Last week, I retold the story of chemist Stanley Miller’s sudden rise to fame with the success of his spark-discharge experiments in the 1950s. Scientists believed that Miller’s experiment proved that life could have formed naturalistically. But subsequent research showed that the chemical conditions on early Earth differed from those used in the experiment.
Miller’s work was relabeled as historically notable, but ultimately irrelevant to the origin-of-life discussion.
The Hopes of a Comeback
Miller died late May 2007. Shortly after his death some of his former students and associates discovered old vials leftover from the initial experiments Miller performed in the 1950s. He had apparently saved them all these years in cardboard boxes, carefully labeled to correspond to his laboratory notebooks.
It turns out that 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.
Miller’s colleagues decided to reanalyze their contents using state-of-the-art analytical methods not available fifty years ago.1 To their surprise, they 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 gases and steam—assuming that the volcanic emissions on early Earth consisted of reducing gases.
Recently, these researchers extended their analysis of Miller’s old vials to include ones from a spark-discharge experiment conducted in 1958.2 Miller used hydrogen sulfide in addition to ammonia, methane, and carbon dioxide as gases in the head space for this particular study. It appears that Miller never analyzed the samples from this experiment. When his former students and colleagues examined them over fifty years later, they discovered that this set up generated a large number of amino acids and other compounds, including two sulfur-containing amino acids.
Miller’s cohorts now argue that this rediscovery 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. These emissions could contain compounds like hydrogen sulfide.
Were Volcanoes the Source of Prebiotic Compounds?
The proposal by Miller’s former associates is not the first time origin-of-life researchers have appealed to volcanoes as the source of prebiotic compounds. 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 Miller used? 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.3 This result means the conditions of Miller’s experiment were not relevant for either early Earth’s atmosphere or volcanic environments.
The fact remains: Miller’s work is not relevant to the origin of life. Though his work and status as a scientist are fixed in a prominent place in the history of science, it is time for biology textbooks to stop referencing this work as evidence for chemical evolution.
1. Adam P. Johnson et al., “The Miller Volcanic Spark Discharge Experiment,” Science 322 (October 17, 2008): 404.
2. Eric T. Parker et al., “Primordial Synthesis of Amines and Amino Acids in a 1958 Miller H2S-Rich Spark Discharge Experiment,” Proceedings of the National Academy of Sciences, USA 108 (April 5, 2011): 5526–31.
3. John W. Delano, “Redox History of Earth’s Interior since ~3900 Ma: Implications for Prebiotic Molecules,” Origins of Life and Evolution of Biospheres 31 (2001): 311–41.