Super-Hero Experiment #1: The Origin of Life

In 1953, two scientists helped to solve the problem of the origin of life on Earth. Their famous experiment, taught in almost every single beginning Chemistry course, has been chosen as the first in this week’s From The Lab Bench series of Super-Hero experiments. Tweet

How organic compounds were synthesized on primitive earth.

In a short 2-page Science article published in 1953, Stanley Miller first and famously demonstrated the spontaneous synthesis of amino acids, organic compounds that contain carbon and that are required as the basis of life on Earth. Amino acids are the ‘pearls’ on the chain macromolecules that we call polypeptides or proteins, the very workhorses of biological activity. Before the Miller-Urey experiments, solving the problem of the origin of life was “one of determining how the first forms of life arose.” (Miller, Science 1953) The problem was that the origin of the first life forms would have required the presence of more complex organic compounds than the elemental carbon, methane, and the small amounts of carbon dioxide and carbon monoxide found on primitive earth. Stanley Miller, in conjunction with fellow scientist Harold Urey, solved the problem of the origin of life by demonstrating spontaneous creation of basic unit life-essential amino acids in conditions mimicking those of primitive earth, from inorganic precursors and the simplest of organic molecules: methane.

For their experiments, Miller and Urey built an apparatus that duplicated the primitive atmosphere on the earth, to test the hypothesis that “organic compounds that serve as the basis of life were formed when the earth had an atmosphere of methane, ammonia, water, and hydrogen instead of carbon dioxide, nitrogen, oxygen, and water.” (Miller, Science 1953) The apparatus was a closed system of glass, supplied with water vapor and gases, and operating at boiling conditions to prevent the growth of organisms that could contaminate the system with amino acids not ‘spontaneously’ produced. The next most important ‘ingredient’ in the Miller and Urey primordial sea and atmosphere was energy. A very large excess of free energy would have been required on early earth to kick start spontaneous organic compound synthesis and the origin of life. This is opposed to the current state of affairs on Earth, where photosynthetic organisms (like plants) themselves harness energy from the sun and produce the organic compounds that higher organisms such as ourselves require.

In his original experiment in 1953, Miller cleverly supplied energy to his primordial atmosphere apparatus with electrodes that produced electric discharges in the form of sparks. These sparks simulated what was believed to be the most important source of free energy in earth earth’s atmosphere and oceans: lightning and corona discharges. Ultraviolet (UV) light from the sun, which penetrated further into early earth’s atmosphere than it can in today’s, is also thought to have contributed significantly to first synthesis of organic compounds, as confirmed by experiments subsequent to those originally conducted by Miller and Urey. With the simple combination of only basic early earth constituent gases (methane, ammonia, water, and hydrogen) and electrical sparks of energy, Stanley Miller watched his primordial sea turn yellow with organic compounds. The results were indeed extraordinary and definitive. Complete analyses by Miller and Urey proved that at least 20 different organic compounds and amino acids were spontaneously synthesized in Miller’s primordial ocean. Modern day analyses indicate that an even more complete list of amino acids were successfully synthesized in the Miller-Urey Experiment.

Miller and Urey concluded that the basis of spontaneous organic compound synthesis or early earth was due to the primarily reducing atmosphere that existed then. A reducing environment would tend to donate electrons to the atmosphere, leading to reactions that form more complex molecules from simpler ones. Today’s oxidizing atmosphere, in which oxygen is present, tends to strip electrons from chemical bonds, leading to less favorable conditions for spontaneous creation of organic compounds from inorganic precursors than those conditions present on primitive Earth. These reducing conditions, as well as high energy levels from UV radiation and strong lightning and volcanic discharges on primitive earth, are believed to have set the state for the origin of life (Campbell and Reece, Biology, 2002 Pearson Education). The reducing-atmosphere hypothesis was very important to understanding the origin of organic compounds on primitive earth, and confirmed the lack of spontaneous generation in an oxidizing environment, as observed in the experiments of Louis Pasteur, who proved that bacteria could not spontaneously grow in a filtered broth under oxidizing conditions.

Miller and Urey’s experiments set the stage for a profoundly more clear understanding of how life originated in the very foreign atmosphere that was primitive earth. The Miller-Urey experiment motivated many future scientists to study the possible modes and conditions of spontaneous synthesis that generated further life-essential molecules including nucleic acid bases, self-replicating genetic material (RNA and DNA), strings of amino acids forming polypeptides and proteins, aromatic compounds, and sugars. The huge impact and significance of their work has led me to choose the Miller-Urey experiment as Super-Hero experiment #1 in this week’s mini blog series!

Check in tomorrow for more famous experiments, and share your thoughts by tweeting @FromTheLabBench!

In Summary:
• The Miller-Urey experiment is known as the classic experiment on the origin of life. It showed spontaneous generation of life-essential organic compounds from simpler inorganic precursors in a primitive atmosphere.
• A reducing environment and higher amounts of free energy (lightning storms and high temperatures) contributed to the origin of life on earth. Since then, the earth has seen a change of atmosphere from a reducing environment to our present day over-all oxidizing environment.
• The Miller-Urey experiment confirmed the hypothesis that “spontaneous generation of the first living organisms might reasonably have taken place if large quantities of organic compounds had been present in the oceans of the primitive earth.” (Oparin, 1957)
• Electric discharge and UV radiation may have ‘played a significant role in the formation of compounds in the primitive atmosphere’. (Miller, Science 1953)
• The origin of life saw the development of simple heterotrophic organisms (organisms that obtain their basic constituents from the environment) before autotrophic organisms (like photosynthetic plants).

References:
1) S. L. Miller. A Production of Amino Acids Under Possible Primitive Earth Conditions. Science 117, 528-559 (1953)
2) S.L. Miller and H.C. Urey. Organic Compound Synthesis on the Primitive Earth. Science 130, 245-251 (1959)
3) A. I. Oparin, The Origin of Life. Macmillan, New York (1938); Academic Press, New York, ed. 3 (1957)
4) Early Earth image: Flickr by ara_gon

Glossary:
• Electrical discharge = flow of electric charge through a gas, liquid or solid
• Amino acid = basic unit of a peptide: contains an amine group, a carboxylic acid group, and a variable side-chain group

• Protein = biochemical compound consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function (wiki commons)
• Carbon = Chemical element with atomic number 6. Has four electrons available to form covalent chemical bonds.
• Photosynthetic organism = organisms that carry out photosynthesis to acquire energy
• Photosynthesis = use the energy from sunlight to convert carbon dioxide and water into organic materials to be utilized in cellular functions of life
• Reduction = gain of electrons or a decrease in oxidation state by a molecule, atom, or ion.
• Oxidization = loss of electrons or an increase in oxidation state by a molecule, atom, or ion.
• Oxygen = Chemical element with atomic number 8. Highly reactive nonmetallic element that readily forms compounds (notably oxides) with almost all other elements. “Elemental O2 only began to accumulate in the atmosphere roughly 2.5 billion years ago.” (NASA Press Release (2007-09-27). NASA Research Indicates Oxygen on Earth 2.5 Billion Years ago)
• Hypothesis = a proposed explanation for a phenomenon

Miller, S. (1953). A Production of Amino Acids Under Possible Primitive Earth Conditions Science, 117 (3046), 528-529 DOI: 10.1126/science.117.3046.528