A study reveals the key role of the reactor surface in the famous Miller’s experiment on the molecular origin of life

A team of researchers from the CSIC and the University of Tuscia (Italy) has demonstrated the role that glass played in the historical experiment carried out by Stanley Miller in 1952 to simulate the conditions that would have given rise to life on the early Earth. The results, published in Scientific Reports, open a new way to study the emergence of life.

Miller built a glass apparatus into which he poured water to simulate the early ocean and a mixture of gases (methane, ammonia, and hydrogen) to emulate the early atmosphere. As a source of energy, he used electric discharges between two electrodes that simulated the rays that must abound in the primordial Earth. Within days, Miller detected in the primitive "ocean" amino acids (the building blocks of proteins) and other prebiotic organic compounds. His work opened the door to experimental studies on the molecular origins of life.

"We have shown that the success of the experiment is largely due to the surface of the glass reactor used by Miller," says CSIC researcher Juan Manuel García-Ruiz from the Instituto Andaluz de Ciencias de la Tierra, in Granada(Spain). “Miller simulated the ocean and the atmosphere of the early Earth but forgot about the rocks. The crucial role of minerals was hidden in the glass walls of the reactor he used, ” García-Ruiz adds.

The reactor Miller built for his experiments, and the reactors subsequently used by other research groups are made of borosilicate glass. “Due to the use of ammonia, the water of the experiment has a basic pH. At that pH, the glass of the reactor slightly dissolves, and the silanol groups of the silica are activated, which means an increase in the reactivity of the surface ”, explains García Ruiz.

Drawing inspiration from their previous research on the role of silica in mineral self-organization phenomena, the researchers wondered what role the borosilicate walls of the reactor might have on the molecular diversity of the compounds synthesized in Miller's experiment.

“We conducted six experiments using three reactors: a glass reactor, another made of Teflon (a chemically inert material), and a third made of Teflon in which we added glass chips to the water. The results unequivocally demonstrate that borosilicate glass plays a key role in Miller's synthesis, in yields, in the number of products synthesized and in their chemical diversity”, explains the researcher. “This result has important geochemical implications because it shows that most of the organic compounds found in the oldest rocks on planets such as Earth or Mars are probably of abiotic, non-biological origin,” he pointed out.”

“Our experiments also show that Miller would have synthesized very few of the organic molecules relevant to the origin of life had he not used the glass reactor, including dipeptides, multi-carbon molecules, dicarboxylic acids, polycyclic aromatic hydrocarbons, or a full panel of biological nucleobases,” indicates García-Ruiz. Glass was a necessary catalyst to synthesize many of these abiotic organic molecules that were key to the emergence of life.

Ultimately, the synthetic pathway discovered by Miller-Urey based on electrical discharge requires expanding the gas phase synthesis scenario to one that includes mineral surfaces. "We thus open a promising avenue of research, exploring the role of mineral evolution and the composition of the early Earth's atmosphere in the synthesis and complexity of prebiotic compounds on early Earth," he adds.

It is possible that the rocks also justify the idea most criticized in Miller's experiments: the existence of a reducing atmosphere, that is, one rich in methane or hydrogen, on the early Earth. “New ideas about the Hadean Earth (the era that began with the formation of the Earth, 4.6 billion years ago and ended 4 billion years ago) suggest the concomitance of a reduced atmosphere, electrical storms, rocky surfaces rich in silicates, meteoric bombardment, and liquid water”, says the researcher.

The reducing atmosphere resulted from the planetary-scale reaction between the earliest rocks in the earth's crust when water condensed in the iron-magnesium silicate crust 4.4 billion years ago, leading to the massive formation of hydrogen and methane. "Certainly, this atmosphere was transient, but it probably lasted almost one hundred million years until it became enriched in CO and CO2," concludes García Ruiz.

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