Miller-Urey experiment
The Miller-Urey experiment is an experiment from 1953 that tried to form organic material using assumed early earth conditions in earth's atmosphere. Some scientists relate it to abiogenesis – the production of life from non-life, but it is generally considered just an attempt to find out a potential scenario of how life, or organic material for life, could have emerged. By recent analysis, the experiment did produce quite a few amino acids, but not many of the ones used for making proteins in a cell or in any significant chemically usable quantity. Furthermore, it produced those acids in a racemic mixture (left and right handed amino acids). Life uses specific chirality (left handed amino acids only). Even secular scientists are not willing to accept this as the definitive proof of how life began.
Contents
The 1953 experiment
In the 1920s, Russian scientist A. I. Oparin and British scientist J. B. S. Haldane suggested that the Earth’s primitive atmosphere had "reducing" (oxygen poor) gases consisted mainly of methane (CH4), ammonia (NH3), hydrogen (H2), and water vapor (H2O).[1] In 1953, Stanley Miller performed an experiment with those exact same reducing gases and reported the production of biomolecules from simple gaseous starting materials using an apparatus constructed to simulate the primordial Earth's atmosphere-ocean system. Miller used what he thought was primitive gases that would be available on a primitive earth: 200 mmHg of CH4, 100 mmHg of H2, 200 mmHg of NH3 and 200 ml of H2O into the equipment. In other words the ratio was 2:1:2:2 for CH4:H2:NH3:H2O. Then he subjected this mixture, under reflux, to an electric discharge simulating lightning for a week while the water was simultaneously heated for that time period.[2]
Results
In his original paper, Miller was able to identify some amino acids glycine, α-alanine and β-alanine while Aspartic acid and α-aminobutyric acid were less certain.[3] Miller also stated "it is estimated that the total yield of amino acids was in the milligram range" which means that very little material formed after a week of continuously running of the experiment non-stop.[3]
Re-analysis of samples and other discharge experiments
Miller had performed other experiments between 1952-1954 using two other apparatuses. A few vials of the samples he made were found in the 2000s and analyzed using modern techniques that have more sensitivity.[4][5] After modern analysis of his 1950s samples (including the 1953 experiment and volcano experiment), 31 amino acids, 6 amines, and 12 dipeptides/cyclodipeptides formed in very small amounts.[4]
NOTE: moles in Table are relative to glycine = 1 mol [where 4.8 mg was the amount of glycine synthesized. The amount of carbon was 710 mg from methane][4]
| Amino Acid | Miller-Urey Experiment 1953[3] | Bada 2016 Re-evaluation of 1953 samples[4]
(moles) |
|---|---|---|
| glycine | Detected | 1.0 |
| α-alanine | Detected | 0.54 |
| β-alanine | Detected | 0.24 |
| Aspartic acid | Unsure but possible | 0.006 |
| α-aminobutyric acid | Unsure but possible | 0.08 |
| a-aminoisobutyric acid | N/A | 0.002 |
| Glycolic acid | N/A | 0.89 |
| Sarcosine (N-methylglycine) | N/A | 0.08 |
| Lactic acid | N/A | 0.49 |
| N-Methylalanine | N/A | 0.02 |
| a-Hydroxybutyric acid | N/A | 0.08 |
| Succinic acid | N/A | 0.06 |
| Glutamic acid | N/A | 0.01 |
| Iminodiacetic acid | N/A | 0.09 |
| Iminoacetic-propionic acid | N/A | 0.02 |
| Formic acid | N/A | 3.70 |
| Acetic acid | N/A | 0.24 |
| Propionic acid | N/A | 0.21 |
| Urea | N/A | 0.03 |
Modern Interpretations
Early earth atmosphere
There is significant uncertainty to what the primitive atmosphere was composed of.[5] The Miller-Urey experiment assumed an a primitive atmosphere was composed of reducing (oxygen poor) gases [e.g. methane (CH4), ammonia (NH3), hydrogen (H2), and water vapor (H2O)], but by the 1980s geochemists believed that early earth atmospheres probably had neutral gases like those emitted by volcanoes [e.g. water vapor (H2O), carbon dioxide (CO2), and nitrogen (N2)] and perhaps some reducing gases like carbon monoxide (CO). Since hydrogen was a light element, there probably would not have been any or perhaps very little in early earth.[1][5] A possible source of lightning could have been "volcanic lightning", which occurs sometimes near volcanic eruption plumes, and some volcanic gases emitted have been hydrogen sulfide (H2S), ammonia (NH3), methane (CH4), and carbon dioxide (CO2).[5] Some scientists believe that early earth atmosphere was similar to earth today.[6]
Chemical yield of amino acids
Amino acid yield in spark discharge experiments depends critically on the pH of the aqueous phase since at lower pH, amino acid production is greatly reduced and hydroxy acids in greatly increased.[5] Hydrothermal vents have been proposed as an alternative prebiotic chemistry source but they face severe limitations and actual synthesis have never been demonstrated using plausible geochemical conditions.[5]
Limited time frames for prebiotic synthesis
Scientists believe that there was a time limit for meteorites for form simple to slightly more complex compounds: greater than 1,000 to less than 1,000,000 years.[5] This poses a problem for hydrothermal vents since they have a very short life span of 100-10,000 years.[5] Leading Miller-Urey experiment researcher Jeffrey Bada admits:[5]
| “ | "An important issue is whether these timescales are compatible with the time required for the transition from abiotic to biotic chemistry. As noted, in carbonaceous meteorites the timescale for amino acid production is between >103 to <106 years, perhaps even as short as 1–10 years, and there is no evidence that the prebiotic chemistry that took place on the meteorite parent bodies during this aqueous alteration period produced anything beyond simple monomeric compounds. It has been suggested that catalysts such as metal sulfides present in vent systems would promote more rapid synthesis, but by the principle of microscopic reversibility, these must also catalyze the decomposition of any synthesized compounds. Moreover, if this was the case there are certainly potential metal catalysts present on meteorite parent bodies and, as has been indicated, nothing beyond simple molecules were apparently synthesized." | ” |
See also
References
- ↑ 1.0 1.1 Jonathan Wells. 2017. Zombie Science: More Icons of Evolution. ISBN: 978-1936599448. 3. Survival of the Fakest
- ↑ Parker, E. T., Cleaves, J. H., Burton, A. S., Glavin, D. P., Dworkin, J. P., Zhou, M., Bada, J. L., Fernández, F. M. Conducting Miller-Urey Experiments. J. Vis. Exp. (83), e51039, doi:10.3791/51039 (2014).
- ↑ 3.0 3.1 3.2 Stanley Miller. 1953. "Production of Amino Acids Under Possible Primitive Earth Conditions". Science. 117 (3046): 528–9.
- ↑ 4.0 4.1 4.2 4.3 Bada, J. L. (2016). One of the foremost experiments of the 20th century: Stanley Miller and the origin of prebiotic chemistry. Metode Science Studies Journal, (6), 183–189. https://doi.org/10.7203/metode.6.4994
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 Bada JL. New insights into prebiotic chemistry from Stanley Miller's spark discharge experiments. Chem Soc Rev. 2013 Mar 7;42(5):2186-96. doi: 10.1039/c3cs35433d
- ↑ Earth's Early Atmosphere: An Update - NASA
