
The Miller–Urey experiment, or Miller experiment, was an experiment in
chemical synthesis
Chemical synthesis (chemical combination) is the artificial execution of chemical reactions to obtain one or several products. This occurs by physical and chemical manipulations usually involving one or more reactions. In modern laboratory uses ...
carried out in 1952 that simulated the conditions thought at the time to be present in the
atmosphere of the early, prebiotic Earth. It is seen as one of the first successful experiments demonstrating the synthesis of
organic compound
Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...
s from
inorganic constituents in an
origin of life
Abiogenesis is the natural process by which life arises from abiotic component, non-living matter, such as simple organic compounds. The prevailing scientific hypothesis is that the transition from non-living to organism, living entities on ...
scenario. The experiment used
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...
(CH
4),
ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
(NH
3),
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
(H
2), in ratio 2:1:2, and water (H
2O). Applying an electric arc (simulating lightning) resulted in the production of
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...
s.
It is regarded as a groundbreaking experiment, and the classic experiment investigating the origin of life (
abiogenesis
Abiogenesis is the natural process by which life arises from non-living matter, such as simple organic compounds. The prevailing scientific hypothesis is that the transition from non-living to living entities on Earth was not a single even ...
). It was performed in 1952 by
Stanley Miller, supervised by Nobel laureate
Harold Urey
Harold Clayton Urey ( ; April 29, 1893 – January 5, 1981) was an American physical chemist whose pioneering work on isotopes earned him the Nobel Prize in Chemistry in 1934 for the discovery of deuterium. He played a significant role in the ...
at the
University of Chicago
The University of Chicago (UChicago, Chicago, or UChi) is a Private university, private research university in Chicago, Illinois, United States. Its main campus is in the Hyde Park, Chicago, Hyde Park neighborhood on Chicago's South Side, Chic ...
, and published the following year. At the time, it supported
Alexander Oparin's and
J. B. S. Haldane's hypothesis that the conditions on the primitive Earth favored chemical reactions that synthesized complex organic compounds from simpler inorganic precursors.
After Miller's death in 2007, scientists examining sealed vials preserved from the original experiments were able to show that more amino acids were produced in the original experiment than Miller was able to report with
paper chromatography
Paper chromatography is an analytical method used to separate colored chemicals or substances. It can also be used for colorless chemicals that can be located by a stain or other visualisation method after separation. It is now primarily used as ...
.
While evidence suggests that Earth's
prebiotic atmosphere
The prebiotic atmosphere is the second atmosphere present on Earth before today's biotic, oxygen-rich ''third atmosphere'', and after the ''first atmosphere'' (which was mainly water vapor and simple hydrides) of Earth's formation. The formation ...
might have typically had a composition different from the gas used in the Miller experiment, prebiotic experiments continue to produce
racemic mixture
In chemistry, a racemic mixture or racemate () is a mixture that has equal amounts (50:50) of left- and right-handed enantiomers of a chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds are produced industrially as r ...
s of simple-to-complex organic compounds, including amino acids, under varying conditions.
Moreover, researchers have shown that transient, hydrogen-rich atmospheres – conducive to Miller-Urey synthesis – would have occurred after large
asteroid
An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
impacts on early Earth.
History
Foundations of organic synthesis and the origin of life
Until the 19th century, there was considerable acceptance of the theory of
spontaneous generation
Spontaneous generation is a superseded scientific theory that held that living creatures could arise from non-living matter and that such processes were commonplace and regular. It was hypothesized that certain forms, such as fleas, could ...
, the idea that "lower" animals, such as insects or rodents, arose from decaying matter. However, several experiments in the 19th century – particularly
Louis Pasteur
Louis Pasteur (, ; 27 December 1822 – 28 September 1895) was a French chemist, pharmacist, and microbiologist renowned for his discoveries of the principles of vaccination, Fermentation, microbial fermentation, and pasteurization, the la ...
's
swan neck flask experiment in 1859 — disproved the theory that life arose from decaying matter.
Charles Darwin
Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English Natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...
published ''
On the Origin of Species
''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life'')The book's full original title was ''On the Origin of Species by M ...
'' that same year, describing the mechanism of
biological evolution
Evolution is the change in the heritable characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, resulting in certai ...
. While Darwin never publicly wrote about the first organism in his theory of evolution, in a letter to
Joseph Dalton Hooker
Sir Joseph Dalton Hooker (30 June 1817 – 10 December 1911) was a British botanist and explorer in the 19th century. He was a founder of geographical botany and Charles Darwin's closest friend. For 20 years he served as director of the Ro ...
, he speculated:
But if (and oh what a big if) we could conceive in some warm little pond with all sorts of ammonia and phosphoric salts, light, heat, electricity etcetera present, that a protein compound was chemically formed, ready to undergo still more complex changes ..

