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Miller-Urey Experiment: Chemical Evolution of Life
Ardent debate between the panspermists and Darwinians continued into the twentieth century. Panspermists, such as Svante Arrhenius, Lord Kelvin, John Helmholtz, Ferdinand Cohn, and Louis Pasteur, believed that life was fundamental to the universe and came only from pre-existing life, and that spontaneous generation (life from non-life) was hogwash. (1) Darwinian Marxists Aleksandr Oparin (1894-1980) and J.B.S. Haldane (1892-1964), and University of Chicago scientists Harold C. Urey (1893-1981), and Stanley L. Miller (1930-2007) believed that life emerged spontaneously on Earth from inorganic chemicals in the ocean, struck by lightening or some other source of energy. Aleksandr Oparin and his “Coacervates”Russian biochemist Aleksandr Oparin was the first person to articulate the twentieth century Darwinian view of the origin of life in a 1924 pamphlet he authored, which he later made into a book that reached the West in the late 1930s. (2)
Oparin wrote, “There is no fundamental difference between a living organism and lifeless matter. The complex combination of manifestations and properties so characteristic of life must have arisen in the process of the evolution of matter.” (2) Furthermore, the “spontaneous generation of the first living organisms might reasonably have taken place if large quantities of organic compounds [e.g., ammonia, methane, hydrogen, and water vapor] had been present in the oceans of the primitive earth.”(3) He continued, “At first there were the simple solutions of organic substances, the behavior of which was governed by the properties of their component atoms and the arrangement of those atoms in the molecular structure. But gradually, as the result of growth and increased complexity of the molecules, new properties have come into being and a new colloidal-chemical order was imposed on the more simple organic chemical relations. These newer properties were determined by the spatial arrangement and mutual relationship of the molecules... In this process, biological orderliness already comes into prominence. Competition, speed of growth, struggle for existence and, finally, natural selection determined such a form of material organization which is characteristic of living things of the present time.” (4) Oparin believed that life came from “coacervates” (from the Latin to assemble or cluster), which are non-living spheres of lipid molecules held together by hydrophobic forces. Coacervates are little balls of organic matter formed by the repulsion of water by something akin to oil. Coacervates form spontaneously, for example, when gelatin reacts with gum Arabic. They provide a locally segregated environment and boundaries that allow absorption of simple organic molecules from the surrounding environment, thereby qualifying them as “heterotrophs” that obtain nutrients from the environment rather than making them, as in photosynthesis. Coacervates cannot reproduce. (5) Oparin suggested that different types of coacervates might have formed in the Earth’s primordial ocean and, subsequently, been subject to a selection process leading eventually to life, as we know it. Miller-Urey ExperimentStanley L. Miller, a graduate student in the laboratory of Harold C. Urey at the University of Chicago, decided to test Oparin’s hypothesis on the origin of life. He reasoned that the primitive Earth’s atmosphere was similar to the “reducing” atmosphere of the outer planets in our solar system, i.e., Jupiter, Saturn, Uranus and Neptune. Organic compounds such as methane, ammonia, water, and hydrogen characterize a reducing atmosphere, rather than the atmosphere composition we know today, which consists of carbon dioxide, nitrogen, oxygen, and water.
Miller in 1952 created a closed system of laboratory glassware, except for tungsten electrodes that provided electric sparks, which mimicked lightening. He added together methane, ammonia, water, and hydrogen, subjected the mixture to a high-frequency spark for a week, and produced milligram quantities of glycine, alanine, and other amino acids. (6,7) Amino acids are building blocks that may hook together to form the long and complex molecules of animal and plant life. Miller lectured in 1961, “It appears likely that reactive organic compounds were formed by electric discharges and by ultraviolet light in the atmosphere of the primitive earth. These compounds were carried on by the rains and reacted in the ocean to give amino acids and other complex organic compounds. It is possible that a significant fraction of the carbon on the surface of the earth was in the form of organic compounds in the oceans. Reactions in this mixture would give a great many of the compounds that are components of present living organisms. “While amino acids are easily synthesized in the laboratory, the synthesis reported here is the first one carried out under conditions that might reasonably be present on the primitive earth. The synthesis of amino acids is not the synthesis of life, nor is it a synthesis of proteins. However, it represents a step toward our understanding of how live matter may have arisen on earth.” (8) Miller was not the first scientist to attempt to create an organic compound from inorganic molecules. In the 1820s, German physician-chemist Frederich Woeller (1800-1882) used ammonium cyanate to synthesize urea, a biological compound secreted by the kidneys. Woeller’s experiment was the first example of the reaction of inorganic compounds to form a biological compound. Today, the distinction between organic (of biological origin, e.g., urea, amino acids) and inorganic (of non-biological origin, e.g., carbon monoxide, graphite) is nebulous. Panspermists Respond to Miller-Urey ExperimentTwo British astronomers fired up the