====== 5.3 Origin of life on earth ======

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The process of life's origin will be described here in seven steps. The first step is **geochemical** carbon dioxide fixation occurring within Hadean alkaline hydrothermal vents. Specifically, these structures featured porous mineral walls separating alkaline vent fluids from acidic seawater. This pH difference created natural proton gradients across thin mineral membranes, serving as a primordial power source. Within these microscopic pores, hydrogen and carbon dioxide reacted via a natural proton-motive force across the inorganic barriers containing catalytic iron-nickel-sulfide minerals.

The second step is **protocell** formation, where organic molecules like long-chain fatty acids became concentrated in rocky pores, leading to spontaneous self-assembly. These compartments consisted of mixed single-chain amphiphile bilayers that enclosed emerging chemical reactions. Unlike modern phospholipid membranes, these fatty-acid bilayers were sufficiently leaky to protons, allowing the continuous influx and efflux of ions required to tap into the geochemically sustained proton-motive force. These protocells provided a boundary that prevented the dissipation of precursors while maintaining internal stability for metabolic reactions.

The third step is **protometabolism**, where flow of energy driven by proton gradients facilitated the rise of complex chemical networks within protocells. This involved essential building blocks like lipids, sugars, and amino acids. Central to this phase was the reverse Krebs cycle, operating non-enzymatically to recycle carbon. Iron-sulfur clusters associated with the membrane acted as redox switches, facilitating carbon fixation through positive feedback loops. This architecture represents a deterministic form of metabolic heredity through the physical inheritance of internal metabolic catalysts.

The fourth step is **RNA polymerization**, where metabolic expansion within protocells favored the formation of long, chain-like molecules. An RNA polymer is composed of repeating nucleotide units that can store genetic information and potentially catalyze vital chemical reactions. Successful polymerization would reinforce metabolic pathways, as these molecules fished amino acids and nucleotides from solution, pulling greater flux through biosynthetic routes. This architectural self-reinforcement marked a shift toward complexity, enabling transition from chemical cycles toward sophisticated informational biological systems for emerging life on Earth.

The fifth step is **genetic coding**, where the emergence of information provided a platform for generating biological functions through direct biophysical interactions. Patterns in the code suggest that amino acids originally associated with cognate RNA sequences based on their biosynthetic distance from carbon dioxide fixation. These nonrandom interactions allowed random RNA sequences to template peptides nonrandomly, bypassing the need for complex translational machinery initially. This leap allowed the protocell to regulate internal chemistry, establishing the first link between a genotype and phenotype.

The sixth step involves the evolution of **polypeptides** and molecular machines, ribosomes and proteins. Random genetic sequences template nonrandom peptides, producing selectable functions like the proto-Ech, which uses cysteine-containing peptides to coordinate iron-sulfur clusters for carbon fixation. These complex machines enabled the protocell to perform specific tasks more robustly than simple mineral catalysts. Ribosomes emerged as factories translating genetic code into functional proteins, allowing for the very precise execution of biological instructions required for the maintenance and growth of autonomous biological evolved entities.

The seventh step is the emergence of the first **living cells**, characterized by full autonomy and self-replication. The last universal common ancestor transitioned from relying on external mineral-bound gradients to generating its own internal chemiosmotic potential across ion-tight membranes. These cells possessed the complete genetic code, transcription, translation machinery, and membrane-integral proteins like ATP synthase. This transition from random chemistry to organized biological blueprints allowed life to leave vents and colonize various environments, establishing the foundation for biological evolution across planet.