At this point, it was known that organic molecules could be formed from inorganic starting materials, as
Friedrich Wöhler
Friedrich Wöhler Royal Society of London, FRS(For) HonFRSE (; 31 July 180023 September 1882) was a German chemist known for his work in both organic chemistry, organic and inorganic chemistry, being the first to isolate the chemical elements be ...
had described
Wöhler synthesis
The Wöhler synthesis is the conversion of ammonium cyanate into urea. This chemical reaction was described in 1828 by Friedrich Wöhler. It is often cited as the starting point of modern organic chemistry. Although the Wöhler reaction concerns ...
of
urea
Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...
from
ammonium cyanate
Ammonium cyanate is an inorganic compound with the formula . It is a colorless, solid salt.
Structure and reactions
The structure of this salt was verified by X-ray crystallography. The respective C–O and C–N distances are 1.174(8) and 1.192 ...
in 1828. Several other early seminal works in the field of
organic synthesis
Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...
followed, including
Alexander Butlerov's
synthesis of sugars from
formaldehyde
Formaldehyde ( , ) (systematic name methanal) is an organic compound with the chemical formula and structure , more precisely . The compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde. It is stored as ...
and
Adolph Strecker's synthesis of the amino acid
alanine
Alanine (symbol Ala or A), or α-alanine, is an α-amino acid that is used in the biosynthesis of proteins. It contains an amine group and a carboxylic acid group, both attached to the central carbon atom which also carries a methyl group sid ...
from
acetaldehyde
Acetaldehyde (IUPAC systematic name ethanal) is an organic compound, organic chemical compound with the chemical formula, formula , sometimes abbreviated as . It is a colorless liquid or gas, boiling near room temperature. It is one of the most ...
,
ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
, and
hydrogen cyanide
Hydrogen cyanide (formerly known as prussic acid) is a chemical compound with the chemical formula, formula HCN and structural formula . It is a highly toxic and flammable liquid that boiling, boils slightly above room temperature, at . HCN is ...
.
[Miller, S. L., & Cleaves, H. J. (2006). Prebiotic chemistry on the primitive Earth. ''Systems biology'', ''1'', 1.] In 1913, Walther Löb synthesized amino acids by exposing
formamide
Formamide is an amide derived from formic acid. It is a colorless liquid which is miscible with water and has an ammonia-like odor. It is chemical feedstock for the manufacture of sulfa drugs and other pharmaceuticals, herbicides and pesticides, ...
to
silent electric discharge, so scientists were beginning to produce the building blocks of life from simpler molecules, but these were not intended to simulate any prebiotic scheme or even considered relevant to origin of life questions.
But the scientific literature of the early 20th century contained speculations on the origin of life.
In 1903, physicist
Svante Arrhenius
Svante August Arrhenius ( , ; 19 February 1859 – 2 October 1927) was a Swedish scientist. Originally a physicist, but often referred to as a chemist, Arrhenius was one of the founders of the science of physical chemistry. In 1903, he received ...
hypothesized that the first microscopic forms of life, driven by the
radiation pressure
Radiation pressure (also known as light pressure) is mechanical pressure exerted upon a surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of ...
of stars, could have arrived on Earth from space in the
panspermia
Panspermia () is the hypothesis that life exists throughout the universe, distributed by space dust, meteoroids, asteroids, comets, and planetoids, as well as by spacecraft carrying unintended contamination by microorganisms,Forward planetary c ...
hypothesis. In the 1920s,
Leonard Troland wrote about a primordial
enzyme
An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
that could have formed by chance in the
primitive ocean and catalyzed reactions, and
Hermann J. Muller suggested that the formation of a
gene
In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...
with catalytic and autoreplicative properties could have set evolution in motion. Around the same time, Alexander Oparin's and J. B. S. Haldane's "
Primordial soup
Primordial soup, also known as prebiotic soup and Haldane soup, is the hypothetical set of conditions present on the Earth around 3.7 to 4.0 billion years ago. It is an aspect of the heterotrophic theory (also known as the Oparin–Haldane hypothes ...
" ideas were emerging, which hypothesized that a
chemically-reducing atmosphere on early Earth would have been conducive to organic synthesis in the presence of sunlight or lightning, gradually concentrating the ocean with random organic molecules until life emerged. In this way, frameworks for the origin of life were coming together, but at the mid-20th century, hypotheses lacked direct experimental evidence.
Stanley Miller and Harold Urey

At the time of the Miller–Urey experiment, Harold Urey was a
Professor of Chemistry at the
University of Chicago
The University of Chicago (UChicago, Chicago, or UChi) is a Private university, private research university in Chicago, Illinois, United States. Its main campus is in the Hyde Park, Chicago, Hyde Park neighborhood on Chicago's South Side, Chic ...
who had a well-renowned career, including receiving the
Nobel Prize in Chemistry
The Nobel Prize in Chemistry () is awarded annually by the Royal Swedish Academy of Sciences to scientists in the various fields of chemistry. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outst ...
in 1934 for his isolation of
deuterium
Deuterium (hydrogen-2, symbol H or D, also known as heavy hydrogen) is one of two stable isotopes of hydrogen; the other is protium, or hydrogen-1, H. The deuterium nucleus (deuteron) contains one proton and one neutron, whereas the far more c ...
and leading efforts to use
gaseous diffusion
Gaseous diffusion is a technology that was used to produce enriched uranium by forcing gaseous uranium hexafluoride (UF6) through microporous membranes. This produces a slight separation (enrichment factor 1.0043) between the molecules containi ...
for
uranium
Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...
isotope enrichment in support of the
Manhattan Project
The Manhattan Project was a research and development program undertaken during World War II to produce the first nuclear weapons. It was led by the United States in collaboration with the United Kingdom and Canada.
