize RNA (Bernhardt and Tate, 2012); and (iii) regulating against high salinity may have been difficult for any protocell (see Chapter 4.3.1.1).

4.3.2.2. Deep subsurface beneath oceanic crust

The deep subsurface (i.e., kilometers beneath the Hadean crust) may have contained an abundance of H₂ formed via serpentinization, mineral catalysts, and elevated temperatures. An origin of life in the deep subsurface (Gold, 1992) would have benefited from incredibly stable conditions (remaining unchanged for millions of years) compared to the more ephemeral hot springs or hydrothermal vents. In fact, the deep crust may have been the only early Earth environment protected from impact events (Maher and Stevenson, 1988; Grimm and Marchi, 2018). Additionally, the deep subsurface was ubiquitous on early Earth with organics that, unlike those produced in the bulk ocean, could be more easily concentrated. However, complex chemical systems, gradients, and mixing, which are critical for establishing metabolism and driving chemical reactions away from equilibrium (see Chapter 4.2.1), would have been hindered in the deep subsurface. Additionally, the deep subsurface would have had limited access to organics brought by exogenous sources or generated at the surface.

4.3.2.3. Terrestrial hot springs

Terrestrial hot springs were first broadly proposed as a potential site for the origins of life by Charles Darwin and have been widely considered since (see Damer and Deamer, 2020 and references therein). Importantly, hot springs differ from deep sea hydrothermal vents in several ways as they (i) act as an interface between atmosphere/minerals/fluids and allow for wet/dry cycling (Damer and Deamer, 2015); (ii) concentrate and make available important prebiotic minerals (borates, nitrates, and phosphates) through water/rock interactions, evaporation, wet-dry cycling, or low pH (Burcar et al., 2016; Ranjan et al., 2019; Steller et al., 2019); (iii) have an abundance of clay minerals that can concentrate and polymerize biologically relevant monomers for the origins of life (Ferris, 2005; Hazen and Sverjensky, 2010); (iv) are exposed to solar and cosmic radiation that may aid in prebiotic synthesis (Ranjan and Sasselov, 2016); and (v) exhibit steep and complex chemical/temperature/pH gradients through mixing zones, thus forming different aqueous bodies with unique chemistries and creating “pools of innovation” to foster the origins of life (Van Kranendonk et al., 2017, 2021). However, some of these benefits may also be detrimental because (i) the low pH may destroy polymers by hydrolyzing bonds (Mungi and Rajamani et al., 2015), (ii) clays could adsorb organics and remove them from solution, therefore impeding reactions (Deamer et al., 2006), and (iii) solar and cosmic radiation could destroy organic molecules (Phoenix et al., 2006; Farr et al., 2023).

Hot spring pools may have been present on early Earth as evidenced by the discovery of geyserite (a mineral only produced in surficial hot springs) interbedded within the 3.5 Ga stromatolite fossils of the Pilbara Craton, Western Australia (Djokic et al., 2017). Further evidence of terrestrial hydrothermal pools stem from ∼4.3 Ga zircons from Jack Hills, Australia, whose rare earth elements and isotopic content suggest that these terrestrial pools may have been fed by relatively oxidized fluids that facilitate the dissolution of metals (e.g., Fe²⁺, Fe³⁺, Mn²⁺, and Zn²⁺) that can drive prebiotic chemistry (Trail and McCollom, 2023). In contrast to hot springs of today which exist over a granitic/felsic crust that is relatively depleted in Fe/Mg and enriched in silica and Na/K (Fig. 4.1), Hadean hot springs over a basaltic bedrock, such as those on Hawaii or Iceland, would have been enriched in magnesium and iron.

4.3.2.4. Other environments

Other environmental niches have been proposed as containing favorable components for the origins of life on Earth, including the shallow subsurface sedimentary environment (Westall et al., 2018). Pumice rafts and microlayers of organic “oily slicks” on the ocean surface could have accumulated suspended mineral grains and served as havens for origins-of-life chemistry (Brasier et al., 2011). Shallow pools on exposed land masses (e.g., basaltic islands) may have formed from rainfall; these pools may have promoted condensation reactions via repeated aerosol formation from splashing (Ruiz-Bermejo et al., 2007; Nam et al., 2017) or formation of semi-anhydrous solvents upon significant evaporation (Burcar et al., 2016). Furthermore, tide pools could have accumulated radioactive detrital grains, which may have aided in prebiotic reactions (Adam, 2007) or concentrated organics upon freezing (see Chapter 4.3.1.2).

It is important to remember that these systems do not work in isolation, and there is a degree of mixing that creates unique and complex settings (Stüeken et al., 2013). Deep sea hydrothermal vents may appear as isolated systems, but clay minerals can transport organics concentrated on the ocean's surface to the seafloor (Kennedy et al., 2002). Thus, multiple environmental combinations may have been required to facilitate the emergence of life. Moreover, different environments could have hosted their own genesis events (i.e., life might have arisen multiple times on early Earth).