Stanley L. Miller’s legendary spark-discharge experiments, conducted in the 1950s, were considered the first experimental validation of chemical evolutionary scenarios for the origin of life. But since that time a number of scientists have raised concerns that question the relevance of the Miller-Urey experiment. Things have now gone from bad to worse. New work by scientists from Japan identifies yet another problem for the Miller-Urey experiment as an explanation for life’s origin.
Truth be told, I love The Three Stooges. While living in Cincinnati, I would get together with my friend John D. and watch episode after episode. Without fail, I found myself laughing as things invariably went from bad to worse whenever Larry, Moe, and Curly were on the job.
Though not a laughing matter, things continue to go wrong when it comes to the famous Miller-Urey experiment and its significance for the origin-of-life question. A recent study by researchers from Japan raises the concern that efforts to revitalize the Miller-Urey experiment have produced misleading results,1 thus dealing yet another “poke in the eyes” at attempts to construct a naturalistic origin-of-life scenario.
A Promising Start
Textbook origin-of-life scenarios claim that chemical reactions in early Earth’s atmosphere produced small organic molecules (prebiotic compounds) that accumulated in oceans to form the so-called “primordial soup.” The prebiotic compounds within the soup reacted to generate life’s building blocks—key steps in the origin-of-life pathway. Stanley L. Miller’s famous spark-discharge experiments, first conducted in the 1950s, provided the impetus for this idea.
The 22-year-old graduate student ostensibly showed that energy discharges passing through early Earth’s atmosphere could have sparked the formation of building blocks such as amino acids and other organic compounds. Miller’s work launched countless prebiotic simulation experiments, and they all seemed to indicate that life’s building blocks could form in a primordial soup, sans Creator.
At the time, scientists believed early Earth’s atmosphere had been composed of hydrogen, methane, ammonia, and water vapor; and as Miller demonstrated, this gas mixture readily yields amino acids. But further insights into our planet’s primordial atmosphere spelled the end of the experiment’s importance.
Going from Bad to Worse
Most origin-of-life researchers now consider Miller’s experiments irrelevant because the consensus view of early Earth’s atmospheric constituents has changed since the 1950s. Scientists now believe the initial atmosphere was composed of carbon dioxide, nitrogen, and water. This gas mixture does not readily produce organic compounds in prebiotic simulation experiments (see “Biology Textbooks Get it Wrong on Life’s Origin”).
Still, a few researchers have tried to elevate the Miller-Urey experiment to renewed prominence. Many scientists think the carbon dioxide-nitrogen-water atmosphere’s failure to generate amino acids stems from the gas mixture’s oxidative properties. However, in recent years, others have suggested that the oxidative potential originates in the laboratory setting when the spark discharge produces nitrite and nitrate. These two compounds have been directly observed breaking down the amino acid precursors, and some people argue this breakdown reduces the reaction yield.
To test this idea more fully, some researchers have added ascorbic acid to the Miller-Urey experiment (employing a carbon dioxide-nitrogen-water mixture). The addition provides an antioxidant compound that counters the effects of the nitrite and nitrate species and increases the yield of amino acids by a factor of several hundred, just as predicted. Other antioxidants, like pyrite and ferrous sulfate, also increase amino acid yields, but only by a slight amount.
These results imply that the Miller-Urey experiment may not be irrelevant after all. Failure to produce amino acids from a carbon dioxide-nitrogen-water mixture may stem from nothing more than an artifact of the experimental design (which unintentionally generates nitrate and nitrite). In other words, this finding improves the likelihood that amino acids (and other organics) would have formed on early Earth via chemical processes taking place in the atmosphere.
In the midst of this seemingly positive turn of events, the research team from Japan raised the possibility that the ascorbic acid, rather than carbon dioxide, may be the carbon source for the amino acid production. To resolve this concern, the team ran two sets of reactions: one with the ascorbic acid labeled with carbon-14 and the other with carbon dioxide labeled with carbon-14, respectively. This distinction allowed them to identify the carbon source for the amino acids. And, sure enough, the experiments demonstrated conclusively that the amino acids derive carbon from the ascorbic acid, not from carbon dioxide. In other words, the addition of ascorbic acid to the Miller-Urey experiment creates artificial success.
To read about other attempts to resurrect the Miller-Urey experiment, check out the following articles:
- “Miller-Urey Redo”
- “A Failed Comeback: Efforts to Reclaim Stanley Miller’s Legacy,” parts 1 and 2
- “Carbon Monoxide Kills Hopes for Primordial Soup”
The inability to reestablish the relevance of the Miller-Urey experiments exemplifies why every abiogenesis model suffers from fundamental and intractable problems. One of the key ways origin-of-life researchers seek to validate these models is by performing prebiotic simulation experiments, in which they attempt to replicate the conditions of early Earth in the laboratory. These studies aim to identify and understand the chemical and physical processes that could conceivably contribute to the various stages of chemical evolution. A cursory survey of the scientific literature from the past 60 years indicates that researchers have identified a number of chemical and physical processes that, in principle, could contribute to life’s emergence.
Yet, as in the case for the Miller-Urey experiment, it is questionable if any of this work bears genuine relevancy to the evolutionary processes that would have taken place on early Earth. In many instances, the laboratory conditions used for the prebiotic studies fail to reflect the physicochemical complexity of our planet’s early days. Laboratory conditions are often carefully rigged and precisely controlled to ensure the success of the experiments in question. In other words, these processes are successful in the laboratory simply because they have been unduly influenced by the researcher. And, of course, highly skilled and knowledgeable chemists would not have been present on early Earth to give oversight to the chemical and physical processes required to generate the first life-form.
For a more detailed discussion of this issue check out my book Creating Life in the Lab.
1. Hideharu Kuwahara et al., “The Use of Ascorbate as an Oxidation Inhibitor in Prebiotic Amino Acid Synthesis: A Cautionary Note,” Origins of Life and Evolution of Biospheres 42 (December 2012): 533–41.