From 1942 to 1946, the ...
. In 1952, Urey postulated that the high temperatures and energies associated with
large impacts in Earth's early history would have provided an atmosphere of
methane
Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...
(CH
4), water (H
2O),
ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
(NH
3), and
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
(H
2), creating the reducing environment necessary for the Oparin-Haldane "primordial soup" scenario.
Stanley Miller arrived at the University of Chicago in 1951 to pursue a PhD under
nuclear physicist
Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter.
Nuclear physics should not be confused with atomic physics, which studies the ...
Edward Teller
Edward Teller (; January 15, 1908 – September 9, 2003) was a Hungarian and American Theoretical physics, theoretical physicist and chemical engineer who is known colloquially as "the father of the hydrogen bomb" and one of the creators of ...
, another prominent figure in the Manhattan Project.
Miller began to work on how different
chemical element
A chemical element is a chemical substance whose atoms all have the same number of protons. The number of protons is called the atomic number of that element. For example, oxygen has an atomic number of 8: each oxygen atom has 8 protons in its ...
s were formed in the early universe, but, after a year of minimal progress, Teller was to leave for California to establish
Lawrence Livermore National Laboratory
Lawrence Livermore National Laboratory (LLNL) is a Federally funded research and development centers, federally funded research and development center in Livermore, California, United States. Originally established in 1952, the laboratory now i ...
and further nuclear weapons research.
Miller, having seen Urey lecture on his 1952 paper, approached him about the possibility of a prebiotic synthesis experiment. While Urey initially discouraged Miller, he agreed to allow Miller to try for a year.
By February 1953, Miller had mailed a manuscript as sole author reporting the results of his experiment to ''
Science
Science is a systematic discipline that builds and organises knowledge in the form of testable hypotheses and predictions about the universe. Modern science is typically divided into twoor threemajor branches: the natural sciences, which stu ...
.''
Urey refused to be listed on the manuscript because he believed his status would cause others to underappreciate Miller's role in designing and conducting the experiment and so encouraged Miller to take full credit for the work. Despite this the set-up is still most commonly referred to including both their names.
After not hearing from ''Science'' for a few weeks, a furious Urey wrote to the editorial board demanding an answer, stating, "If ''Science'' does not wish to publish this promptly we will send it to the ''
Journal of the American Chemical Society
The ''Journal of the American Chemical Society'' (also known as JACS) is a weekly peer-reviewed scientific journal that was established in 1879 by the American Chemical Society. The journal has absorbed two other publications in its history, the ...
''."
Miller's manuscript was eventually published in ''Science'' in May 1953.
Experiment
In the original 1952 experiment, methane (CH
4), ammonia (NH
3), and hydrogen (H
2) were all sealed together in a 2:2:1 ratio (1 part H
2) inside a sterile 5-L glass flask connected to a 500-mL flask half-full of water (H
2O). The gas chamber was intended to represent
Earth's prebiotic atmosphere, while the water simulated an ocean. The water in the smaller flask was boiled such that water vapor entered the gas chamber and mixed with the "atmosphere". A continuous electrical spark was discharged between a pair of electrodes in the larger flask. The spark passed through the mixture of gases and water vapor, simulating lightning. A
condenser below the gas chamber allowed
aqueous solution
An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, also known as sodium chloride (NaCl), in water ...
to accumulate into a U-shaped trap at the bottom of the apparatus, which was sampled.
After a day, the solution that had collected at the trap was pink, and after a week of continuous operation the solution was deep red and
turbid
Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of both water clarity and wate ...
, which Miller attributed to organic matter adsorbed onto
colloidal silica
Colloidal silicas are suspensions of fine amorphous, nonporous, and typically spherical silica particles in a liquid phase. It may be produced by Stöber process from Tetraethyl orthosilicate (TEOS).
Properties
Usually they are suspended in an ...
.
The boiling flask was then removed, and
mercuric chloride
Mercury(II) chloride (mercury bichloride, mercury dichloride, mercuric chloride), historically also sulema or corrosive sublimate, is the inorganic chemical compound of mercury and chlorine with the formula HgCl2, used as a laboratory reagent. ...
(a poison) was added to prevent microbial contamination. The reaction was stopped by adding
barium hydroxide
Barium hydroxide is a chemical compound with the chemical formula Ba(OH)2. The monohydrate (''x'' = 1), known as baryta or baryta-water, is one of the principal compounds of barium. This white granular monohydrate is the usual commercial form.
...
and
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...
