Chemical Evolution and the Origin of Life

Introduction

From chemistry to life: the prebiotic steps that produced the first self-replicating molecules

In 1953, Stanley Miller and Harold Urey sealed a glass flask containing water, methane (CH₄), ammonia (NH₃), and hydrogen (H₂) — gases thought to represent the early Earth's atmosphere — and passed electrical sparks through the mixture to simulate lightning. Within a week, the flask had turned reddish-brown with a rich brew of organic compounds, including five of the twenty standard amino acids. The result was electrifying: if so many of life's building blocks could appear spontaneously from simple inorganic precursors in just a few days in a laboratory flask, then 100 million years of early Earth chemistry seemed ample time for life to arise. Subsequent analyses of the original Miller-Urey samples, conducted decades later using modern mass spectrometry, revealed that more than 20 amino acids had actually been produced — far more than initially detected. Later variants of the experiment substituting CO₂/N₂/H₂O for the more reducing atmosphere yielded similar, if somewhat lower, organic yields, confirming that prebiotic synthesis is robust across a range of atmospheric compositions. Beyond the laboratory, carbonaceous chondrite meteorites such as the Murchison meteorite (fell Australia, 1969) have been found to contain over 70 amino acids, including many that are rare or absent in biology — direct evidence that organic chemistry is widespread in the cosmos and that the raw materials of life were delivered to early Earth from space.

The Oparin-Haldane hypothesis, formulated independently by Soviet biochemist Alexander Oparin (1924) and British geneticist J. B. S. Haldane (1929), proposed that the early Earth's reducing atmosphere and sunlit oceans provided a natural reactor for the synthesis of organic molecules — the so-called primordial soup. In this framework, monomers (small organic molecules such as amino acids and nucleotides) accumulated in the oceans or tidal pools and gradually polymerised into larger chains: peptides and polynucleotides. Heating and wetting cycles in tidal pools or hydrothermal environments could drive polymerisation even without enzymes, producing short peptides and RNA-like oligomers on mineral surfaces. Experiments by Sidney Fox in the 1950s–60s demonstrated that dry amino acids, when heated, spontaneously form microsphere-like proteinoid structures with some catalytic activity — a tantalising hint that protein-like polymers could self-organise under simple geochemical conditions.

Three steps stand between the primordial soup and the first living cells: (1) the formation of monomers from inorganic precursors, (2) the polymerisation of monomers into information-carrying and catalytic polymers, and (3) the encapsulation of those polymers within a membrane boundary to form a protocell. Of these, step (2) was the deepest conceptual puzzle until Francis Crick and Leslie Orgel proposed the RNA World hypothesis in the 1960s, later championed by Carl Woese and Thomas Cech. RNA is unique because it can store genetic information (like DNA) and also fold into three-dimensional shapes that catalyse chemical reactions (like proteins). Ribozymes — catalytic RNA molecules — were discovered by Thomas Cech and Sidney Altman in the 1980s, earning them the 1989 Nobel Prize in Chemistry, and they demonstrated beyond doubt that the chicken-and-egg paradox of which came first — genes or enzymes — could be resolved by a single molecule that does both. In the RNA World model, self-replicating RNA molecules were the first Darwinian entities; proteins and DNA came later as refinements.

The question of where life originated has increasingly focused on hydrothermal systems on the ocean floor. Two contrasting environments have been proposed: high-temperature, acidic black smokers (discovered 1977 on the Galápagos Rift), where superheated (>300°C (572°F)), mineral-rich fluids emerge from volcanic activity; and cooler (~40–90°C (194°F)), alkaline Lost City-type vents (discovered 2000 on the Mid-Atlantic Ridge), where serpentinisation reactions between seawater and mantle rocks produce hydrogen-rich, alkaline fluids that naturally generate pH and electrical gradients strikingly similar to those used by living cells in their membranes. The Lost City alkaline vent model, championed by Mike Russell and Nick Lane, is currently favoured for several reasons: the gentler temperatures are more compatible with fragile early RNA chemistry, the natural proton gradient across thin iron-sulfide mineral membranes could have directly driven the first energy-conserving reactions, and the labyrinthine micropores of carbonate chimneys provide cell-sized compartments that could have served as proto-membranes before true lipid bilayers evolved.

Step (3) — the formation of protocells — requires that organic polymers become enclosed within a lipid membrane. Phospholipids and simpler fatty acids are amphipathic molecules: they have a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. In water, these molecules spontaneously self-assemble into bilayer vesicles — hollow spheres bounded by a two-molecule-thick membrane — driven purely by thermodynamics with no biological machinery required. Jack Szostak's laboratory has shown that fatty acid vesicles can grow, divide, and even take up RNA oligomers from the surrounding solution, making them compelling models for the first protocells. The competing lipid-world, metabolism-first, and replication-first models disagree about which came first: the membrane container, the catalytic metabolic network, or the self-replicating polymer. Current evidence suggests these components co-evolved and were mutually reinforcing rather than arising in strict sequence. All roads eventually lead to LUCA — the Last Universal Common Ancestor — the population of organisms from which all Bacteria, Archaea, and Eukarya descend, estimated to have lived roughly 3.5–4.0 billion years ago, and reconstructed by comparative genomics to have already possessed a full complement of ribosomes, ATP synthase, and genetic code.

Key Terms