The
Miller–Urey experiment (or
Urey–Miller experiment) was an
experimentIn scientific research, an experiment is a method of investigating causal relationships among variables, or to test a hypothesis. An experiment is a cornerstone of the empirical approach to acquiring data about the world and is used in both natural sciences and social sciences...
that simulated hypothetical conditions thought at the time to be present on the
early EarthThe "early Earth" is a term usually defined as Earth's first billion years, or gigayear. On the geologic time scale, the "early Earth" comprises all of the Hadean eon , as well as the Eoarchean and part of the Paleoarchean eras of the Archean eon.This period of Earth's history, being its earliest,...
, and tested for the occurrence of
chemical evolutionIn the natural sciences, abiogenesis, or "chemical evolution", is the study of how life on Earth could have arisen from inanimate matter. It should not be confused with evolution, which is the study of how groups of living things change over time...
. Specifically, the experiment tested Soviet scientist Alexander Oparin's and
J. B. S. HaldaneJohn Burdon Sanderson Haldane FRS , known as Jack , was a British-born geneticist and evolutionary biologist...
's
hypothesisA hypothesis is a proposed explanation for an observable phenomenon. The term derives from the Greek, ὑποτιθέναι - hypotithenai meaning "to put under" or "to suppose." For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
that conditions on the primitive Earth favored chemical reactions that synthesized
organic compoundAn organic compound is any member of a large class of chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of compounds such as carbonates, simple oxides of carbon and cyanides, as well as the allotropes of carbon, are considered inorganic...
s from inorganic precursors. Considered to be the classic experiment on the
origin of lifeIn the natural sciences, abiogenesis, or "chemical evolution", is the study of how life on Earth could have arisen from inanimate matter. It should not be confused with evolution, which is the study of how groups of living things change over time...
, it was conducted in 1952 and published in 1953 by
Stanley MillerStanley Lloyd Miller was an American chemist and biologist who is known for his studies into the origin of life, particularly the Miller-Urey experiment which demonstrated that organic compounds can be created by fairly simple physical processes from inorganic substances...
and
Harold UreyHarold Clayton Urey was an American physical chemist whose pioneering work on isotopes earned him the Nobel Prize in Chemistry in 1934 and later led him to theories of planetary evolution.-Biography:...
at the
University of ChicagoThe University of Chicago is a private, coeducational research university in Chicago, Illinois, USA. It was founded by oil magnate and benefactor John D...
.
In 2008, a re-analysis of Miller's archived solutions from the original experiments showed 22
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
s rather than 5 were actually created in one of the apparatus used.
Experiment and interpretation
The experiment used
waterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
(H
2O),
methaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees. Burning methane in the presence of oxygen produces carbon dioxide and water. The relative abundance of methane and its clean...
(CH
4),
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula NH
3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers...
(NH
3), and
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
(H
2). The chemicals were all sealed inside a sterile array of glass tubes and flasks connected in a loop, with one flask half-full of liquid water and another flask containing a pair of electrodes. The liquid water was heated to induce
evaporationEvaporation is the vaporization of a liquid and the reverse, of condensation. A type of phase transition, it is the process by which molecules in a liquid state spontaneously become gaseous . Generally, evaporation can be seen by the gradual disappearance of a liquid from a substance when exposed...
, sparks were fired between the electrodes to simulate
lightningLightning is an atmospheric discharge of electricity accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...
through the
atmosphereThe Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
and
water vaporWater vapor or water vapour , also aqueous vapor, is the gas phase of water. Water vapor is one state of the water cycle within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice...
, and then the atmosphere was cooled again so that the water could condense and trickle back into the first flask in a continuous cycle.
At the end of one week of continuous operation, Miller and Urey observed that as much as 10–15% of the
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
within the system was now in the form of organic compounds. Two percent of the carbon had formed amino acids that are used to make
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
s in living cells, with
glycineGlycine is an organic compound with the formula NH2CH2COOH. With only a hydrogen atom as its side chain, glycine is the smallest of the 20 amino acids commonly found in proteins....
as the most abundant. Sugars, lipids, and some of the building blocks for nucleic acids were also formed.