, and evaporated to remove impurities. Using
paper chromatography
Paper chromatography is an analytical method used to separate colored chemicals or substances. It can also be used for colorless chemicals that can be located by a stain or other visualisation method after separation. It is now primarily used as ...
, Miller identified five amino acids present in the solution:
glycine
Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid. Glycine is one of the proteinogenic amino acids. It is encoded by all the codons starting with GG (G ...
,
α-alanine and
β-alanine were positively identified, while
aspartic acid
Aspartic acid (symbol Asp or D; the ionic form is known as aspartate), is an α-amino acid that is used in the biosynthesis of proteins. The L-isomer of aspartic acid is one of the 22 proteinogenic amino acids, i.e., the building blocks of protei ...
and
α-aminobutyric acid (AABA) were less certain, due to the spots being faint.
Materials and samples from the original experiments remained in 2017 under the care of Miller's former student,
Jeffrey Bada, a professor at the
UCSD
The University of California, San Diego (UC San Diego in communications material, formerly and colloquially UCSD) is a public land-grant research university in San Diego, California, United States. Established in 1960 near the pre-existing ...
,
Scripps Institution of Oceanography
Scripps Institution of Oceanography (SIO) is the center for oceanography and Earth science at the University of California, San Diego. Its main campus is located in La Jolla, with additional facilities in Point Loma.
Founded in 1903 and incorpo ...
who also conducts origin of life research.
, the apparatus used to conduct the experiment was on display at the
Denver Museum of Nature and Science.
Chemistry of experiment
In 1957 Miller published research describing the chemical processes occurring inside his experiment.
Hydrogen cyanide (HCN) and
aldehyde
In organic chemistry, an aldehyde () (lat. ''al''cohol ''dehyd''rogenatum, dehydrogenated alcohol) is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred ...
s (e.g., formaldehyde) were demonstrated to form as intermediates early on in the experiment due to the electric discharge.
This agrees with current understanding of
atmospheric chemistry
Atmospheric chemistry is a branch of atmospheric science that studies the chemistry of the Earth's atmosphere and that of other planets. This multidisciplinary approach of research draws on environmental chemistry, physics, meteorology, comput ...
, as HCN can generally be produced from reactive
radical species in the atmosphere that arise when CH
4 and nitrogen break apart under
ultraviolet (UV) light.
Similarly, aldehydes can be generated in the atmosphere from radicals resulting from CH
4 and H
2O decomposition and other intermediates like
methanol
Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical compound and the simplest aliphatic Alcohol (chemistry), alcohol, with the chemical formula (a methyl group linked to a hydroxyl group, often ab ...
.
Several energy sources in planetary atmospheres can induce these dissociation reactions and subsequent hydrogen cyanide or aldehyde formation, including lightning, ultraviolet light,
and
galactic cosmic rays.
For example, here is a set
photochemical reactions of species in the Miller-Urey atmosphere that can result in formaldehyde:
: H
2O + ''
hv'' → H + OH
: CH
4 + OH → CH
3 + HOH
: CH
3 + OH → CH
3OH
: CH
3OH + ''hv'' → CH
2O (formaldehyde) + H
2

A photochemical path to HCN from NH
3 and CH
4 is:
: NH
3 + ''hv'' → NH
2 + H
: NH
2 + CH
4 → NH
3 + CH
3
: NH
2 + CH
3 → CH
5N
: CH
5N + ''hv'' → HCN + 2H
2
Other active intermediate compounds (
acetylene
Acetylene (Chemical nomenclature, systematic name: ethyne) is a chemical compound with the formula and structure . It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is u ...
,
cyanoacetylene, etc.) have been detected in the aqueous solution of Miller–Urey-type experiments, but the immediate HCN and aldehyde production, the production of amino acids accompanying the plateau in HCN and aldehyde concentrations, and slowing of amino acid production rate during HCN and aldehyde depletion provided strong evidence that
Strecker amino acid synthesis was occurring in the aqueous solution.
Strecker synthesis describes the reaction of an aldehyde, ammonia, and HCN to a simple amino acid through an
aminoacetonitrile intermediate:
: CH
2O + HCN + NH
3 → NH
2-CH
2-CN (aminoacetonitrile) + H
2O
: NH
2-CH
2-CN + 2H
2O → NH
3 + NH
2-CH
2-COOH (
glycine
Glycine (symbol Gly or G; ) is an amino acid that has a single hydrogen atom as its side chain. It is the simplest stable amino acid. Glycine is one of the proteinogenic amino acids. It is encoded by all the codons starting with GG (G ...
)
Furthermore, water and formaldehyde can react via
Butlerov's reaction to produce various sugars like
ribose
Ribose is a simple sugar and carbohydrate with molecular formula C5H10O5 and the linear-form composition H−(C=O)−(CHOH)4−H. The naturally occurring form, , is a component of the ribonucleotides from which RNA is built, and so this comp ...
.
The experiments showed that simple organic compounds, including the building blocks of proteins and other macromolecules, can abiotically be formed from gases with the addition of energy.
