Miller–Urey experiment

The Miller–Urey experiment^[1] (or Miller experiment^[2]) was an experiment in chemical synthesis 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 compounds from inorganic constituents in an origin of life scenario. The experiment used methane (CH₄), ammonia (NH₃), hydrogen (H₂), in ratio 2:2:1, and water (H₂O). Applying an electric arc (the latter simulating lightning) resulted in the production of amino acids.

It is regarded as a groundbreaking experiment, and the classic experiment investigating the origin of life (abiogenesis). It was performed in 1952 by Stanley Miller, supervised by Nobel laureate Harold Urey at the University of Chicago, 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.^[3][4][5]

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.^[6] While evidence suggests that Earth's prebiotic atmosphere might have typically had a composition different from the gas used in the Miller experiment, prebiotic experiments continue to produce racemic mixtures of simple-to-complex organic compounds, including amino acids, under varying conditions.^[7] Moreover, researchers have shown that transient, hydrogen-rich atmospheres – conducive to Miller-Urey synthesis – would have occurred after large asteroid impacts on early Earth.^[8][9]

History

Foundations of organic synthesis and the origin of life

Until the 19th century, there was considerable acceptance of the theory of spontaneous generation, the idea that "lower" animals, such as insects or rodents, arose from decaying matter.^[10] However, several experiments in the 19th century – particularly Louis Pasteur's swan neck flask experiment in 1859^[11] — disproved the theory that life arose from decaying matter. Charles Darwin published On the Origin of Species that same year, describing the mechanism of biological evolution.^[12] While Darwin never publicly wrote about the first organism in his theory of evolution, in a letter to Joseph Dalton Hooker, 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 [...]"^[13]

At this point, it was known that organic molecules could be formed from inorganic starting materials, as

But the scientific literature of the early 20th century contained speculations on the origin of life.[15]^[17] In 1903, physicist Svante Arrhenius hypothesized that the first microscopic forms of life, driven by the radiation pressure of stars, could have arrived on Earth from space in the panspermia hypothesis.^[18] In the 1920s, Leonard Troland wrote about a primordial enzyme that could have formed by chance in the primitive ocean and catalyzed reactions, and Hermann J. Muller suggested that the formation of a gene with catalytic and autoreplicative properties could have set evolution in motion.^[19] Around the same time, Alexander Oparin's and J. B. S. Haldane's "Primordial soup" 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.^[20] 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

Stanley Miller arrived at the University of Chicago in 1951 to pursue a PhD under nuclear physicist Edward Teller, another prominent figure in the Manhattan Project.^[24] Miller began to work on how different chemical elements 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 and further nuclear weapons research.^[24] 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.^[24] By February 1953, Miller had mailed a manuscript as sole author reporting the results of his experiment to Science.^[25] 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.^[25][26] 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."^[25] Miller's manuscript was eventually published in Science in May 1953.^[25]

Experiment

In the original 1952 experiment, methane (CH₄), ammonia (NH₃), and hydrogen (H₂) were all sealed together in a 2:2:1 ratio (1 part H₂) inside a sterile 5-L glass flask connected to a 500-mL flask half-full of water (H₂O). 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 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

For example, here is a set photochemical reactions of species in the Miller-Urey atmosphere that can result in formaldehyde:^[31]

H₂O + hv → H + OH^[34]

CH₄ + OH → CH₃ + HOH^[35]

CH₃ + OH → CH₃OH^[36]

CH₃OH + hv → CH₂O (formaldehyde) + H₂^[37]

A photochemical path to HCN from NH₃ and CH₄ is:^[39]

NH₃ + hv → NH₂ + H

NH₂ + CH₄ → NH₃ + CH₃

NH₂ + CH₃ → CH₅N

CH₅N + hv → HCN + 2H₂

Other active intermediate compounds (acetylene, cyanoacetylene, etc.) have been detected in the aqueous solution of Miller–Urey-type experiments,^[40] 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.^[29]

Strecker synthesis describes the reaction of an aldehyde, ammonia, and HCN to a simple amino acid through an aminoacetonitrile intermediate:

CH₂O + HCN + NH₃ → NH₂-CH₂-CN (aminoacetonitrile) + H₂O

NH₂-CH₂-CN + 2H₂O → NH₃ + NH₂-CH₂-COOH (glycine)

Furthermore, water and formaldehyde can react via Butlerov's reaction to produce various sugars like ribose.^[41]

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 (March 8, 1953) titled "Looking Back Two Billion Years" describes the work of Wollman M. MacNevin at Ohio State University, before the Miller Science paper was published in May 1953. MacNevin was passing 100,000V sparks through methane and water vapor and produced "resinous solids" that were "too complex for analysis."^[25][42][43] 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 (CO₂) and water in a flow system and did not note any significant reduction products.^[44] 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.^[25]

By introducing an experimental framework to test prebiotic chemistry, the Miller–Urey experiment paved the way for future origin of life research.

