The Miller–Urey experiment (or Miller experiment) was a chemical experiment that simulated the conditions thought at the time to be present on the early Earth, and tested the chemical origin of life under those conditions. The experiment supported Alexander Oparin's and J. B. S. Haldane's hypothesis that putative conditions on the primitive Earth favoured chemical reactions that synthesized more complex organic compounds from simpler inorganic precursors. Considered to be the classic experiment investigating abiogenesis, it was conducted in 1952 by Stanley Miller, with assistance from Harold Urey, at the University of Chicago and later the University of California, San Diego and published the following year.
After Miller's death in 2007, scientists examining sealed vials preserved from the original experiments were able to show that there were actually well over 20 different amino acids produced in Miller's original experiments. That is considerably more than what Miller originally reported, and more than the 20 that naturally occur in life. More recent evidence suggests that Earth's original atmosphere might have had a composition different from the gas used in the Miller experiment, but prebiotic experiments continue to produce racemic mixtures of simple to complex compounds under varying conditions.
The experiment used water (H2O), methane (CH4), ammonia (NH3), and hydrogen (H2). The chemicals were all sealed inside a sterile 5-liter glass flask connected to a 500 ml flask half-full of water. The water in the smaller flask was heated to induce evaporation, and the water vapour was allowed to enter the larger flask. Continuous electrical sparks were fired between the electrodes to simulate lightning in the water vapour and gaseous mixture, and then the simulated atmosphere was cooled again so that the water condensed and trickled into a U-shaped trap at the bottom of the apparatus.
After a day, the solution collected at the trap had turned pink in colour. At the end of one week of continuous operation, the boiling flask was removed, and mercuric chloride was added to prevent microbial contamination. The reaction was stopped by adding barium hydroxide and sulfuric acid, and evaporated to remove impurities. Using paper chromatography, Miller identified five amino acids present in the solution: glycine, α-alanine and β-alanine were positively identified, while aspartic acid and α-aminobutyric acid (AABA) were less certain, due to the spots being faint.
In a 1996 interview, Stanley Miller recollected his lifelong experiments following his original work and 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. In biological systems, almost all of the compounds are non-racemic, or homochiral.
The original experiment remains today under the care of Miller and Urey's former student Jeffrey Bada, a professor at the UCSD, Scripps Institution of Oceanography. The apparatus used to conduct the experiment is on display at the Denver Museum of Nature and Science.
Chemistry of experiment
One-step reactions among the mixture components can produce hydrogen cyanide (HCN), formaldehyde (CH2O), and other active intermediate compounds (acetylene, cyanoacetylene, etc.):
- CO2 → CO + [O] (atomic oxygen)
- CH4 + 2[O] → CH2O + H2O
- CO + NH3 → HCN + H2O
- CH4 + NH3 → HCN + 3H2 (BMA process)
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)
The experiments showed that simple organic compounds of building blocks of proteins and other macromolecules can be formed from gases with the addition of energy.
This experiment inspired many others. In 1961, Joan Oró found that the nucleotide base adenine could be made from hydrogen cyanide (HCN) and ammonia in a water solution. His experiment produced a large amount of adenine, the molecules of which 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 nucleobases could be obtained through simulated prebiotic chemistry with a reducing atmosphere.
There also had been similar electric discharge experiments related to the origin of life contemporaneous with Miller–Urey. An article in The New York Times (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 10, 1953. Wilde's paper was published on July 10, 1953. Wilde used voltages up to only 600 V on a binary mixture of carbon dioxide (CO2) 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 monoxide. They have found that various alcohols, aldehydes and organic acids were synthesized in reaction mixture.
More recent experiments by chemists Jeffrey Bada, one of Miller's graduate students, and Jim Cleaves at Scripps Institution of Oceanography of the University of California, San Diego were similar to those performed by Miller. However, Bada noted that in current models of early Earth conditions, carbon dioxide and nitrogen (N2) create nitrites, which destroy amino acids as fast as they form. 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 sulfide (H2S), and sulfur dioxide (SO2) 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 enantiomers) 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 CO2 with perhaps some CO and the nitrogen mostly N2. In practice gas mixtures containing CO, CO2, N2, etc. give much the same products as those containing CH4 and NH3 so long as there is no O2. 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 made 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 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.