Related experiments and follow-up work
Contemporary experiments

There were a few similar spark discharge experiments contemporaneous with Miller-Urey. An article in ''
The New York Times
''The New York Times'' (''NYT'') is an American daily newspaper based in New York City. ''The New York Times'' covers domestic, national, and international news, and publishes opinion pieces, investigative reports, and reviews. As one of ...
'' (March 8, 1953) titled "Looking Back Two Billion Years" describes the work of Wollman M. MacNevin at
Ohio State University
The Ohio State University (Ohio State or OSU) is a public university, public Land-grant university, land-grant research university in Columbus, Ohio, United States. A member of the University System of Ohio, it was founded in 1870. It is one ...
, before the Miller Science paper was published in May 1953. MacNevin was passing 100,000V sparks through methane and water vapor and produced "
resin
A resin is a solid or highly viscous liquid that can be converted into a polymer. Resins may be biological or synthetic in origin, but are typically harvested from plants. Resins are mixtures of organic compounds, predominantly terpenes. Commo ...
ous solids" that were "too complex for analysis."
Furthermore, K. A. Wilde submitted a manuscript to ''Science'' on December 15, 1952, before Miller submitted his paper to the same journal in February 1953. Wilde's work, published on July 10, 1953, used voltages up to only 600V on a binary mixture of
carbon dioxide
Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
(CO
2) and water in a flow system and did not note any significant reduction products. According to some, the reports of these experiments explain why Urey was rushing Miller's manuscript through ''Science'' and threatening to submit to the ''Journal of the American Chemical Society.''
By introducing an experimental framework to test prebiotic chemistry, the Miller–Urey experiment paved the way for future origin of life research. In 1961,
Joan Oró produced milligrams of the
nucleobase
Nucleotide bases (also nucleobases, nitrogenous bases) are nitrogen-containing biological compounds that form nucleosides, which, in turn, are components of nucleotides, with all of these monomers constituting the basic building blocks of nuc ...
adenine
Adenine (, ) (nucleoside#List of nucleosides and corresponding nucleobases, symbol A or Ade) is a purine nucleotide base that is found in DNA, RNA, and Adenosine triphosphate, ATP. Usually a white crystalline subtance. The shape of adenine is ...
from a concentrated solution of HCN and NH
3 in water. Oró found that several amino acids were also formed from HCN and ammonia under those conditions. Experiments conducted later showed that the other
RNA and DNA nucleobases could be obtained through simulated prebiotic chemistry with a
reducing atmosphere
A reducing atmosphere is an atmosphere in which oxidation is prevented by the absence of oxygen and other oxidizing gases or vapours, and which may contain actively reductant gases such as hydrogen, carbon monoxide, methane and hydrogen sulfide ...
. Other researchers also began using
UV-
photolysis
Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by absorption of light or photons. It is defined as the interaction of one or more photons wi ...
in prebiotic schemes, as the UV flux would have been much higher on early Earth. For example, UV-photolysis of water vapor with
carbon monoxide
Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
was found to yield various
alcohols
In chemistry, an alcohol (), is a type of organic compound that carries at least one hydroxyl () functional group bound to a Saturated and unsaturated compounds, saturated carbon atom. Alcohols range from the simple, like methanol and ethanol ...
, aldehydes, and
organic acid
An organic acid is an organic compound with acidic properties. The most common organic acids are the carboxylic acids, whose acidity is associated with their carboxyl group –COOH. Sulfonic acids, containing the group –SO2OH, are re ...
s. In the 1970s,
Carl Sagan
Carl Edward Sagan (; ; November 9, 1934December 20, 1996) was an American astronomer, planetary scientist and science communicator. His best known scientific contribution is his research on the possibility of extraterrestrial life, including e ...
used Miller-Urey-type reactions to synthesize and experiment with complex organic particles dubbed "
tholin
Tholins (after the Greek (') "hazy" or "muddy"; from the ancient Greek word meaning "sepia ink") are a wide variety of organic compounds formed by solar ultraviolet or cosmic rays, cosmic ray irradiation of simple carbon-containing compounds su ...
s", which likely resemble particles formed in hazy atmospheres like that of
Titan
Titan most often refers to:
* Titan (moon), the largest moon of Saturn
* Titans, a race of deities in Greek mythology
Titan or Titans may also refer to:
Arts and entertainment
Fictional entities
Fictional locations
* Titan in fiction, fictiona ...
.
Modified Miller–Urey experiments
Much work has been done since the 1950s toward understanding how Miller-Urey chemistry behaves in various environmental settings. In 1983, testing different atmospheric compositions, Miller and another researcher repeated experiments with varying proportions of H
2, H
2O, N
2, CO
2 or CH
4, and sometimes NH
3.
They found that the presence or absence of NH
3 in the mixture did not significantly impact amino acid yield, as NH
3 was generated from N
2 during the spark discharge.
Additionally, CH
4 proved to be one of the most important atmospheric ingredients for high yields, likely due to its role in HCN formation.
Much lower yields were obtained with more oxidized carbon species in place of CH
4, but similar yields could be reached with a high H
2/CO
2 ratio.