In 1999, after Miller suffered a stroke, he donated the contents of his laboratory to Bada.[27] In an old cardboard box, Bada discovered unanalyzed samples from modified experiments that Miller had conducted in the 1950s.^[27] In a "volcanic" apparatus, Miller had amended an aspirating nozzle to shoot a jet of steam into the reaction chamber.^[7][55] Using high-performance liquid chromatography and mass spectrometry, 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.^[7][55] 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.^[7][55] In a separate set of experiments, Miller added hydrogen sulfide (H₂S) to the reducing atmosphere, and Bada's analyses of the products suggested order-of-magnitude higher yields, including some amino acids with sulfur moieties.^[7][56]

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.^[57]

After comparing Miller–Urey experiments conducted in borosilicate glassware with those conducted in Teflon apparatuses, a 2021 paper suggests that the glass reaction vessel acts as a mineral catalyst, implicating silicate rocks as important surfaces in prebiotic Miller-Urey reactions.^[58]

Early Earth's prebiotic atmosphere

See also:

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.^[60]

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.[7]^[53][54] Furthermore, harkening back to Urey's original hypothesis of a "post-impact" reducing atmosphere,^[23] a recent atmospheric modeling study has shown that an iron-rich impactor with a minimum mass around 4×10²⁰ – 5×10²¹ kg would be enough to transiently reduce the entire prebiotic atmosphere, resulting in a Miller-Urey-esque H₂-, CH₄-, and NH₃-dominated atmosphere that persists for millions of years.^[9] 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.^[8][9][62]

CC-BY 4.0

.

A large factor controlling the redox budget of early Earth's atmosphere is the rate of atmospheric escape of H₂ after Earth's formation. Atmospheric escape – common to young, rocky planets — occurs when gases in the atmosphere have sufficient kinetic energy to overcome gravitational energy.^[63] It is generally accepted that the timescale of hydrogen escape is short enough such that H₂ made up < 1% of the atmosphere of prebiotic Earth,^[60] 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 of 30%.^[64] A hydrogen-rich prebiotic atmosphere would have large implications for Miller-Urey synthesis in the Hadean and Archean, but later work suggests solutions in that model might have violated conservation of mass and energy.^[63][65] That said, during hydrodynamic escape, lighter molecules like hydrogen can "drag" heavier molecules with them through collisions, and recent modeling of xenon escape has pointed to a hydrogen atmospheric mixing ratio of at least 1% or higher at times during the Archean.^[66]

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.^[9][23][60]

Extraterrestrial sources of amino acids

Conditions similar to those of the Miller–Urey experiments are present in other regions of the

ppm) concentrations.^[71][72]

In this way, the organic composition of the Murchison meteorite is seen as evidence of Miller-Urey synthesis outside Earth.

Comets 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.^[52][73] 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,^[74][75] however very low concentrations of biologically-relevant material combined with uncertainty surrounding the survival of organic matter upon impact make this difficult to determine.^[15]

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 (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.^[76] Thus, while creationist arguments focus on the fact that Miller–Urey experiments have not generated all 22 genetically-encoded amino acids,^[77] this does not actually conflict with the evolutionary perspective on the origin of life.^[76]

CC-BY 4.0

.