torch of panspermia in the early 1960s: the late Sir Frederick Hoyle (1915-2001) and Nalin Chandra Wickramasinghe (born 1939, Sri Lanka). (9) They acknowledged that many scientists, including Woeller and Miller, had developed ingenious experiments to show the relative ease of making relevant monomers by inorganic processes, e.g., electric sparks. A monomer is a small molecule that may chemically bond to another monomer to form a polymer. For example, amino acids are monomers that may polymerize to form proteins, and glucose molecules are monomers that may polymerize to form starches. Hoyle and Wickramasinghe, however, cautioned, “[S]uch experiments do not come remotely near the desired goal of producing life from non-life…What is relevant for the origin of life is not just the formation of the chemical building blocks, but the emergence of highly specific arrangements of these molecules into structures such as enzymes.” (10) Some 1000-2000 enzymes sustain life, and, surprisingly, the variation of amino
acid sequences in these enzymes from one species to another is, overall, rather
minor, noted What does this mean for the origin of life? Hoyle and Wickramasinghe laid out three possible deductions. First, “Life is a cosmic phenomenon, and we are forced to accept panspermia.” Second, “life is terrestrial, but its information content contains enormous redundancy by a factor ~ (20)2000 for the case of enzymes.” Third, “Life is terrestrial. It occurs with such a miniscule probability that it is unique to the Earth.” (12) What is the contemporary consensus among most scientists—the first, second, or third deduction, according to Hoyle and Wickramasinghe? Most scientists say the first option is unacceptable. Either the second or the third then must be so. Hoyle and Wickramasinghe respond, “There is no evidence [for the second option], while the third one is “distinctly preCopernican.” Therefore, they conclude, the only realistic option is the first option, panspermia. (12) Astronomer Nicolaus Copernicus (1473-1543) formulated the first modern Sun-centered, rather than Earth-centered, model of the solar system. He so doing, he contradicted Aristotle. Stanley L. Miller Responds to PanspermistsMiller, interviewed in 1996, explained his work in relation to the theory of panspermia, which, he pointed out, had been renamed exobiology by microbiologist Joshua Lederberg (born 1925). (13) Miller defined exobiology as “the study of life beyond the Earth.” He continued: “But since there’s no known life beyond the Earth people say it’s a subject with no subject matter. It refers to the search for life elsewhere, Mars, the satellites of Jupiter and in other solar sytsems.” He then said, “It is also used to describe studies of the origin of life on Earth, that is, the study of pre-biotic Earth and what chemical reactions might have taken place as the setting for life’s origin.” (13) In reference to his reducing atmosphere work in his graduate student experiment in 1952 he said, “Although there is a dispute over the composition of the primitive atmosphere, we’ve shown that either you have a reducing atmosphere or you are not going to have the organic compounds required for life. If you don’t make them on Earth, you have to bring them in on comets, meteorites or dust [theory of panspermia]. Certainly some material did come from these sources. In my opinion the amount from these sources would have been too small to effectively contribute to the origin of life.” (13) He continued, “I’m skeptical that you are going to get more than a few percent of organic compounds from comets and dust. It ultimately doesn’t make much diference where [compounds] comes from. I happen to think priobiotic synthesis happened on the Earth, but I admit I could be wrong. There is another part of the story. In 1969 a carbonaceous meteorite fell in Murchison, Australia. It turned out the meteorite had high concentrations of amino acids, about 100 parts per million, and they were the same kind of amino acids you get in prebiotic expermients like mine. This discovery made it plausbile that a similar process could have happened on primitive Earth, on an asteroid, or for that matter, anywhere else the proper conditions exist.” (13) Warm Little Pond RevisitedEnglish naturalist Charles Robert Darwin (1809-1882) espoused in 1859 his theory of the origin of species through natural selection. He omitted, however, a starting point, some type of primordial cell from which all present forms of life descended. Darwin broke his silence briefly on the origin of life in a famous letter to his friend Joseph Hooker in 1871: “It is often said that all the conditions for the first production of a living organism are present, which could ever have been present. But if (and Oh! what a big if!) we could conceive in some warm little pond [italics by author], with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc., present, that a protein compound was chemically formed ready to undergo still more complex changes, at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed.” (14) Miller, who had originally believed the spontaneous generation of life occurred in Earth’s oceans, said in his 1996 interview, “The primitive Earth had big oceans, but it also had lakes, lagoons and beaches. Our [new] hypothesis is that the conditions may have been ideal on these beaches or drying lagoons for preiobiotic reactions to occur, for the simple reason that the chemicals were more concentrated in these sites than in the middle of the ocean.” (13) A century after Darwin proposed a “warm little pond” for the spontaneous origin of life, his descendants, including Oparin, Haldane, Miller, and Urey, among others, remained true to the concept, which panspermists, however, viewed as “distinctly preCopernican.” Notes:
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