Thus, Miller-Urey reactions work in atmospheres of other compositions as well, depending on the ratio of reducing and oxidizing gases. More recently,
Jeffrey Bada and H. James Cleaves, graduate students of Miller, hypothesized that the production of nitrites, which destroy amino acids, in CO
2 and N
2-rich atmospheres may explain low amino acids yields.
In a Miller-Urey setup with a less-reducing (CO
2 + N
2 + H
2O) atmosphere, when they added
calcium carbonate
Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...
to
buffer the aqueous solution and
ascorbic acid
Ascorbic acid is an organic compound with formula , originally called hexuronic acid. It is a white solid, but impure samples can appear yellowish. It dissolves freely in water to give mildly acidic solutions. It is a mild reducing agent.
Asco ...
to inhibit oxidation, yields of amino acids greatly increased, demonstrating that amino acids can still be formed in more neutral atmospheres under the right
geochemical
Geochemistry is the science that uses the tools and principles of chemistry to explain the mechanisms behind major geological systems such as the Earth's crust and its oceans. The realm of geochemistry extends beyond the Earth, encompassing the ...
conditions.
In a prebiotic context, they argued that seawater would likely still be buffered and
ferrous iron could inhibit oxidation.
In 1999, after Miller suffered a stroke, he donated the contents of his laboratory to Bada.
In an old cardboard box, Bada discovered unanalyzed samples from modified experiments that Miller had conducted in the 1950s.
In a "
volcanic
A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.
On Earth, volcanoes are most often fo ...
" apparatus, Miller had amended an aspirating nozzle to shoot a jet of steam into the reaction chamber.
Using
high-performance liquid chromatography
High-performance liquid chromatography (HPLC), formerly referred to as high-pressure liquid chromatography, is a technique in analytical chemistry used to separate, identify, and quantify specific components in mixtures. The mixtures can origin ...
and
mass spectrometry
Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used ...
, Bada's lab analyzed old samples from a set of experiments Miller conducted with this apparatus and found some higher yields and a more diverse suite of amino acids.
Bada speculated that injecting the steam into the spark could have split water into H and OH radicals, leading to more
hydroxylated amino acids during Strecker synthesis.
In a separate set of experiments, Miller added
hydrogen sulfide
Hydrogen sulfide is a chemical compound with the formula . It is a colorless chalcogen-hydride gas, and is toxic, corrosive, and flammable. Trace amounts in ambient atmosphere have a characteristic foul odor of rotten eggs. Swedish chemist ...
(H
2S) to the reducing atmosphere, and Bada's analyses of the products suggested order-of-magnitude higher yields, including some amino acids with
sulfur
Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...
moieties.
A 2021 work highlighted the importance of the high-energy free electrons present in the experiment. It is these electrons that produce ions and radicals, and represent an aspect of the experiment that needs to be better understood.
After comparing Miller–Urey experiments conducted in
borosilicate glass
Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10−6 K−1 at 20 °C), ma ...
ware with those conducted in
Teflon
Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene, and has numerous applications because it is chemically inert. The commonly known brand name of PTFE-based composition is Teflon by Chemours, a spin-off from ...
apparatuses, a 2021 paper suggests that the glass reaction vessel acts as a mineral
catalyst
Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...
, implicating silicate rocks as important surfaces in prebiotic Miller-Urey reactions.
Early Earth's prebiotic atmosphere
While there is a lack of geochemical observations to constrain the exact composition of the prebiotic atmosphere, recent models point to an early "weakly reducing" atmosphere; that is, early Earth's atmosphere was likely dominated by CO
2 and N
2 and not CH
4 and NH
3 as used in the original Miller–Urey experiment.
This is explained, in part, by the chemical composition of volcanic outgassing. Geologist
William Rubey was one of the first to compile data on gases emitted from modern volcanoes and concluded that they are rich in CO
2, H
2O, and likely N
2, with varying amounts of H
2,
sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is r ...
(SO
2), and H
2S.
Therefore, if the redox state of
Earth's mantle
Earth's mantle is a layer of silicate mineral, silicate rock between the Earth's crust, crust and the Earth's outer core, outer core. It has a mass of and makes up 67% of the mass of Earth. It has a thickness of making up about 46% of Earth's ...
— which dictates the composition of outgassing – has been constant since
formation, then the atmosphere of early Earth was likely weakly reducing, but there are some arguments for a more-reducing atmosphere for the first few hundred million years.
While the prebiotic atmosphere could have had a different redox condition than that of the Miller–Urey atmosphere, the modified Miller–Urey experiments described in the above section demonstrated that amino acids can still be abiotically produced in less-reducing atmospheres under specific geochemical conditions.
Furthermore, harkening back to Urey's original hypothesis of a "
post-impact" reducing atmosphere,
a recent atmospheric modeling study has shown that an iron-rich impactor with a minimum mass around 4×10
20 – 5×10
21 kg would be enough to transiently reduce the entire prebiotic atmosphere, resulting in a Miller-Urey-esque H
2-, CH
4-, and NH
3-dominated atmosphere that persists for millions of years.
