Stanley Miller Working in Laboratory

Photograph

By: Anonymous

Date: July 19, 1953

Source: Corbis

About the Photographer: The Bettmann/Corbis Archive is one of the largest photographic collections in the world, and originated with a single man, Otto Bettmann, who left Germany in 1935 for the United States with his original collection of photographs packed into two steamer trunks.

INTRODUCTION

In 1952–1953, a graduate student in chemistry at the University of Chicago named Stanley Miller (1930–) performed a series of historic experiments to test the notion that chemicals essential to the origin of life might arise by simple chemistry from non-living ingredients. Miller described his hypothesis as "the idea that the organic [carbon-containing] compounds that serve as the basis of life were formed [billions of years ago] when the earth had an atmosphere of methane, ammonia, water, and hydrogen instead of carbon dioxide, nitrogen, oxygen, and water," as it does today. "In order to test this hypothesis," Miller wrote in the journal Science in 1953, "an apparatus was built to circulate CH₄ [carbon dioxide], NH₃ [ammonia], H₂ [molecular hydrogen], and H₂O [water] past an electric discharge"—a spark. Miller's idea was that energy from the spark could cause molecules to form that would otherwise not be able to do so. Lightning would have been a natural source of such discharges on the primitive Earth.

Miller based his ideas about the Earth's early atmosphere on the work of Nobel laureate Harold Urey (1893–1981), discoverer of deuterium, who in 1950 was studying the chemistry of the early solar system. This led Urey to wonder about the origin of life. The Earth must have been sterile when it first formed at high temperature from the primitive dust cloud that became the Sun and planets; later, there was life. How did it arise? Urey, like virtually all modern scientists, sought a chemical rather than a miraculous explanation. In particular, he suggested that life may have arisen on Earth thanks to the chemistry made possible by a highly "reducing" atmosphere—a chemical environment in which substances strongly tend to separate from oxygen or bind with hydrogen (gaining an electron in so doing). The present-day atmosphere of the Earth is the opposite of reducing, "oxidizing," meaning that it encourages substances to combine with oxygen. It does so because it consists of 21 percent molecular oxygen, O₂: rusting, breathing, and burning are all examples of oxidation processes that the oxygen-rich atmosphere of Earth makes possible today. This oxygen is almost all produced by green plants using photosynthesis.

Miller, a second-year graduate student at the time, boldly contacted the world-famous Urey about testing Urey's ideas in the laboratory. Urey not only encouraged Miller to do so, but designed a series of experiments with him. All three experiments involved mixing carbon dioxide, ammonia, hydrogen, and water at various pressures and temperatures and subjecting them to electric sparks. Miller began running his experiments in the fall of 1952. After the first experiment had been running for a week, the inside of the glass container in which the mixture was being subjected to electric sparks became coated with an oily, brownish material.

Analysis of the goo produced by Miller's apparatus astonished the scientific world: it contained thirteen of the amino acids that are the "building blocks" of all proteins, including DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), the molecules that code for heredity in all terrestrial life. Miller had gone much further down the road to life than anyone would have thought possible. He had proved that extremely simple chemical processes could have produced the basic molecules of life on the Earth.

PRIMARY SOURCE

STANLEY MILLER WORKING IN LABORATORY

See primary source image.

SIGNIFICANCE

Miller's experiment, often referred to as the Miller-Urey (or Urey-Miller) experiment, shifted scientific thinking about the chemical origins of life from speculation to experiment. However, scientific thought about the origins of life has not proceeded in a straight line from Miller's work. First, simply running the Miller experiment does not produce life. For life to exist, self-replication must occur, and the chemicals in Miller's experiment are too simple to spontaneously produce a self-replicating system. Second, scientific opinion about whether the Earth's early atmosphere was in fact reducing has varied over time. Third, alternative chemical pathways for the origin of life have been proposed. One is that organic chemicals might have been lined up to form self-replicating structures by adhering to the surfaces of certain minerals (minerals have organized atomic structures comparable to the patterns in which tiles may be laid on a floor). Moreover, amino acids might have been supplied to the early Earth by meteorites rather than by terrestrial chemistry.

Nevertheless, the Miller-Urey experiment has not been rendered irrelevant. Recent geological and computational work has shown that the early Earth's environment may indeed have been favorable to the formation of the kind of "prebiotic soup" hypothesized by Urey and Miller. Experiments using atmospheric compositions different from Miller's experiment have produced similar results. Studies of living DNA designed to unravel the order in which amino acids were added by evolution to the set of twenty-one now used by all life show that the earliest life was probably based mostly on a smaller number of amino acids than produced in the Miller-Urey experiment. Regardless of whether the precise chemistry produced by Miller was that which actually gave rise to life on Earth, Miller's work had the important effect of making the chemical origin of life a topic for hard-core, quantitative research rather than hand-waving. Much important work has been done in the origin-of-life field since 1953, although scientists still cannot describe exactly how life originated from simple chemicals. Scientists have shown that non-living organic molecules can self-replicate in appropriate environments, that natural selection can modify non-living organic chemicals in the laboratory, and that simpler forms of life based on RNA only, rather than on a combination of RNA and DNA such as is used by all modern life, probably preceded the "last universal ancestor" of all modern species—the microorganism from which all living things today have descended.

Miller's work has been attacked in recent years by creationists such as Jonathan Wells, author of Icons of Evolution (2000). The origin of life remains a scientific mystery, but since the Miller experiment the size of the mystery has shrunk considerably.

FURTHER RESOURCES

Periodicals

Bada, Jeffrey L. and Antonio Lazcano. "Prebiotic Soiup—Revisiting the Miller Experiment." Science, vol. 300, May 2, 2003, pp. 745-746. Available online at: 〈http://www.issol.org/miller/BadaLazcano2003.pdf〉 (accessed February 19, 2006).

Miller, Stanley L. "A Production of Amino Acids Under Possible Primitive Earth Conditions." Science, vol. 117, May 13, 1953, pp. 117-118. Available online at: 〈http://www.issol.org/miller/miller1953.pdf〉 (accessed February 19, 2006).

Web sites

Gishlick, Alan. National Center for Science Education. "Icons of Evolution? Miller-Urey Experiment." 〈http://www.ncseweb.org/icons/icon1millerurey.html〉 (accessed February 19, 2006).