In an interview, Stanley Miller stated: "Just turning on the spark in a basic pre-biotic experiment will yield 11 out of 20 amino acids."
As observed in all subsequent experiments, both left-handed (L) and right-handed (D) optical isomers were created in a racemic mixture.
Chemistry of experiment
One-step reactions among the mixture components can produce
hydrogen cyanideHydrogen cyanide is a chemical compound with chemical formula HCN. A solution of hydrogen cyanide in water is called hydrocyanic acid. Hydrogen cyanide is a colorless, extremely poisonous, and highly volatile liquid that boils slightly above room temperature at 26 °C...
(HCN),
formaldehydeFormaldehyde is a chemical compound with the formula CH2O. It is the simplest aldehyde. Formaldehyde also exists as the cyclic trimer trioxane and the polymer paraformaldehyde. It exists in water as the hydrate H2C2. Aqueous solutions of formaldehyde are referred...
(CH
2O), and other active intermediate compounds (
acetyleneAcetylene is the chemical compound with the formula HC
2H. It is a hydrocarbon and the simplest alkyne. This colourless gas is widely used as a fuel and a chemical building block...
, cyanoacetylene, etc.):
- CO2 → CO + [O] (atomic oxygen)
- CH4 + 2[O] → CH2O + H2O
- CO + NH3 → HCN + H2O
- CH4 + NH3 → HCN + 3H2 (BMA process
The BMA process is a chemical process developed by Degussa for the production of hydrogen cyanide from methane and ammonia in presence of a platinum catalyst...
)
The formaldehyde, ammonia, and HCN then react by Strecker synthesis to form amino acids and other biomolecules:
- CH2O + HCN + NH3 → NH2-CH2-CN + H2O
- NH2-CH2-CN + 2H2O → NH3 + NH2-CH2-COOH (glycine
Glycine is an organic compound with the formula NH2CH2COOH. With only a hydrogen atom as its side chain, glycine is the smallest of the 20 amino acids commonly found in proteins....
)
Furthermore, water and formaldehyde can react via
Butlerov's reactionThe formose reaction, discovered by Aleksandr Butlerov in 1861, involves the formation of sugars from formaldehyde. Formose is a contraction of formaldehyde and aldose.-Reaction and mechanism:...
to produce various
sugarSugar is a class of edible crystalline substances, mainly sucrose, lactose, and fructose. Human taste buds interpret its flavor as sweet. Sugar as a basic food carbohydrate primarily comes from sugar cane and from sugar beet, but also appears in fruit, honey, sorghum, sugar maple , and in many...
s like
riboseRibose, primarily occurring as
D-ribose, is an organic compound that occurs widely in nature. It is an aldopentose, that is a monosaccharide containing five carbon atoms that, in its acyclic form, has an aldehyde functional group at one end. This species predominantly exists in the...
.
Other experiments
This experiment inspired many others. In 1961,
Joan OróJoan Oró Florensa was a biochemist from Catalonia , Marquess of Oró, whose research has been of importance in understanding the origin of life....
found that the
nucleotideNucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides play central roles in metabolism...
base
adenineAdenine is a nucleobase with a variety of roles in biochemistry including cellular respiration, in the form of both the energy-rich adenosine triphosphate and the cofactors nicotinamide adenine dinucleotide and flavin adenine dinucleotide , and protein synthesis, as a chemical component of DNA...
could be made from
hydrogen cyanideHydrogen cyanide is a chemical compound with chemical formula HCN. A solution of hydrogen cyanide in water is called hydrocyanic acid. Hydrogen cyanide is a colorless, extremely poisonous, and highly volatile liquid that boils slightly above room temperature at 26 °C...
(HCN) and
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula NH
3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers...
in a water solution. His experiment produced a large amount of adenine, which molecules were formed from 5 molecules of HCN.
Also, many amino acids are formed from HCN and ammonia under these conditions.
