**4.1 Availability of metals in the Hadean ocean**

It is not surprising that many bio-essential metal elements are also Earthabundant, considering the high reliance of the biosphere on the geosphere. These include mainly d-block elements (vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), molybdenum (Mo), tungsten (W), and zinc (Zn)) and exist in numerous oxidation states and be bonded by various ligands (O, S, etc.) with varied crystal structures and stoichiometries in minerals. The variety and relative abundances of minerals evolve with the Earth's history and also depend on the geological type of the locality [14]. In the ocean, changes in elemental abundances on geological time scales are intimately linked to evolutionary processes [87]. The availability of soluble transition metals changes progressively with time, with the greatest change in the redox-sensitive elements. However, this has provided environments enriched with an immobilized form of minerals, which would have provided the active surface for promoting prebiotic organic synthesis. The redox state of the environment evolved through at least three stages (adapted from ref. [87]), with major oxygenation events occurring ~2.4 billion to 1.8 billion years ago during the first of these stages, the ocean was largely devoid of dissolved O2, and iron was abundant in the form of dissolved Fe2+ complexes. Much of the sulfur at that time was in the form of insoluble sulfide minerals locked in the continental crust. Besides Fe, the ocean abundances of transition metals such as manganese, cobalt, nickel, copper, zinc, and molybdenum are sensitive to environmental redox conditions, and also precipitate as sulfide minerals.

The scarcity of many bio-essential transition metals due to precipitation as insoluble sulfides has been considered to limit the size and shape the metabolism of the primordial biosphere [87, 88]. However, in terms of prebiotic chemistry, both the soluble and precipitated forms of transition metals could contribute to promoting the reactions, as have discussed in Section 3. The immobilized form of metal sulfides could have provided active surfaces with enormous potentials for catalytic functions. Therefore, for future prebiotic synthesis studies, more work using non-iron elements should be conducted to screen optimal geological catalysts.

### **4.2 Chemical diversity of metal sulfides**

Since transition metals mainly existed in sulfides during Hadean and early Archean eon, these sulfides have been studied for prebiotic synthesis, as described in Section 3. However, in most cases, the activities of these minerals are low compared with their enzymatic counterparts and their contribution to prebiotic synthesis has rarely been quantitatively constrained based on their kinetics with some exceptions [73]. A possible reason accounting for the low reactivity is that prebiotic synthesis researches have been heavily focused on the most earth-abundant minerals (e.g., FeS, and NiS). To explore the chemical diversity of sulfide minerals, Li et al. evaluated the chemical diversity of metal sulfides of Co, Cu, Fe, Mn, Mo, Ni, V, and/or W with 135,434 species-locality pairs recorded in the mineralogy database (http://rruff. info/ima/) [88]. The diversity and distribution of these metal sulfides were analyzed in terms of the following aspects: locality frequency, multiple metal composition, crystal structure, and valence state of dominating elements. It was found that natural

### **Figure 3.**

*(A) Metal sulfide distribution in natural environments. The 20 most frequently observed species are ranked in order of locality counts. The chemical composition of each species is shown. (B) Distribution and chemical diversity (chemical composition, Fe/S valence states, and crystal symmetry) of Fe single-metal sulfides. (C) Relative abundances and locality distribution of Fe2+-, Fe3+-, and Fe2+ plus Fe3+-sulfides. Adapted from ref. [88].*

metal sulfides show marked variations in chemical composition, crystal structure, and metal/sulfur valence states, suggesting a large chemical space associating with chemical variations of sulfide minerals still waits for exploration (**Figure 3**). For Fe sulfides, unexpectedly, mackinawite (FeS) is not among the top-ten mostly frequently observed species. This suggest that it may be problematic to use this mineral as a dominant target for prebiotic synthesis. Rather, pyrite is the most frequently observed species. The observation of the S2 2− state suggests that not only metals can mediate redox change, sulfur ligands can also participate in the redox reaction with the valence change (S2 2−/S2−).