In contrast to the general notion of early earth's reducing atmosphere, researchers at the Rensselaer Polytechnic Institute in New York reported the possibility of oxygen available around 4.3 billion years ago. Their study reported in 2011 on the assessment of Hadean zircons from the earth's interior (magma) indicated the presence of oxygen traces similar to modern-day lavas. This study suggests that oxygen could have been released in the earth's atmosphere earlier than generally believed.
Conditions similar to those of the Miller–Urey experiments are present in other regions of the solar system, often substituting ultraviolet light for lightning as the energy source for chemical reactions. The Murchison meteorite that fell near Murchison, Victoria, Australia in 1969 was found to contain over 90 different amino acids, nineteen of which are found in Earth life. Comets and other icy outer-solar-system bodies are thought to contain large amounts of complex carbon compounds (such as tholins) 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 panspermia hypothesis.
In recent years, studies have been made of the amino acid 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 species, assumed to share only the last universal ancestor (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.
Jeffrey Bada, himself Miller's student, inherited the original equipment from the experiment when Miller died in 2007. Based on sealed vials from the original experiment, scientists have been able to show that although successful, Miller was never able to find out, with the equipment available to him, the full extent of the experiment's success. Later researchers have been able to isolate even more different amino acids, 25 altogether. Bada has estimated that more accurate measurements could easily bring out 30 or 40 more amino acids in very low concentrations, but the researchers have since discontinued the testing. Miller's experiment was therefore a remarkable success at synthesizing complex organic molecules from simpler chemicals, considering that all life uses just 20 different amino acids.
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 Darwin's envisioned "warm little pond", Miller had also performed more experiments, including one with conditions similar to those of volcanic eruptions. This experiment had a nozzle spraying a jet of steam at the spark discharge. By using high-performance liquid chromatography and mass spectrometry, the group found more organic molecules than Miller had. They found that the volcano-like experiment had produced the most organic molecules, 22 amino acids, 5 amines and many hydroxylated molecules, which could have been formed by hydroxyl radicals 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 sulfide there could have helped these molecules form peptides.
The main problem of theories based around amino acids is the difficulty in obtaining spontaneous formation of peptides. Since John Desmond Bernal's suggestion that clay surfaces could have played a role in abiogenesis, scientific efforts have been dedicated to investigating clay-mediated peptide bond formation, with limited success. Peptides formed remained over-protected and shown no evidence of inheritance or metabolism. In December 2017 a theoretical model developed by Erastova and collaborators  suggested that peptides could form at the interlayers of layered double hydroxides such as green rust in early earth conditions. According to the model, drying of the intercalated layered material should provide energy and co-alignment required for peptide bond formation in a ribosome-like fashion, while re-wetting should allow mobilising the newly formed peptides and repopulate the interlayer with new amino acids. This mechanism is expected to lead to the formation of 12+ amino acid-long peptides within 15-20 washes. Researches also observed slightly different adsorption preferences for different amino acids, and postulated that, if coupled to a diluted solution of mixed amino acids, such preferences could lead to sequencing.
In October 2018, researchers at McMaster University on behalf of the Origins Institute announced the development of a new technology, called a Planet Simulator, to help study the origin of life on planet Earth and beyond.
Amino acids identified
Below is a table of amino acids produced and identified in the "classic" 1952 experiment, as published by Miller in 1953, the 2008 re-analysis of vials from the volcanic spark discharge experiment, and the 2010 re-analysis of vials from the H2S-rich spark discharge experiment.
|Amino acid||Produced in experiment||Proteinogenic|
|Volcanic spark discharge
|H2S-rich spark discharge|
- Hill HG, Nuth JA (2003). "The catalytic potential of cosmic dust: implications for prebiotic chemistry in the solar nebula and other protoplanetary systems". Astrobiology. 3 (2): 291–304. Bibcode:2003AsBio...3..291H. doi:10.1089/153110703769016389. PMID 14577878.