Experiments conducted later showed that the other
RNA and DNA nucleobasesNucleobases are the parts of DNA and RNA that may be involved in pairing . The main ones are cytosine, guanine, adenine , thymine and uracil , abbreviated as C, G, A, T, and U, respectively. They are usually simply called bases in genetics...
could be obtained through simulated prebiotic chemistry with a
reducing atmosphereA reducing atmosphere, also known as a reduction atmosphere, is an atmospheric condition in which oxidation is prevented by removal of oxygen and other oxidising gases or vapours. Usually nitrogen is used or in more extreme conditions hydrogen gas. In that case the atmosphere is reducing...
.
There also had been similar electric discharge experiments related to the origin of life contemporaneous with Miller–Urey. An article in
The New York TimesThe New York Times is an American daily newspaper founded in 1851 and published in New York City. The largest metropolitan newspaper in the United States, "The Gray Lady"—named for its staid appearance and style—is regarded as a national newspaper of record...
(March 8, 1953:E9), titled "Looking Back Two Billion Years" describes the work of Wollman (William) M. MacNevin at The Ohio State University, before the Miller
Science paper was published in May 1953. MacNevin was passing 100,000 volt sparks through methane and water vapor and produced "resinous solids" that were "too complex for analysis." The article describes other early earth experiments being done by MacNevin. It is not clear if he ever published any of these results in the primary scientific literature.
K. A. Wilde submitted a paper to
Science on December 15, 1952, before Miller submitted his paper to the same journal on February 14, 1953. Wilde's paper was published on July 10, 1953. Wilde used voltages up to only 600 V on a binary mixture of
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
(CO
2) and water in a flow system. He observed only small amounts of carbon dioxide reduction to carbon monoxide, and no other significant reduction products or newly formed carbon compounds.
Other researchers were studying UV-photolysis of water vapor with
carbon monoxideCarbon monoxide is a colorless, odorless and tasteless gas, yet very toxic to humans. It consists of one carbon atom and one oxygen atom, connected by a covalent double bond and a dative covalent bond...
. They have found that various alcohols, aldehydes and organic acids were synthesized in reaction mixture .
More recent experiments by chemist Jeffrey Bada at
Scripps Institution of OceanographyScripps Institution of Oceanography in La Jolla, California, is one of the oldest and largest centers for ocean and earth science research, graduate training, and public service in the world...
(in La Jolla, CA) were similar to those performed by Miller. However, Bada noted that in current models of early Earth conditions, carbon dioxide and
nitrogenNitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
(N
2) create
nitriteThe nitrite ion is NO2−. The anion is bent, being isoelectronic with ozone. More generally, a nitrite compound is either a salt or an ester of nitrous acid.-Examples:* nitrous acid, HNO2...
s, which destroy amino acids as fast as they form. However, the early Earth may have had significant amounts of iron and
carbonate mineralsCarbonate minerals are those minerals containing the carbonate ion: CO32-.-Anhydrous carbonates:*Calcite group: Trigonal**Calcite CaCO3**Gaspeite CO3**Magnesite MgCO3...
able to neutralize the effects of the nitrites. When Bada performed the Miller-type experiment with the addition of iron and carbonate minerals, the products were rich in amino acids. This suggests the origin of significant amounts of amino acids may have occurred on Earth even with an atmosphere containing carbon dioxide and nitrogen.
Earth's early atmosphere
Some evidence suggests that Earth's original atmosphere might have contained fewer of the reducing molecules than was thought at the time of the Miller–Urey experiment. There is abundant evidence of major volcanic eruptions 4 billion years ago, which would have released carbon dioxide, nitrogen,
hydrogen sulfideHydrogen sulfide is the chemical compound with the formula H
2S. This colorless, toxic and flammable gas is partially responsible for the foul odor of rotten eggs and flatulence....
