Miller-Urey Redo

Miller-Urey Redo

Discovery of Old Lab Vials Erupts New Interest in a Famous Origin-of-Life Experiment

It never ceases to amaze me what turns up when I clean out our garage: forgotten stuff that brings back memories, and occasionally, old things that still have value.

And this is exactly what some former students and associates of the late origin-of-life researcher Stanley Miller discovered when they cleaned out his lab after his death. Old vials from leftover experiments that bring back memories of his famous spark-discharge experiment may shed valuable new light on how prebiotic materials could have formed on the early Earth.

The Miller-Urey Experiment

Miller’s work, conducted in the early 1950s, was the first experimental validation of the Oparin-Haldane hypothesis. Based on the principles of chemical evolution, this model was one of the first scientific theories to describe a mechanistic pathway between simple chemical compounds and life.

To test this hypothesis Miller filled the confines of a carefully assembled glass apparatus with methane, ammonia, and hydrogen after diligently excluding oxygen from the system. At that time, scientists thought the gases Miller used in his experiment existed in early Earth’s atmosphere. A boiling flask of water connected to the glassware introduced water vapor into the headspace and simulated early Earth’s oceans. Miller passed a continuous electric discharge through the gas mix to simulate lightning. The results showed that the primitive atmosphere of the early Earth could, in principle, generate amino acids, one of the key building blocks of life.

Status of the Miller-Urey Experiment

Today, the Miller-Urey experiment is generally considered to be irrelevant to the origin-of-life question. Current understanding of the composition of early Earth’s atmosphere differs significantly from the thinking of the 1950s. Most planetary scientists now believe the Earth’s primeval atmosphere consisted of carbon dioxide, nitrogen, and water vapor. Laboratory experiments indicate that this gas mixture is incapable of yielding organic materials in Miller-Urey-type experiments.

In May 2003 origin-of-life researchers Jeffrey Bada and Antonio Lazcano, long-time associates of Miller, wrote an essay for Science commemorating the 50th anniversary of the publication of Miller’s initial results. They pointed out that the Miller-Urey experiment has historical significance, but not scientific importance in contemporary origin-of-life thought. Bada and Lazcano wrote:

Is the “prebiotic soup” theory a reasonable explanation for the emergence of life? Contemporary geoscientists tend to doubt that the primitive atmosphere had the highly reducing composition used by Miller in 1953.

In his book Biogenesis, origin-of-life researcher Noam Lahav passes similar judgment:

The prebiotic conditions assumed by Miller and Urey were essentially those of a reducing atmosphere. Under slightly reducing conditions, the Miller-Urey reaction does not produce amino acids, nor does it produce the chemicals that may serve as the predecessors of other important biopolymer building blocks. Thus, by challenging the assumption of a reducing atmosphere, we challenge the very existence of the “prebiotic soup,” with its richness of biologically important organic compounds.

Revived Interest in Miller’s Experiment

By sifting through the items left behind in Stanley Miller’s laboratory, his former students and associates uncovered vials of material from his original experiments that they think gives new importance to the Miller-Urey experiment.

Miller actually performed three versions of the spark-discharge experiment. All three permutations yielded amino acids and other organic compounds. Miller decided to focus his efforts, however, on the version that now appears in biology textbooks because he thought that it most closely modeled the atmosphere of early Earth.

Still, Miller held on to cartons of vials containing materials from the other two variations of the spark-discharge experiment along with notebooks that carefully documented the experimental work he performed.

After stumbling upon the vials and corresponding notebooks, Miller’s colleagues decided to re-analyze their contents using state-of-the-art analytical methods not available to Miller fifty years ago.

To their surprise, Miller’s associates discovered that the “textbook” version of the Miller-Urey experiment wasn’t the most successful. The most productive synthesis was one that introduced water into the headspace as a fine mist using an aspirator. This particular experimental rig produced more amino acids with a greater chemical diversity than the textbook experiment.

The design of this forgotten experiment intrigued Miller’s collaborators because it models volcanic emissions that could have occurred on early Earth. Accordingly, volcanic lightning would have served as the energy source that generated prebiotic compounds as it passed through volcanic gases and steam—assuming that the volcanic emissions on early Earth consisted of reducing gases.

Miller’s cohorts now argue that this re-discovery gives new relevance to Miller’s old experiment. Perhaps the sources of prebiotic materials on early Earth were volcanic emissions, not chemical reactions taking place in the atmosphere.

Were Volcanoes the Source of Prebiotic Compounds?

The proposal by Miller’s former associates is not the first time that origin-of-life researchers have appealed to volcanoes as the source of prebiotic compounds. As Hugh Ross and I describe in our book Origins of Life, other scientists have suggested this possibility.

Based on the chemical composition of volcanic emissions today, there doesn’t seem to be much hope that prebiotic materials could form in this environment. The gases spewing from volcanoes today consist primarily of water, carbon dioxide, and sulfur dioxide. This is a highly oxidizing mixture of gases that will not generate prebiotic materials in laboratory simulation experiments like the ones that Miller performed.

But were the gaseous emissions of volcanoes on early Earth different? Did they consist of gases like the ones used by Miller in his spark-discharge experiments? Research conducted a few years ago indicates the opposite. It appears as if the gaseous emissions of volcanoes 3.9 billion years ago were identical to the emissions today. This result means that the conditions of Miller’s experiment were not relevant for either the atmosphere of the early Earth or volcanic environments at that time.

Miller’s work and his status as a scientist remain fixed in a prominent place in the history of science. However, perhaps it’s best that Miller’s vials are removed from the lab once and for all, and sent to a museum for posterity.