- Balm SP; Hare J.P.; Kroto HW (1991). "The analysis of comet mass spectrometric data". Space Science Reviews. 56 (1–2): 185–9. Bibcode:1991SSRv...56..185B. doi:10.1007/BF00178408.
- Bada, Jeffrey L. (2000). "Stanley Miller's 70th Birthday" (PDF). Origins of Life and Evolution of the Biosphere. 30 (2/4): 107–12. doi:10.1023/A:1006746205180. Archived from the original (PDF) on February 27, 2009.
- Miller, Stanley L. (1953). "Production of Amino Acids Under Possible Primitive Earth Conditions" (PDF). Science. 117 (3046): 528–9. Bibcode:1953Sci...117..528M. doi:10.1126/science.117.3046.528. PMID 13056598. Archived (PDF) from the original on 2012-03-17.
- Miller, Stanley L.; Harold C. Urey (1959). "Organic Compound Synthesis on the Primitive Earth". Science. 130 (3370): 245–51. Bibcode:1959Sci...130..245M. doi:10.1126/science.130.3370.245. PMID 13668555. Miller states that he made "A more complete analysis of the products" in the 1953 experiment, listing additional results.
- A. Lazcano; J. L. Bada (2004). "The 1953 Stanley L. Miller Experiment: Fifty Years of Prebiotic Organic Chemistry". Origins of Life and Evolution of Biospheres. 33 (3): 235–242. doi:10.1023/A:1024807125069. PMID 14515862.
- "The Spark of Life". BBC Four. 26 August 2009. Archived from the original on 2010-11-13. TV Documentary.
- Bada, Jeffrey L. (2013). "New insights into prebiotic chemistry from Stanley Miller's spark discharge experiments". Chemical Society Reviews. 42 (5): 2186–96. doi:10.1039/c3cs35433d. PMID 23340907.
- Asimov, Isaac (1981). Extraterrestrial Civilizations. Pan Books Ltd. p. 178.
- "Exobiology: An Interview with Stanley L. Miller". Accessexcellence.org. Archived from the original on May 18, 2008. Retrieved 2009-08-20.
- Dreifus, Claudia (2010-05-17). "A Conversation With Jeffrey L. Bada: A Marine Chemist Studies How Life Began". nytimes.com. Archived from the original on 2017-01-18.
- "Astrobiology Collection: Miller-Urey Apparatus". Denver Museum of Nature & Science. Archived from the original on 2013-05-24.
- https://www.webcitation.org/query?url=http://www.geocities.com/capecanaveral/lab/2948/orgel.html&date=2009-10-25+16:53:26 Origin of Life on Earth by Leslie E. Orgel
- Council, National Research; Studies, Division on Earth Life; Technology, Board on Chemical Sciences and; Sciences, Division on Engineering Physical; Board, Space Studies; System, Task Group on Organic Environments in the Solar (2007). Read "Exploring Organic Environments in the Solar System" at NAP.edu. doi:10.17226/11860. ISBN 978-0-309-10235-3. Archived from the original on 2009-06-21. Retrieved 2008-10-25. Exploring Organic Environments in the Solar System (2007)
- Oró J, Kimball AP (August 1961). "Synthesis of purines under possible primitive earth conditions. I. Adenine from hydrogen cyanide". Archives of Biochemistry and Biophysics. 94 (2): 217–27. doi:10.1016/0003-9861(61)90033-9. PMID 13731263.
- Oró J, Kamat SS (April 1961). "Amino-acid synthesis from hydrogen cyanide under possible primitive earth conditions". Nature. 190 (4774): 442–3. Bibcode:1961Natur.190..442O. doi:10.1038/190442a0. PMID 13731262.
- Oró J (1967). Fox SW, ed. Origins of Prebiological Systems and of Their Molecular Matrices. New York Academic Press. p. 137.
- Krehl, Peter O. K. (2009). History of Shock Waves, Explosions and Impact: A Chronological and Biographical Reference. Springer-Verlag. p. 603.
