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. Specifically, the experiment tested Alexander Oparin's and J. B. S. Haldane's hypothesis that 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, under the supervision of 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. There is abundant evidence of major volcanic eruptions 4 billion years ago, which would have released carbon dioxide (CO2), nitrogen (N2), hydrogen sulfide (H2S), and sulfur dioxide (SO2) into the atmosphere. Experiments using these gases in addition to the ones in the original Miller experiment have produced more diverse molecules. More-recent evidence suggests that Earth's original atmosphere might have had a different composition 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 liquid water. The liquid 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. Paper chromatography revealed the presence of glycine, α- and β-alanine. Miller could not ascertain aspartic acid and GABA, due to faint spots.
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 UCSD, at the University of California, San Diego, 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
- 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 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 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. However, the early Earth may have had significant amounts of iron and carbonate minerals 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 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.
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.
Professor 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. Professor 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. Interestingly, 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.
- 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: 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 (Netherlands: Kluwer Academic Publishers) 30: 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.
- 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.
- BBC: The Spark of Life. TV Documentary, BBC 4, 26 August 2009.
- 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". Origins of Life and Evolution of Biospheres 38 (2): 105–115. doi:10.1007/s11084-007-9120-3. PMID 18204914. pdf
- Bada, Jeffrey L. (2013). "New insights into prebiotic chemistry from Stanley Miller's spark discharge experiments". Chemical Society Reviews 42 (5): 2186. 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. Retrieved 2009-08-20.
- Dreifus, Claudia (2010-05-17). "A Conversation With Jeffrey L. Bada: A Marine Chemist Studies How Life Began". nytimes.com.
- "Astrobiology Collection: Miller-Urey Apparatus".
- http://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
- http://books.nap.edu/openbook.php?record_id=11860&page=85 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: 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. Retrieved 2008-07-09.
- Synthesis of organic compounds from carbon monoxide and water by UV photolysis
- 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. 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. Retrieved 2005-12-17.
- Fitzpatrick, Tony (2005). "Calculations favor reducing atmosphere for early earth – Was Miller–Urey experiment correct?". Washington University in St. Louis. Retrieved 2005-12-17.
- Nunn, JF (1998). "Evolution of the atmosphere". Proceedings of the Geologists' Association. Geologists' Association 109 (1): 1–13. PMID 11543127.
- Raulin, F; Bossard, A (1984). "Organic syntheses in gas phase and chemical evolution in planetary atmospheres.". Advances in Sspace Research 4 (12): 75–82. PMID 11537798.
- Raulin, François; Brassé, Coralie; Poch, Olivier; Coll, Patrice (2012). "Prebiotic-like chemistry on Titan". Chemical Society Reviews 41 (16): 5380. 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. doi:10.1111/j.1945-5100.1999.tb01409.x. Retrieved 25 September 2013.
- 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.
- 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. Retrieved 2008-10-18.
- Miller, Stanley L. (May 15, 1953). "A Production of Amino Acids Under Possible Primitive Earth Conditions" (PDF). Science 117. pp. 528–529. Archived from the original (PDF) on 2009-02-05.
- 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.
- http://www.chem.duke.edu/~jds/cruise_chem/Exobiology/miller.html - Miller–Urey experiment explained.
- http://www.althofer.de/miller-experiment-with-lego.html - Miller experiment with Lego bricks.
- Long-Neglected Experiment Gives New Clues to Origin of Life
- Primordial synthesis of amines and amino acids in a 1958 Miller H2S-rich spark discharge experiment