Oparin-Haldane Hypothesis and the Miller-Urey Experiment

By the early twentieth century, biologists broadly accepted that life must have arisen from non-living matter at some point in Earth's distant past, but no one had proposed a scientifically testable mechanism for exactly how that transition could have happened, until the Russian biochemist Alexander Oparin and, independently, the British scientist J. B. S. Haldane put forward closely similar hypotheses in the 1920s that came to be known jointly as the Oparin-Haldane hypothesis. Both proposed that the very first atmosphere surrounding Earth was fundamentally different in composition from today's atmosphere: rather than containing abundant free oxygen (as our atmosphere does today, thanks to billions of years of subsequent photosynthesis by living organisms), the primitive atmosphere was chemically reducing, dominated by gases such as methane (CH₄), ammonia (NH₃), hydrogen gas (H₂) and water vapour (H₂O), with essentially no free molecular oxygen present at all.

This absence of free oxygen was, in Oparin and Haldane's view, absolutely essential to their hypothesis, and for a very specific chemical reason: oxygen is a highly reactive gas that readily breaks down (oxidises) simple organic molecules almost as fast as they can form. In an oxygen-free, reducing atmosphere, by contrast, simple molecules such as methane, ammonia, hydrogen and water vapour could instead react with one another, driven by external energy sources — intense ultraviolet radiation from the sun (unfiltered by an ozone layer that had not yet formed, since ozone itself is a product of atmospheric oxygen), frequent lightning discharges in the primitive atmosphere, and heat released from ongoing volcanic activity — to slowly build up progressively more complex organic molecules: first simple building blocks such as amino acids and simple sugars, and eventually, over a longer span of time, larger molecules such as short chains of nucleotides. Oparin and Haldane proposed that these organic molecules gradually accumulated in the early oceans, forming what has sometimes been called a 'primordial soup' or 'prebiotic soup', within which the first steps toward biological organisation could eventually take place.

For three decades after Oparin and Haldane first proposed their hypothesis, it remained an untested, if plausible, theoretical idea, until the American chemist Stanley Miller, working under the supervision of Harold Urey at the University of Chicago in 1953, devised a now-famous laboratory experiment to test it directly. Miller and Urey built a sealed glass apparatus (Fig. 6.1) designed to simulate the conditions Oparin and Haldane had proposed for primitive Earth: a lower flask of boiling water represented the primitive ocean, its rising water vapour carried into an upper sealed flask containing a mixture of the gases methane, ammonia, hydrogen and water vapour, meant to represent the reducing early atmosphere, while a pair of electrodes fitted inside this upper flask produced continuous, repeated electric sparks to simulate the energy of lightning discharges. The apparatus was run continuously in this way for about a week, after which Miller and Urey drew off and chemically analysed the liquid that had accumulated in the system. Remarkably, this liquid was found to contain several of the simple amino acids that are the essential building blocks of proteins in every living cell, formed spontaneously from nothing more than simple inorganic starting gases, water, and an electrical energy source — providing striking, direct experimental support for the central chemical claim of the Oparin-Haldane hypothesis, that the organic building blocks of life could indeed have arisen spontaneously under the conditions believed to have existed on the primitive Earth.

The next conceptual step proposed in this line of thinking concerns how these simple organic molecules, once formed and accumulated in the primitive ocean, might have begun to organise themselves into structures resembling living cells. Oparin himself proposed that such molecules could spontaneously aggregate into small, bounded droplets called coacervates (also sometimes called protobionts) — clusters of organic molecules, particularly proteins and lipids, held together within a loosely organised boundary that could selectively concentrate certain molecules from the surrounding medium, much as a primitive membrane does, without yet being a true, fully functional living cell. Such protobionts are proposed as a plausible intermediate stage bridging simple organic chemistry and the first true living cell: once a protobiont acquired the additional capacity to catalyse its own chemical reactions and to replicate its own contents with reasonable fidelity — most likely first achieved through self-replicating RNA molecules, capable of both storing genetic information and catalysing chemical reactions, according to what is often called the 'RNA world' hypothesis — the transition from mere chemistry to the first true, though extremely simple, living cell would have been essentially complete. …