- Wilde, Kenneth A.; Zwolinski, Bruno J.; Parlin, Ransom B. (July 1953). "The Reaction Occurring in CO2, 2O Mixtures in a High-Frequency Electric Arc". Science. 118 (3054): 43–44. Bibcode:1953Sci...118...43W. doi:10.1126/science.118.3054.43-a. PMID 13076175. Archived from the original on 2009-06-21. Retrieved 2008-07-09.
- Synthesis of organic compounds from carbon monoxide and water by UV photolysis Origins of Life. December 1978, Volume 9, Issue 2, pp 93-101 Akiva Bar-nun, Hyman Hartman.
- Fox, Douglas (2007-03-28). "Primordial Soup's On: Scientists Repeat Evolution's Most Famous Experiment". Scientific American. History of Science. Scientific American Inc. Retrieved 2008-07-09.
Cleaves, H. J.; Chalmers, J. H.; Lazcano, A.; Miller, S. L.; Bada, J. L. (2008). "A Reassessment of Prebiotic Organic Synthesis in Neutral Planetary Atmospheres" (PDF). Origins of Life and Evolution of Biospheres. 38 (2): 105–115. Bibcode:2008OLEB...38..105C. doi:10.1007/s11084-007-9120-3. PMID 18204914. Archived from the original (PDF) on 2013-11-07.
- Green, Jack (2011). "Academic Aspects of Lunar Water Resources and Their Relevance to Lunar Protolife". International Journal of Molecular Sciences. 12 (9): 6051–6076. doi:10.3390/ijms12096051. PMC 3189768. PMID 22016644.
- "Right-handed amino acids were left behind". New Scientist (2554). Reed Business Information Ltd. 2006-06-02. p. 18. Archived from the original on 2008-10-24. Retrieved 2008-07-09.
- Kojo, Shosuke; Uchino, Hiromi; Yoshimura, Mayu; Tanaka, Kyoko (October 2004). "Racemic D,L-asparagine causes enantiomeric excess of other coexisting racemic D,L-amino acids during recrystallization: a hypothesis accounting for the origin of L-amino acids in the biosphere". Chemical Communications (19): 2146–2147. doi:10.1039/b409941a. PMID 15467844.
- Ruiz-Mirazo, Kepa; Briones, Carlos; de la Escosura, Andrés (2014). "Prebiotic Systems Chemistry: New Perspectives for the Origins of Life". Chemical Reviews. 114 (1): 285–366. doi:10.1021/cr2004844. PMID 24171674.
- "Early Earth atmosphere favorable to life: study". University of Waterloo. Archived from the original on 2005-12-14. Retrieved 2005-12-17.
- Fitzpatrick, Tony (2005). "Calculations favor reducing atmosphere for early earth – Was Miller–Urey experiment correct?". Washington University in St. Louis. Archived from the original on 2008-07-20. Retrieved 2005-12-17.
- Trail, Dustin; Watson, E. Bruce; Tailby, Nicholas D. (2011). "The oxidation state of Hadean magmas and implications for early Earth's atmosphere". Nature. 480 (7375): 79–82. Bibcode:2011Natur.480...79T. doi:10.1038/nature10655. PMID 22129728.
- Scaillet, Bruno; Gaillard, Fabrice (2011). "Earth science: Redox state of early magmas" (PDF). Nature. 480 (7375): 48–49. Bibcode:2011Natur.480...48S. CiteSeerX 10.1.1.659.2086. doi:10.1038/480048a. PMID 22129723. Archived (PDF) from the original on 2017-10-26.
- Nunn, JF (1998). "Evolution of the atmosphere". Proceedings of the Geologists' Association. Geologists' Association. 109 (1): 1–13. doi:10.1016/s0016-7878(98)80001-1. PMID 11543127.
- Raulin, F; Bossard, A (1984). "Organic syntheses in gas phase and chemical evolution in planetary atmospheres". Advances in Space Research. 4 (12): 75–82. Bibcode:1984AdSpR...4...75R. doi:10.1016/0273-1177(84)90547-7. PMID 11537798.
- Raulin, François; Brassé, Coralie; Poch, Olivier; Coll, Patrice (2012). "Prebiotic-like chemistry on Titan". Chemical Society Reviews. 41 (16): 5380–93. doi:10.1039/c2cs35014a. PMID 22481630.