(H
2S), and
sulfur dioxideSulfur dioxide is the chemical compound with the formula SO
2. It is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide...
(SO
2) into the atmosphere. Experiments using these gases in addition to the ones in the original Miller–Urey experiment have produced more diverse molecules. The experiment created a mixture that was racemic (containing both L and D
enantiomerIn stereochemistry, an enantiomer is one of two stereoisomers that are mirror images of each other that are "non-superposable" , much as one's left and right hands are "the same" but opposite....
s) and experiments since have shown that "in the lab the two versions are equally likely to appear." However, in nature, L amino acids dominate; later experiments have confirmed disproportionate amounts of L or D oriented enantiomers are possible.
Originally it was thought that the primitive secondary atmosphere contained mostly ammonia and methane. However, it is likely that most of the atmospheric carbon was CO
2 with perhaps some CO and the nitrogen mostly N
2. In practice gas mixtures containing CO, CO
2, N
2, etc. give much the same products as those containing CH
4 and NH
3 so long as there is no O
2. The hydrogen atoms come mostly from water vapor. In fact, in order to generate aromatic amino acids under primitive earth conditions it is necessary to use less hydrogen-rich gaseous mixtures. Most of the natural amino acids, hydroxyacids, purines, pyrimidines, and sugars have been produced in variants of the Miller experiment.
More recent results may question these conclusions. The University of Waterloo and University of Colorado conducted simulations in 2005 that indicated that the early atmosphere of Earth could have contained up to 40 percent hydrogen—implying a possibly much more hospitable environment for the formation of prebiotic organic molecules. The escape of hydrogen from Earth's atmosphere into space may have occurred at only one percent of the rate previously believed based on revised estimates of the upper atmosphere's temperature. One of the authors, Owen Toon notes: "In this new scenario, organics can be produced efficiently in the early atmosphere, leading us back to the organic-rich soup-in-the-ocean concept... I think this study makes the experiments by Miller and others relevant again." Outgassing calculations using a chondritic model for the early earth complement the Waterloo/Colorado results in re-establishing the importance of the Miller–Urey experiment.
However, when
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
gas is added to this mixture, no organic molecules are formed. Critics of the Miller–Urey hypothesis point out recent research that shows the presence of uranium in sediments dated to 3.7 Ga and indicates it was transported in solution by oxygenated water (otherwise it would have precipitated out). These critics argue that this presence of oxygen precludes the formation of prebiotic molecules via a Miller–Urey-like scenario, attempting to invalidate the hypothesis of
abiogenesisIn the natural sciences, abiogenesis, or "chemical evolution", is the study of how life on Earth could have arisen from inanimate matter. It should not be confused with evolution, which is the study of how groups of living things change over time...
. However, the authors of the paper are arguing that this presence of oxygen merely evidences the existence of
photosyntheticPhotosynthesis is a process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of Bacteria, but not in Archaea...
organisms 3.7 Ga ago (a date about 200 Ma earlier than previous estimates) a conclusion which while pushing back the time frame in which Miller–Urey reactions and abiogenesis could potentially have occurred, would not preclude them. Though there is somewhat controversial evidence for very small (less than 0.1%) amounts of oxygen in the atmosphere almost as old as Earth's oldest rocks, the authors are not in any way arguing for the existence of an oxygen-rich atmosphere any earlier than previously thought, and they state: ". . . In fact most evidence suggests that oxygenic photosynthesis was present during time periods from which there is evidence for a non-oxygenic atmosphere".
Conditions similar to those of the Miller–Urey experiments are present in other regions of the
solar systemThe Solar System consists of the Sun and those celestial objects bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago...
, often substituting
ultravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light for lightning as the energy source for chemical reactions. The
Murchison meteoriteThe Murchison meteorite is named after Murchison, Victoria, in Australia. It is one of the most studied meteorites due to its large mass , the fact that it was an observed fall, and it belongs to a group of meteorites rich in organic compounds....
that fell near
Murchison, VictoriaMurchison is a small rural village located on the Goulburn River in Victoria, Australia. Murchison is located 167 kilometres from Melbourne and is just to the west of the Goulburn Valley Highway between Shepparton and Nagambie. The surrounding countryside contains orchards, vineyards and dairy...