Each metal also has the dominant valent states. In Fe-S species, more than 84.86% of localities feature only Fe2+ species, followed by those containing both Fe2+ and Fe3+(15.12%), and by three sulfides that only contain Fe3+ (0.02%). Generally, metals with low valence states predominate the library of metal sulfides, except for Co, allowing these minerals to act as an electron source or a catalytic center for charge accumulation during redox conversion. The minerals with mixed-valence states could exhibit unique functions due to special electron transfer and surface adsorption properties [89, 90]. Moreover, binary metal compositions are ubiquitous in natural sulfides. For example, Ni-Fe sulfides have ten species, and seven of them contain Ni and Fe as substitutional cations with a wide Ni/Fe ratio range (0 ~ 35 at%), with pentlandite (Ni,Fe)9S8 being the most prevalent form. The other three species contain fixed Fe/Ni ratios. The capability of Ni-Fe binary sulfides to have both fixed and varied Ni/Fe ratios in their structures is a unique characteristic different from that of Cu-Fe binary sulfides, in which Cu and Fe tend to form specific structures with fixed stoichiometries. The great chemical diversity provides a wide variety of catalytic functions and suggests that there is still a large chemical space of minerals for the exploration of unknown reactivities.

### **4.3 Enzyme analog minerals**

Based on the discussion above, screening suitable geological catalysts is challenging and requires rational approaches. In this regard, machine learning or big-data mining could provide promising solutions.

Given an envisioned evolution from geochemistry to biochemistry, the mineral-and enzyme-based catalytic systems could be compared to understand the evolutionary link between them. In 2014, Michael Russell and his colleagues proposed that minerals sharing similar metal sites and ligands with enzymes could serve as a prebiotic catalyst for activating small geological molecules [4] (**Figure 4**) because the similar structure

*Minerals as Prebiotic Catalysts for Chemical Evolution towards the Origin of Life DOI: http://dx.doi.org/10.5772/intechopen.102389*

**Figure 4.**

*Structural comparison between minerals and enzyme active centers. Adapted from ref. [4].*

could exhibit similar chemical affinity towards the same substrate. Many prebiotic syntheses are influenced by similar perceptions and pursue the prebiotic carbon and nitrogen fixation using enzyme mimetic mineral catalysts [27, 45, 48, 49, 51, 52, 54, 69]. This idea could narrow down the candidates of geo-catalysts, however, an inherent difficulty in studying the property of minerals is the wide range of data and parameters to consider when searching for an appropriate catalyst for a specific enzymatic reaction [91]. Especially, since the structure (particularly the first coordination structure) alone doesn't dictate the overall catalytic property, the structural resemblance between minerals and enzymes doesn't ensure a definite functional similarity.

To solve this problem, a computational approach has been employed to systematically compare the metal–ligand structure of minerals and enzymes, as reported in recent work by Zhao et al. [91]. They compared the metalloenzyme cluster structure recorded in the protein database and the mineral structural data in the mineralogy database, using molecular similarity metrics. Therefore, this is probably the first attempt to quantify the structural similarity between biological machineries and minerals. In this study, iron–sulfur and nickel–iron–sulfur ligands were analyzed. Except for greigite and mackinawite, other iron sulfide species (marcasite and troilite) that were less studied previously were also predicted to have high structural similarity with iron–sulfur clusters in biology. Therefore, these results highlight the predictability of the modeling method for searching less studied minerals that hold potential as early prebiotic catalysts.

## **5. Conclusions**

Minerals have been considered as the key player for prebiotic synthesis, and up-to-date researches have verified the catalytic property of many prebiotic mineral catalysts towards the carbon and nitrogen fixation reactions. Based on these discussions, mineral-mediated processes are probably critical for the evolution of protometabolism towards autotrophic origins of life. However, the rare demonstration of the formation of other types of life's building blocks, such as sugars, nucleobases, etc., accompanied by the difficulty in polymerization in water, pose several obvious challenges for many related origins of life scenarios [92]. To solve this problem, extensive screening of prebiotic catalysts based on the mineralogy database combined with the numerical prediction of structure–function relations should be helpful. Alternatively, other types of membrane and replicating systems have been proposed [16]. Additionally, the evolution of the mineral catalytic system could be further explored, by considering the hybridization of organic ligands with minerals. These organic ligands could be short peptides that can be formed under geologically plausible conditions [93, 94], or other types of polymers (e.g., polyesters [95, 96]). The hybridization of these organic ligands introduces stereochemical and electronic control on the whole reaction systems, which could help with overcoming the kinetic problems for certain reactions.