- Thompson WR, Murray BG, Khare BN, Sagan C (December 1987). "Coloration and darkening of methane clathrate and other ices by charged particle irradiation: applications to the outer solar system". Journal of Geophysical Research. 92 (A13): 14933–47. Bibcode:1987JGR....9214933T. doi:10.1029/JA092iA13p14933. PMID 11542127.
- PIERAZZO, E.; CHYBA C.F. (2010). "Amino acid survival in large cometary impacts". Meteoritics & Planetary Science. 34 (6): 909–918. Bibcode:1999M&PS...34..909P. doi:10.1111/j.1945-5100.1999.tb01409.x.
- Brooks D.J.; Fresco J.R.; Lesk A.M.; Singh M. (October 1, 2002). "Evolution of amino acid frequencies in proteins over deep time: inferred order of introduction of amino acids into the genetic code". Molecular Biology and Evolution. 19 (10): 1645–55. doi:10.1093/oxfordjournals.molbev.a003988. PMID 12270892. Archived from the original on December 13, 2004.
- Johnson AP, Cleaves HJ, Dworkin JP, Glavin DP, Lazcano A, Bada JL (October 2008). "The Miller volcanic spark discharge experiment". Science. 322 (5900): 404. Bibcode:2008Sci...322..404J. doi:10.1126/science.1161527. PMID 18927386.
- "'Lost' Miller–Urey Experiment Created More Of Life's Building Blocks". Science Daily. October 17, 2008. Archived from the original on October 19, 2008. Retrieved 2008-10-18.
- Bernal JD (1949). "The physical basis of life". Proc. Phys. Soc. A. 62 (9): 537–558.
- "'How did life form from rocks?' Protein puzzle reveals secrets of Earth's evolution". RT. January 2017.
- Erastova V, Degiacomi MT, Fraser D, Greenwell HC (December 2017). "Mineral surface chemistry control for origin of prebiotic peptides". Nature Communications. 8 (1): 2033. doi:10.1038/s41467-017-02248-y. PMC 5725419. PMID 29229963.
- Balch, Erica (4 October 2018). "Ground-breaking lab poised to unlock the mystery of the origins of life on Earth and beyond". McMaster University. Retrieved 4 October 2018.
- Staff (4 October 2018). "Ground-breaking lab poised to unlock the mystery of the origins of life". EurekAlert!. Retrieved 14 October 2018.
- Staff (2018). "Planet Simulator". IntraVisionGroup.com. Retrieved 14 October 2018.
- Anderson, Paul Scott (14 October 2018). "New technology may help solve mystery of life's origins - How did life on Earth begin? A new technology, called Planet Simulator, might finally help solve the mystery". EarthSky. Retrieved 14 October 2018.
- Myers, P. Z. (October 16, 2008). "Old scientists never clean out their refrigerators". Pharyngula. Archived from the original on October 17, 2008. Retrieved 7 April 2016.
- Parker, ET; Cleaves, HJ; Dworkin, JP; et al. (February 14, 2011). "Primordial synthesis of amines and amino acids in a 1958 Miller H2S-rich spark discharge experiment". Proceedings of the National Academy of Sciences. 108 (14): 5526–31. Bibcode:2011PNAS..108.5526P. doi:10.1073/pnas.1019191108. PMC 3078417. PMID 21422282. Archived from the original on 3 March 2016. Retrieved 7 April 2016.
- A simulation of the Miller–Urey Experiment along with a video Interview with Stanley Miller by Scott Ellis from CalSpace (UCSD)
- Origin-Of-Life Chemistry Revisited: Reanalysis of famous spark-discharge experiments reveals a richer collection of amino acids were formed.[dead link]
- Miller–Urey experiment explained
- Miller experiment with Lego bricks
- "Stanley Miller's Experiment: Sparking the Building Blocks of Life" on PBS
- The Miller-Urey experiment website
- Cairns-Smith clay-crystal hypothesis of 1966
- Details of 2008 re-analysis