Another common misconception is that the racemic (containing both L and D enantiomers) mixture of amino acids produced in a Miller–Urey experiment is also problematic for abiogenesis theories,^[77] as life on Earth today uses L-amino acids.^[79] While it is true that Miller-Urey setups produce racemic mixtures,^[80] the origin of homochirality is a separate area in origin of life research.^[81]

Recent work demonstrates that

enantioselective crystallization of chiral molecules, including RNA precursors, due to the chiral-induced spin selectivity (CISS) effect.^[82][83] Once an enantioselective bias is introduced, homochirality can then propagate through biological systems in various ways.^[84]

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 monomers; polymerization of these building blocks to form peptides and other more complex structures is the next step of prebiotic chemistry schemes.^[85] Polymerization requires condensation reactions, which are thermodynamically unfavored in aqueous solutions because they expel water molecules.^[86] Scientists as far back as John Desmond Bernal in the late 1940s thus speculated that clay surfaces would play a large role in abiogenesis, as they might concentrate monomers.^[87] Several such models for mineral-mediated polymerization have emerged, such as the interlayers of layered double hydroxides like green rust over wet-dry cycles.^[88] Some scenarios for peptide formation have been proposed that are even compatible with aqueous solutions, such as the hydrophobic air-water interface^[86] and a novel "sulfide-mediated α-aminonitrile ligation" scheme, where amino acid precursors come together to form peptides.^[89] Polymerization of life's building blocks is an active area of research in prebiotic chemistry.

Amino acids identified

See also: Category:Chemical synthesis of amino acids

Below is a table of amino acids produced and identified in the "classic" 1952 experiment, as analyzed by Miller in 1952^[3] and more recently by Bada and collaborators with modern mass spectrometry,^[7] the 2008 re-analysis of vials from the volcanic spark discharge experiment,^[7][55] and the 2010 re-analysis of vials from the H₂S-rich spark discharge experiment.^[7][56] 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.^[76]

Amino acid	Produced in experiment				Proteinogenic
	Miller–Urey (1952)	Bada Reanalysis of 1950s product (2008–)	Volcanic spark discharge (2008)	H2S-rich spark discharge (2010)
Glycine					Yes
α-Alanine					Yes
β-Alanine					No
Aspartic acid					Yes
α-Aminobutyric acid					No
Serine					Yes
Isoserine					No
α-Aminoisobutyric acid					No
β-Aminoisobutyric acid					No
β-Aminobutyric acid					No
γ-Aminobutyric acid					No
Valine					Yes
Isovaline					No
Glutamic acid					Yes
Norvaline					No
Methylamine					No
Ethylamine					No
Ethanolamine					No
Isopropylamine					No
n-Propylamine					No
α-Aminoadipic acid					No
Homoserine					No
2-Methylserine					No
β-Hydroxyaspartic acid					No
Ornithine					No
2-Methylglutamic acid					No
Phenylalanine					Yes
Homocysteic acid					No
S-Methylcysteine					No
Methionine					Yes
Methionine sulfoxide					No
Methionine sulfone					No
Isoleucine					Yes
Leucine					Yes
Ethionine					No
Cysteine					Yes
Histidine					Yes
Lysine					Yes
Asparagine					Yes
Pyrrolysine					Yes
Proline					Yes
Glutamine					Yes
Arginine					Yes
Threonine					Yes
Selenocysteine					Yes
Tryptophan					Yes
Tyrosine					Yes