Previous work has estimated from the
lunar cratering record and composition of Earth's mantle that between four and seven such impactors reached the Hadean Earth.

A large factor controlling the redox budget of early Earth's atmosphere is the rate of
atmospheric escape of H
2 after Earth's formation. Atmospheric escape – common to young,
rocky planets — occurs when gases in the atmosphere have sufficient
kinetic energy
In physics, the kinetic energy of an object is the form of energy that it possesses due to its motion.
In classical mechanics, the kinetic energy of a non-rotating object of mass ''m'' traveling at a speed ''v'' is \fracmv^2.Resnick, Rober ...
to overcome
gravitational energy
Gravitational energy or gravitational potential energy is the potential energy an object with mass has due to the gravitational potential of its position in a gravitational field. Mathematically, it is the minimum mechanical work that has to be do ...
.
[Catling, D., & Kasting, J. (2017). Escape of Atmospheres to Space. In ''Atmospheric Evolution on Inhabited and Lifeless Worlds'' (pp. 129–168). Cambridge: Cambridge University Press. ] It is generally accepted that the timescale of hydrogen escape is short enough such that H
2 made up < 1% of the atmosphere of prebiotic Earth,
but, in 2005, a
hydrodynamic model of hydrogen escape predicted escape rates two orders of magnitude lower than previously thought, maintaining a hydrogen
mixing ratio
In chemistry and physics, the dimensionless mixing ratio is the abundance of one component of a mixture relative to that of all other components. The term can refer either to mole ratio (see concentration) or mass ratio (see stoichiometry).
In a ...
of 30%. A hydrogen-rich prebiotic atmosphere would have large implications for Miller-Urey synthesis in the
Hadean
The Hadean ( ) is the first and oldest of the four geologic eons of Earth's history, starting with the planet's formation about 4.6 billion years ago (estimated 4567.30 ± 0.16 million years ago set by the age of the oldest solid material ...
and
Archean
The Archean ( , also spelled Archaean or Archæan), in older sources sometimes called the Archaeozoic, is the second of the four geologic eons of Earth's history of Earth, history, preceded by the Hadean Eon and followed by the Proterozoic and t ...
, but later work suggests solutions in that model might have violated conservation of mass and energy.
That said, during hydrodynamic escape, lighter molecules like hydrogen can "drag" heavier molecules with them through collisions, and recent modeling of
xenon
Xenon is a chemical element; it has symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
escape has pointed to a hydrogen atmospheric mixing ratio of at least 1% or higher at times during the Archean.
Taken together, the view that early Earth's atmosphere was weakly reducing, with transient instances of highly-reducing compositions following large impacts is generally supported.
Extraterrestrial sources of amino acids

Conditions similar to those of the Miller–Urey experiments are present in other regions of the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
, often substituting
ultraviolet
Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
light for lightning as the energy source for chemical reactions. The
Murchison meteorite
The Murchison meteorite is a meteorite that fell in Australia in 1969 near Murchison, Victoria. It belongs to the carbonaceous chondrite class, a group of meteorites rich in organic compounds. Due to its mass (over ) and the fact that it was ...
that fell near
Murchison, Victoria, Australia in 1969 was found to contain an amino acid distribution remarkably similar to Miller-Urey discharge products.
Analysis of the organic fraction of the Murchison meteorite with
Fourier-transform ion cyclotron resonance mass spectrometry detected over 10,000 unique compounds, albeit at very low (
ppb–
ppm) concentrations. In this way, the organic composition of the Murchison meteorite is seen as evidence of Miller-Urey synthesis outside Earth.
Comet
A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or Coma (cometary), coma surrounding ...
s and other
icy outer-solar-system bodies are thought to contain large amounts of complex carbon compounds (such as tholins) formed by processes akin to Miller-Urey setups, darkening surfaces of these bodies.
Some argue that comets bombarding the early Earth could have provided a large supply of complex organic molecules along with the water and other volatiles, however very low concentrations of biologically-relevant material combined with uncertainty surrounding the survival of organic matter upon impact make this difficult to determine.
Relevance to the origin of life
The Miller–Urey experiment was proof that the building blocks of life could be synthesized abiotically from gases, and introduced a new prebiotic chemistry framework through which to study the origin of life. Simulations of protein sequences present in the
last universal common ancestor
The last universal common ancestor (LUCA) is the hypothesized common ancestral cell from which the three domains of life, the Bacteria, the Archaea, and the Eukarya originated. The cell had a lipid bilayer; it possessed the genetic code a ...
(LUCA), or the last shared ancestor of all extant species today, show an enrichment in simple amino acids that were available in the prebiotic environment according to Miller-Urey chemistry. This suggests that the genetic code from which all life evolved was rooted in a smaller suite of amino acids than those used today.
Thus, while
creationist arguments focus on the fact that Miller–Urey experiments have not generated all 22
genetically-encoded amino acids,
this does not actually conflict with the evolutionary perspective on the origin of life.

