**1. Introduction**

Origins of life remain a mystery for our humankind. The concept of "chemical evolution" describes a general evolutionary route from the abiotic to the biotic world through a variety of chemical and physical processes. Kitadai. et al. summarized the reactions explored in the lab for the chemical evolution (**Figure 1**). Starting from geologically abundant molecules (e.g., N2, CO2, H2, PO4, NO3 − , HCN, etc.), high energy input drives the synthesis of small organic molecules as precursors. These small organic molecules react with each other to form life's building blocks (e.g., amino acids, nucleobases, sugars, aliphatic acids, etc.). Subsequently, these monomers polymerize into functional polymers which assemble into the so-called protocell.

**Figure 1.** *Overview of the chemical evolution of life, adapted from ref. [1].*

As many life's building blocks are not stable at temperatures higher than 100°C [2], a geological setting with moderate temperature is considered to be more favorable for life's emergence. In addition, a moderate temperature can render the chemical system a kinetic control that will otherwise only generate the most stable products following thermodynamics under a high-temperature regime. Kinetic control is required to form metastable products. There are many challenges in chemical evolution, two of which are caused by reaction kinetics. First, in the beginning, how geologically abundant inorganic molecules were activated and converted into small organic molecules? Second, how the chemical reactions are directed towards a high molecular complexity and product diversity for selectively generating life's building blocks? In this regard, catalysis is at the center of chemical evolution. A catalyst lowers the energetic barrier and enhances the reaction rate of activation of inert molecules. Different catalysts with tuned surface property and electronic characteristics can regulate the reaction pathways by adjusting the transition states of the intermediates.

Geological molecules, as the feedstocks of prebiotic synthesis, are typically chemically inert despite their high abundances. The activation of small geological molecules, such as H2, CO2, etc., requires redox processes. H2 needs to be oxidized to release the chemical energy while CO2 needs to be reduced to synthesize organics which usually show intermediate valence states of carbon (from +3 to −3) [3]. Similarly, methane (CH4), which was considered to be abundant on the early Earth in some scenarios [4], needs to be oxidized to synthesize useful organics. Therefore, in general, redox processes play an important role in chemical evolution.

The importance of redox processes for energy conservation is also reflected in modern biology, where modern living organisms are relying on enzymes for catalyzing biochemical reactions and maintaining homeostasis. In particular, redox

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

enzymes are important for organisms to harvest energy from geologically available molecules in their surrounding environments. For example, methanogens convert CO2 and H2 into methane, with the generation of proton gradient for ATP synthesis [5, 6]. Nitrogen-fixing organisms use redox enzyme nitrogenase to reduce N2 into ammonia for N assimilation, which is a 6e− /6H+ reaction (N2 + 6e− + 6H+ = 2NH3) [7, 8]. These metabolic reaction pathways were considered to be very ancient based on phylogenetic analysis, which could have appeared in the last universal common ancestor (LUCA) [9–11]. Redox enzymes accounting for these reactions highly rely on earth-abundant transition metals (e.g., Fe, Ni, Mo, etc.) due to the electron-shuttling property of these metal sites and the relatively high affinity of the *d*-orbital electrons with the small molecules. Meanwhile, the interaction between amino acid residuals from the surrounding peptides and the metallic center, and the ligands in the first coordination sphere, also plays an important role in the catalytic processes [12, 13]. This includes mediating proton transfers, stabilizing the intermediates through electronic interaction, etc.

However, before life emerged, it has long been considered that enzymes are too complex to be readily available. What are the geo-catalysts responsible for activating small molecules (including C-, N-, and S-related compounds)? Earth owns more than 5700 known species of minerals, with new species being identified every year (e.g., https://rruff.info/ima/). Both the variety and relative abundances of minerals have changed dramatically over the Earth's history, through various chemical, physical, and biological processes [14]. To understand the role of minerals in the origin of life, determining the first place to spawn the first life is an essential question. There are two dominating and contrasting scenarios of origins of life: those predicting that life emerged in the submarine, alkaline hydrothermal vent systems where the redox, pH, and T gradients keep the system far from equilibrium and serve as energy sources for prebiotic synthesis, as pioneered by Russell, et al. [4, 9, 15–18]; and those predicting that life emerged within subaerial environments with prebiotic synthesis driven by UV photolysis pioneered by Sutherland et al. [3, 19–23]. Both of these two scenarios implicitly emphasize the importance of redox processes for activating inert molecules. The former scenario proposed minerals (such as sulfides and hydroxides) as key players, while the latter relies on radicals and solvated electrons for redox conversions. Recently, an alternative scenario of origins of life in volcanic hot-spring water or the so-called "land-based pool" scenario was proposed by Damer and Deamer [24–26], to solve the self-assembly problem for membrane formation in the salty ocean while allowing condensation/polymerization through wet-dry cycle provided by the fluctuating boundary conditions. This scenario has been testified with self-assembly experiments simulating the hot-spring conditions [25]. Since the role of minerals hasn't been explicitly considered in the scenarios by Sutherland et al. and Deamer et al., only the scenarios involving mineral catalysis (e.g., alkaline hydrothermal vent (AHV) theory, iron–sulfur world theory by Wächtershäuser [27–31]) will be discussed in this chapter.

AHV theory was proposed based on the notion that the far-from-equilibrium condition in alkaline hydrothermal vent systems resembles biochemistry in the following aspects: (1) the large chemical disequilibrium is akin to the conditions the biology tends to live on and stably maintained through Earth's geological time; (2) the pH gradient sustained by the chimney rock wall resembles the chemiosmotic energy conservation shared by all life forms; (3) the transition metal-bearing mineral walls are rich in sulfides, which share the similar metal center and sulfur ligands with the modern Ni-, and Fe-bearing redox enzymes (e.g., carbon monoxide dehydrogenases, hydrogenase, ferredoxin, etc.) [9], thus could have catalyzed similar chemical conversions; (4) many

chemoautotrophic microorganisms were discovered in the deep-sea hydrothermal vents and their metabolism is suggested to be phylogenetically old and energetically fueled by the chemicals in the vents; (5) different from the acidic type, high temperature hydrothermal vents, the low temperature (<120°C), alkaline, lost-city type hydrothermal vents renders kinetic control and could stabilize biomolecules formed in-situ.

Regardless of the scenarios, minerals have shown special functions in different types of prebiotic synthesis. Here in this chapter, a special focus will be posted on the redox catalysis mediated by minerals for the prebiotic synthesis, involving C, N, S, which are the fundamental elements of life and involved in a variety of redox conversions.