References

1. PMID 14577878
   .
2. S2CID 123124418
   .
3. ^
   PMID 13056598. Archived from the original
   (PDF) on 2012-03-17. Retrieved 2011-01-17.
4. PMID 13668555
   . Miller states that he made "A more complete analysis of the products" in the 1953 experiment, listing additional results.
5. S2CID 19515024
   .
6. ^ "The Spark of Life". BBC Four. 26 August 2009. Archived from the original on 2010-11-13. TV Documentary.{{cite web}}: CS1 maint: postscript (link)
7. ^
   S2CID 12230177
   .
8. ^
   ISSN 2632-3338
   .
9. ^
   ISSN 2632-3338
   .
10. S2CID 44194609
    .
11. ^ "Pasteur's "col de cygnet" (1859) | British Society for Immunology". 2022-05-27. Archived from the original on 2022-05-27. Retrieved 2023-11-11.
12. ^ Darwin, Charles (1859). On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. London: John Murray.
13. ^ Darwin, Charles. Darwin Correspondence Project, "Letter No. 7471", 1871. Available online: http://www.darwinproject.ac.uk/DCP-LETT-7471
14. ^ Friedrich Wöhler (1828). "Ueber künstliche Bildung des Harnstoffs". Annalen der Physik und Chemie. 88 (2): 253–256
15. ^ ^{a b c d} Miller, S. L., & Cleaves, H. J. (2006). Prebiotic chemistry on the primitive Earth. Systems biology, 1, 1.
16. ^ Löb, W. (1913). Über das Verhalten des Formamids unter der Wirkung der stillen Entladung Ein Beitrag zur Frage der Stickstoff‐Assimilation. Berichte der deutschen chemischen Gesellschaft, 46(1), 684–697.
17. ISBN 978-0-520-23391-1
    .
18. ^ Arrhenius, Svante (1903). "Die Verbreitung des Lebens im Weltenraum" [The Distribution of Life in Space]. Die Umschau.
19. PMID 20534710
    .
20. PMID 33081930
21. ^ Harold C. Urey – Biographical. NobelPrize.org. Nobel Prize Outreach AB 2023. Mon. 13 Nov 2023. https://www.nobelprize.org/prizes/chemistry/1934/urey/biographical/
22. ^ "Harold C. Urey". www.nndb.com. Retrieved 2023-11-13.
23. ^
    PMID 16589104
    .
24. ^
    S2CID 1167340
    .
25. ^
    ISSN 1573-0875
    .
26. New Scientist Magazine
    . Australia: New Scientist.
27. ^ ^{a b c d} 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.
28. ^ "Astrobiology Collection: Miller-Urey Apparatus". Denver Museum of Nature & Science. Archived from the original on 2013-05-24.
29. ^
    ISSN 0006-3002
    .
30. ^
    ISBN 978-0-521-82421-7
    .
31. ^
    PMID 1133344
    .
32. ISSN 1538-4365
    .
33. ISSN 0021-9584
    .
34. S2CID 97474816
    .
35. ISSN 0047-2689
    .
36. ISSN 0538-8066
    .
37. ISSN 0021-7689
    .
38. ISSN 1936-6434
    .
39. ISSN 0004-637X
    .
40. S2CID 4998122
    .
41. ^ Mense, Thorben H. (December 2019). "A Closer Look at Reactions in the Miller-Urey-Experiment using Coupled Gas Chromatography – Mass Spectrometry" (PDF). Bielefeld University.
42. Springer-Verlag
    . p. 603.
43. ^ "Looking Back Two Billion Years". The New York Times. Retrieved 2023-11-14.
44. S2CID 11170339
    .
45. ISBN 978-1-78801-749-7
    .
46. PMID 13731263
    .
47. S2CID 4219284
    .
48. ^ Oró J (1967). Fox SW (ed.). Origins of Prebiological Systems and of Their Molecular Matrices. New York Academic Press. p. 137.
49. PMID 28396441
    .
50. ISSN 0301-9268
    .
51. S2CID 33972427
    .
52. ^
    S2CID 4261076
    .
53. ^
    ISSN 0273-1177
    .
54. ^
    S2CID 7731172
    .
55. ^
    S2CID 10134423
    .
56. ^
    PMID 21422282
    .
57. doi:10.3390/molecules26123663
    .
58. PMC 8545935
    .
59. PMID 20573713
    .
60. ^
    ISBN 978-0-521-84412-3
    .
61. doi:10.1130/spe62-p631
    , retrieved 2023-11-15
62. S2CID 205239647
    .
63. ^
    doi:10.1017/9781139020558.006
64. S2CID 262262244
    .
65. ISSN 0012-821X
    .
66. ISSN 0016-7037
    .
67. PMID 11543127
    .
68. PMID 11537798
    .
69. PMID 22481630
    .
70. PMID 20160129
    .
71. PMID 11541223
    .
72. PMC 5428853
    .
73. PMID 11542127
    .
74. S2CID 97334519
    .
75. S2CID 4346044
    .
76. ^
    PMID 12270892. Archived from the original
    on December 13, 2004.
77. ^ ^{a b} "Why the Miller Urey research argues against abiogenesis". creation.com. Retrieved 2023-11-15.
78. PMID 35787048
    .
79. ^ Nelson, D. L., & Cox, M. M. (2017). Lehninger principles of biochemistry (7th ed.). W.H. Freeman.
80. PMID 24473135
    .
81. PMC 6396334
    .
82. PMID 37285423
    .
83. ISSN 2041-1723
    .
84. PMID 37551802
    .
85. S2CID 199541746
    .
86. ^
    PMID 22927374
    .
87. doi:10.1007/978-3-642-11274-4_158
    ]]
88. PMID 29229963
    .
89. S2CID 195873596
    .

External links

• 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.
• 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, A.G. (1966). "The origin of life and the nature of the primitive gene". Journal of Theoretical Biology. 10 (1): 53–88.
  PMID 5964688
  .
• Details of 2008 re-analysis