Another common criticism is that the
racemic
In chemistry, a racemic mixture or racemate () is a mixture that has equal amounts (50:50) of left- and right-handed enantiomers of a chiral molecule or salt. Racemic mixtures are rare in nature, but many compounds are produced industrially as r ...
(containing both L and D
enantiomer
In chemistry, an enantiomer (Help:IPA/English, /ɪˈnænti.əmər, ɛ-, -oʊ-/ Help:Pronunciation respelling key, ''ih-NAN-tee-ə-mər''), also known as an optical isomer, antipode, or optical antipode, is one of a pair of molecular entities whi ...
s) mixture of amino acids produced in a Miller–Urey experiment is not exemplary of abiogenesis theories,
as life on Earth today uses almost exclusively L-amino acids. While it is true that Miller-Urey setups produce racemic mixtures, the origin of
homochirality
Homochirality is a uniformity of chirality, or handedness. Objects are chiral when they cannot be superposed on their mirror images. For example, the left and right hands of a human are approximately mirror images of each other but are not their ow ...
is a separate area in origin of life research.
Recent work demonstrates that
magnetic
Magnetism is the class of physical attributes that occur through a magnetic field, which allows objects to attract or repel each other. Because both electric currents and magnetic moments of elementary particles give rise to a magnetic field, m ...
mineral surfaces like
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula . It is one of the iron oxide, oxides of iron, and is ferrimagnetism, ferrimagnetic; it is attracted to a magnet and can be magnetization, magnetized to become a ...
can be templates for the
enantioselective crystallization
Crystallization is a process that leads to solids with highly organized Atom, atoms or Molecule, molecules, i.e. a crystal. The ordered nature of a crystalline solid can be contrasted with amorphous solids in which atoms or molecules lack regu ...
of chiral molecules, including
RNA
Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...
precursors, due to the
chiral-induced spin selectivity (CISS) effect. Once an enantioselective bias is introduced, homochirality can then propagate through biological systems in various ways. In this way, enantioselective synthesis is not required of Miller-Urey reactions if other geochemical processes in the environment are introducing homochirality.
Finally, Miller-Urey and similar experiments primarily deal with the synthesis of
monomer
A monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization.
Classification
Chemis ...
s;
polymerization
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...
of these building blocks to form
peptide
Peptides are short chains of amino acids linked by peptide bonds. A polypeptide is a longer, continuous, unbranched peptide chain. Polypeptides that have a molecular mass of 10,000 Da or more are called proteins. Chains of fewer than twenty am ...
s and other more complex structures is the next step of prebiotic chemistry schemes. Polymerization requires
condensation reaction
In organic chemistry, a condensation reaction is a type of chemical reaction in which two molecules are combined to form a single molecule, usually with the loss of a small molecule such as water. If water is lost, the reaction is also known as a ...
s, which are thermodynamically unfavored in aqueous solutions because they expel water molecules.
Scientists as far back as
John Desmond Bernal
John Desmond Bernal (; 10 May 1901 – 15 September 1971) was an Irish scientist who pioneered the use of X-ray crystallography in molecular biology. He published extensively on the history of science. In addition, Bernal wrote popular boo ...
in the late 1940s thus speculated that clay surfaces would play a large role in abiogenesis, as they might concentrate monomers. Several such models for mineral-mediated polymerization have emerged, such as the interlayers of
layered double hydroxides like
green rust
Green rust is a generic name for various green crystalline chemical compounds containing iron(II) and iron(III) cations, the hydroxide () anion, and another anion such as carbonate (), chloride (), or sulfate (), in a layered double hydroxide (LDH ...
over wet-dry cycles.
Some scenarios for peptide formation have been proposed that are even compatible with aqueous solutions, such as the hydrophobic air-water interface
and a novel "
sulfide
Sulfide (also sulphide in British English) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to large families o ...
-mediated α-aminonitrile ligation" scheme, where amino acid precursors come together to form peptides.
Polymerization of life's building blocks is an active area of research in prebiotic chemistry.
Amino acids identified
Below is a table of amino acids produced and identified in the "classic" 1952 experiment, as analyzed by Miller in 1952
and more recently by Bada and collaborators with modern mass spectrometry,
the 2008 re-analysis of vials from the volcanic spark discharge experiment,
and the 2010 re-analysis of vials from the H
2S-rich spark discharge experiment.
While not all proteinogenic amino acids have been produced in spark discharge experiments, it is generally accepted that early life used a simpler set of prebiotically-available amino acids.
References
External links
A simulation of the Miller–Urey Experiment along with a video Interview with Stanley Millerby Scott Ellis from CalSpace (UCSD)
*
ttps://web.archive.org/web/20090821213017/http://www.chem.duke.edu/~jds/cruise_chem/Exobiology/miller.html Miller–Urey experiment explainedMiller experiment with Lego bricks*
ttp://www.millerureyexperiment.com/ The Miller-Urey experiment website*
Details of 2008 re-analysis
{{DEFAULTSORT:Miller-Urey Experiment
Articles containing video clips
Biology experiments
Chemical synthesis of amino acids
Chemistry experiments
Origin of life
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