- Miller-Urey experiment
The Miller-Urey experiment (or Urey-Miller experiment) was an
experiment that simulated hypothetical conditions present on theearly Earth and tested for the occurrence ofchemical evolution . Specifically, the experiment tested Oparin and Haldane'shypothesis that conditions on the primitive Earth favored chemical reactions that synthesizedorganic compound s from inorganic precursors. Considered to be the classic experiment on theorigin of life , it was conducted in 1953 byStanley L. Miller andHarold C. Urey at theUniversity of Chicago . [cite journal |last=Miller |first=Stanley L. |url=http://www.issol.org/miller/miller1953.pdf |title=Production of Amino Acids Under Possible Primitive Earth Conditions|journal=Science |year=1953 |month=May |volume=117 |pages=528 |doi=10.1126/science.117.3046.528 |pmid=13056598] [cite journal |last=Miller |first=Stanley L. |coauthors=Harold C. Urey |title=Organic Compound Synthesis on the Primitive Earth |journal=Science |year=1959 |month=July |volume=130 |pages=245 |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.] [cite journal |title=The 1953 Stanley L. Miller Experiment: Fifty Years of Prebiotic Organic Chemistry |author=A. Lazcano, J. L. Bada |journal=Origins of Life and Evolution of Biospheres |volume=33 |year=2004 |month=June |pages=235–242 |doi=10.1023/A:1024807125069 |pmid=14515862]Experiment and interpretation
The experiment used
water (H2O),methane (CH4),ammonia (NH3) andhydrogen (H2). The chemicals were all sealed inside a sterile array of glass tubes and flasks connected together 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 induceevaporation , sparks were fired between the electrodes to simulatelightning through the atmosphere andwater vapor , 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
carbon within the system was now in the form of organic compounds. Two percent of the carbon had formedamino acids that are used to makeprotein s in living cells, withglycine 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 [http://www.accessexcellence.org/WN/NM/miller.php will yield 11] out of 20 amino acids."
As observed in all consequent experiments, both left-handed (L) and right-handed (D) optical isomers were created in a
racemic mixture.Other experiments
This experiment inspired many experiments in a similar vein. In 1961,
Joan Oró found that amino acids could be made fromhydrogen cyanide (HCN) and ammonia in a water solution. He also found that his experiment produced a large amount of thenucleotide baseadenine . Experiments conducted later showed that the otherRNA andDNA bases 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 inThe New York Times (March 8, 1953:E9), titled "Looking Back Two Billion Years" describes the work of Wollman (William) M. MacNevin atOhio 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. [cite journal |last=Wilde |first=Kenneth A. |authorlink= |coauthors=Bruno J. Zwolinski and Ransom B. Parlin |year=1953 |month=July |title=The Reaction Occurring in CO2, 2O Mixtures in a High-Frequency Electric Arc |journal=Science |volume=118 |issue=3054 |pages=43–44 |id= |url=http://www.sciencemag.org/cgi/content/citation/118/3054/43-a |accessdate=2008-07-09 |doi=10.1126/science.118.3054.43-a |pmid=13076175] Wilde only used voltages up to 600 V on a binary mixture of carbon dioxide and water in a flow system. He only observed small amounts of carbon dioxide reduction to carbon monoxide and no other significant reduction products or newly formed carbon compounds.
More recent experiments by chemist Jeffrey Bada at Scripps Institution of Oceanography, La Jolla, Calif. were similar to those performed by Miller. However, Bada noted that in current models of early Earth conditions carbon dioxide and nitrogen create
nitrite s, which destroy amino acids as fast as they form. However, the early Earth may have had significant amounts of iron andcarbonate 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. [Citation |last=Fox |first=Douglas |date=2007-03-28 |title=Primordial Soup's On: Scientists Repeat Evolution's Most Famous Experiment |periodical=Scientific American |series=History of Science |publisher=Scientific American Inc. |url=http://www.sciam.com/article.cfm?id=primordial-soup-urey-miller-evolution-experiment-repeated |accessdate=2008-07-09]In 2006 another experiment showed that a thick organic haze might have blanketed
Early Earth . [cite journal |last=Trainer |first=Melissa G. |coauthors=Alexander A. Pavlov, H. Langley DeWitt, Jose L. Jimenez, Christopher P. McKay, Owen B. Toon and Margaret A. Tolbert |year=2006 |month=November |title=Organic haze on Titan and the early Earth |journal=Proceedings of the National Academy of Sciences |volume=103 |issue=48 |pages=18035–18042 |url=http://www.pnas.org/content/103/48/18035.short |accessdate=2008-07-09 |doi=10.1073/pnas.0608561103] An organic haze can form over a wide range of methane and carbon dioxide concentrations, believed to be present in the atmosphere of Early Earth. After forming, these organic molecules would have floated down all over the Earth, allowing life to flourish globally. [Citation |last=Hayes |first=Jacqui |date=2006-11-07 |title=Hazy origins of life on Earth |periodical=Cosmos |publisher=Luna Media Pty Ltd |url=http://www.cosmosmagazine.com/node/829 |accessdate=2008-07-09]Earth's early atmosphere