, Australia in 1969 was found to contain over 90 different amino acids, nineteen of which are found in Earth life.
CometA comet is a Small Solar System Body that has coma and is bigger than a meteoroid. When close enough to the Sun, a comet exhibits a visible coma , and sometimes a tail, both because of the effects of solar radiation upon the comet's nucleus...
s and other
icy outer-solar-system bodiesA trans-Neptunian object is any object in the Solar System that orbits the Sun at a greater distance on average than Neptune. The Kuiper belt, scattered disk, and Oort cloud are three divisions of this volume of space....
are thought to contain large amounts of complex carbon compounds (such as
tholinTholin , is a heteropolymer molecule formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane. Tholins do not form naturally on modern-day Earth, but are found in great abundance on the surface of icy bodies in the outer solar system...
s) formed by these processes, darkening surfaces of these bodies. The early Earth was bombarded heavily by comets, possibly providing a large supply of complex organic molecules along with the water and other volatiles they contributed. This has been used to infer an origin of life outside of Earth: the
panspermiaPanspermia is the hypothesis that "seeds" of life exist already all over the Universe, that life on Earth may have originated through these "seeds", and that they may deliver or have delivered life to other habitable bodies....
hypothesis.
Recent related studies
In recent years, studies have been made of the
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
composition of the products of "old" areas in "old" genes, defined as those that are found to be common to organisms from several widely separated
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
, assumed to share only the
last universal ancestorThe last universal ancestor is the most recent organism from which all organisms now living on Earth descend. Thus it is the most recent common ancestor of all current life on Earth...
(LUA) of all extant species. These studies found that the products of these areas are enriched in those amino acids that are also most readily produced in the Miller–Urey experiment. This suggests that the original genetic code was based on a smaller number of amino acids – only those available in prebiotic nature – than the current one.
In 2008, a group of scientists examined 11 vials left over from Miller's experiments of the early 1950s. In addition to the classic experiment, reminiscent of
Charles DarwinCharles Robert Darwin FRS was an English naturalist who realised and presented compelling evidence that all species of life have evolved over time from common ancestors, through the process he called natural selection...
's envisioned "warm little pond", Miller had also performed more experiments, including one with conditions similar to those of
volcanic3. Conduit
4. Base
5. Sill
6. Dike
7. Layers of ash emitted by the volcano
8. Flank| 9. Layers of lava emitted by the volcano
10. Throat
11. Parasitic cone
12. Lava flow
13. Vent
14. Crater
15...
eruptions. This experiment had a nozzle spraying a jet of steam at the spark discharge. By using high-performance liquid chromatography and
mass spectrometryMass spectrometry is an analytical technique for the determination of the elemental composition of a sample or molecule. It is also used for elucidating the chemical structures of molecules, such as peptides and other chemical compounds...
, the group found more organic molecules than Miller had. Interestingly, they found that the volcano-like experiment had produced the most organic molecules, 22 amino acids, 5
amineAmines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
s and many hydroxylated molecules, which could have been formed by
hydroxyl radicalThe hydroxyl radical, OH·, is the neutral form of the hydroxide ion . Hydroxyl radicals are highly reactive and consequently short-lived; however, they form an important part of radical chemistry...
s produced by the electrified steam. The group suggested that volcanic island systems became rich in organic molecules in this way, and that the presence of
carbonyl sulfideCarbonyl sulfide is the chemical compound with the formula OCS. Commonly written as COS, it is a colourless flammable gas with an unpleasant odor. It is a linear molecule consisting of a carbonyl group double bonded to a sulfur atom...
there could have helped these molecules form
peptidePeptides are short polymers formed from the linking, in a defined order, of α-amino acids. The link between one amino acid residue and the next is called an amide bond or a peptide bond....
s.
See also
- Abiogenesis
In the natural sciences, abiogenesis, or "chemical evolution", is the study of how life on Earth could have arisen from inanimate matter. It should not be confused with evolution, which is the study of how groups of living things change over time...
, the study of how life on Earth emerged from inanimate organic and inorganic molecules.
External links