New Analysis of Iconic Miller-Urey Origin of Life Experiment Asks More Questions Than It Answers
How did we get here? It’s no small question. Scientists have been hacking away at the origin of life ever since we opened up “science” in the human skill tree. In 1952, two chemists conducted an experiment designed to brew up a kettle of primordial soup, and in doing so, they began to probe the circumstances under which life arose on Earth. Their work still bears their names: the Miller-Urey experiment inspired countless other studies, and it’s in every freshman biology text. But a new analysis of the OG experiment has concluded that one component of the primordial soup must have come from an unexpected source. The analysis is compelling and peer-reviewed, and it raises more questions than it answers.
The groundbreaking Miller-Urey experiment was designed to test the idea of “abiogenesis,” which is the notion that life could come from that which was not previously living. The idea goes like this: if life came from the primordial soup, and we’re living but the soup wasn’t alive, then at some point there must have been some kind of transition from nonlife to life. Life is made of cells. (Don’t get me started on viruses.) Cells are made of polymers, which are made of monomers, which are made of yet smaller, simpler, building-block molecules. There should be some knowable transition from life to unlife, somewhere in the cosmos we live in, for us to witness and understand.
But the early Earth was very different from the one we occupy, enough so that it confounded our experiments at first. One important difference is the atmosphere: before the Great Oxygenation Event, our planet had a reducing atmosphere, made of things like hydrogen, methane, and ammonia. The difference is so great between our current atmosphere and the primordial atmosphere, in fact, that entirely separate classes of chemical reactions are favored. Still, methane and ammonia contain carbon and nitrogen, which are the necessary raw materials for the backbone of all known proteins and amino acids. Add in gaseous hydrogen and you’ve got the materials to make hydrocarbon chains, sugars, and even nucleic acids. So Stanley Miller and his advisor Harold Urey sealed those gases inside a sterile glass vessel, which was connected to another smaller glass bubble containing water. Heating the water made steam, which mingled with the reducing gases to make a microcosm of what we believed the atmosphere was on the primordial Earth. The resulting clouds swirled around electrodes that sent a spark across a gap, over and over, mimicking lightning from ancient storms. A cooling clamp allowed the vapors to condense into a tiny, domesticated version of the corrosive primordial rain, which puddled in a collection chamber below.
The resulting solution contained five amino acids, reported the two chemists; of that the chromatography was unequivocal. Their report acknowledged also that there was weak evidence for the presence of two other amino acids, but too weak to make a definitive claim. In 2007, Miller’s scientific successor, Jeffrey Bada, and several colleagues examined the original chromatography slips from the 1952 experiment. They determined that the original report had been, if anything, too conservative: There had been not just five or seven amino acids created in the reaction chamber, the 2007 analysis reported, there were twenty-five. But looking at the intact setup from Miller and Urey’s work, one coauthor of the 2007 analysis, Joaquin Criado-Reyes, realized that even more must have been going on inside those sealed reaction vessels.
Borosilicate glass, sometimes called Pyrex, is a kind of extremely tough glass often used in labware because of its resistance to breakage, corrosion, and other kinds of abuse. Usually a good scrub with some Alconox is enough to clean up lab glass so that it shines like the day it came out of the box. What’s more, glass is often what you store the acids and solvents in before you use them to clean off other, lesser tools. But in the 1952 experiment, in addition to making amino acids, electrifying those reducing vapors produced a highly corrosive, highly alkaline stew of chemicals sufficient to etch and pit the glassware itself. To get a better look, Criado-Reyes and colleagues ran the Miller-Urey experiment again, but in three parallel trials. One used the original glassware from the 1952 work, one used a Teflon vessel, and one used a Teflon vessel with broken chips of borosilicate dropped in. (Teflon is waxy, hard, and obnoxiously nonreactive, and when even glass can’t safely contain a chemical, often Teflon can.) At the end of the experiment, the kinds of organic molecules created inside all three reaction vessels matched. But the quantities didn’t. The Teflon vessels contained less of everything.
The imbalance still supports Miller and Urey’s original work. Etching the glass would have dissolved some of the silicon dioxide. Liberating some silicate into solution creates a twofold catalyst, by way of the silicate molecules themselves as well as the corroded, pitted surface they leave behind. So the Teflon vessels have extra reason not to turn as much yield as the glass. But the results from the 2021 analysis do call other things into question.
One answer to the Fermi paradox is a dismissive hand-wave at the sheer improbability of life. (Never mind that to the best of our knowledge, life does in fact have a 100% probability of existing, which we know because we are here to scratch our collective heads about it.) While chemistry teaches us that higher-order reactions become less likely and therefore harder to find, by virtue of having to line up several different sets of circumstances, the raw components of the primordial soup experiments are not rare. We’ve observed lightning on Jupiter, which has an atmosphere of hydrogen, helium, methane, and ammonia. Rocky planets are common, and with them, the same silicon dioxide that’s in Pyrex. And nearly every star has a “snow line” in which it might be possible to find liquid water. According to the most recent (2018) NASA analysis of data from Kepler, up to half the stars in our galaxy could have terrestrial planets within their habitable zones. The ubiquity of these elements weakens the “rare earth” hypothesis, without necessarily being courteous enough to provide any more information. To begin the Rube Goldberg chain of reactions that eventually produces amino acids from dissociated organic and inorganic molecules, what you need is just rocks, water, and lightning, but coastal storms are not hard to find. So what gives?
The answer is also one big reason we have a scientific interest in exploring places like Titan and Enceladus. Polymers, which are long molecules which can encode information in their sequence, are thought to be essential to life. But there may be another chapter to the story. Miller and Urey didn’t stop work with just one batch of primordial soup. In another experiment, they shot vapors from a nozzle at a spark gap. That work produced almost two dozen amino acids, plus amines and other hydroxylated species. Where else in the universe do you find lightning getting shot through pressurized water vapor? Cryovolcanoes, like those found on the moons of Saturn, represent one place where conditions exist that can produce polymers. We may well need to cast a scientific eye toward the colder reaches of our solar system, to understand how life formed here on Earth.