Some evidence suggests that Earth's original atmosphere might have contained less of the reducing molecules than was thought at the time of Miller-Urey experiment. There is abundant evidence of major volcanic eruptions 4 billion years ago, which would have released carbon dioxide, nitrogen, hydrogen sulfide, and sulfur dioxide into the atmosphere. Experiments using these gases in addition to the ones in the original Miller-Urey experiment have produced more diverse molecules. Although the experiment created a mixture that was racemic (containing both L, D enantiomers), experiments since have shown that "when made from scratch in the lab the two versions are equally likely to appear, but in nature, L amino acids dominate." [Citation |date=
2006-06-02 |title=Right-handed amino acids were left behind |periodical=New Scientist |publisher=Reed Business Information Ltd |issue=2554 |pages=18 |url=http://www.newscientist.com/channel/life/mg19025545.200-righthanded-amino-acids-were-left-behind.html |accessdate=2008-07-09] Other experiments have confirmed disproportionate amounts of L or D oriented enantiomers are possible. [cite journal |last=Kojo |first=Shosuke |coauthors=Hiromi Uchino, Mayu Yoshimura and Kyoko Tanaka |year=2004 |month=October |title=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 |journal=Chemical Communications |volume= |issue=19 |pages=2146–2147 |pmid=15467844 |accessdate=2008-07-09 |doi=10.1039/b409941a]Originally it was thought that the primitive secondary atmosphere contained mostly NH3 and CH4. 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 H 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. [cite web |url=http://www.science.siu.edu/microbiology/micr425/425Notes/14-OriginLife.html |title=MICR 425: PHYSIOLOGY & BIOCHEMISTRY of MICROORGANISMS: The Origin of Life |accessdate=2005-12-17 |publisher=SIUC / College of Science]
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. [cite web |url=http://newsrelease.uwaterloo.ca/news.php?id=4348 |accessdate=2005-12-17 |title=Early Earth atmosphere favourable to life: study |publisher=University of Waterloo] 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. [cite web |url=http://news-info.wustl.edu/news/page/normal/5513.html |accessdate=2005-12-17 |title=Calculations favor reducing atmosphere for early earth - Was Miller-Urey experiment correct? |first=Tony |last=Fitzpatrick |publisher=Washington University in St. Louis |year=2005]
Although lightning storms are thought to have been very common in the
primordial atmosphere, they are not thought to have been as common as the amount ofelectricity used by the Miller-Urey experiment implied. These factors suggest that much lower concentrations of biochemicals would have been produced on Earth than was originally predicted (although the time scale would be 100 million years instead of a week). Similar experiments, both with different sources of energy and with different mixtures of gases, have resulted in amino andhydroxy acid s being produced; it is likely that at least some organic compounds would have been generated on the early Earth.Fact|date=October 2007However, when
oxygen gas is added to this mixture, no organic molecules are formed. Opponents of Miller-Urey hypothesis seized upon 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).cite journal |author=Rosing M.T. & Frei R. |url=http://www.geol.ku.dk/pershps/robertfrei/WEB/Rosing%20and%20Frei%20-%20EPSL-2004-1.pdf |title=U-rich Archaean sea-floor sediments from Greenland—indications of >3700 Ma oxygenic photosynthesis |journal=Earth and Planetary Science Letters |year=2004 |volume=217 |pages=237–244 |doi=10.1016/S0012-821X(03)00609-5] These opponents argue that this presence of oxygen precludes the formation of prebiotic molecules via a Miller-Urey-like scenario, attempting to invalidate the hypothesis ofabiogenesis . However, the authors of the paper are arguing that this presence of oxygen merely evidences the existence of photosynthetic organisms 3.7 Ga ago (a date about 200 Ma earlier than previous estimates [cite web |url=http://www.windows.ucar.edu/tour/link=/earth/past/oxygen_buildup.html |accessdate=2005-12-17 |title=The slow build up of Oxygen in the Earth's Atmosphere |publisher=University Corporation for Atmospheric Research |author=Windows to the Universe |year=1999] ) 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 system , often substitutingultraviolet light for lightning as the driving force for chemical reactions. TheMurchison meteorite that fell nearMurchison, Victoria ,Australia in 1969 was found to contain over 90 different amino acids, nineteen of which are found in Earth life.Comet s and other icy outer-solar-system bodies are thought to contain large amounts of complex carbon compounds (such astholin s) formed by these processes, in some cases so much so that the surfaces of these bodies are turned dark red or as black asasphalt .Fact|date=October 2007 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: thePanspermia hypothesis.Recent related studies
During 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 separatedspecies , assumed to share only thelast 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. [cite journal
author=Brooks D.J., Fresco J.R., Lesk A.M. & Singh M.
url = http://mbe.oupjournals.org/cgi/content/full/19/10/1645
title = Evolution of amino acid frequencies in proteins over deep time: inferred order of introduction of amino acids into the genetic code
journal=Molecular Biology and Evolution
year=2002 | volume=19 |pages=1645–55
pmid = 12270892]ee also
*
Origin of life References
External links
* [http://issol.org/miller/miller1953.pdf A Production of Amino Acids Under Possible Primitive Earth Conditions] by
Stanley L. Miller
* [http://millerureyexperiment.com A simulation of the Miller-Urey Experiment along with a video Interview with Stanley Miller] by Scott Ellis from CalSpace(UCSD)
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