The Miller-Urey experiment is a simulation of the process in which thunder and lightning in the reducing atmosphere of the primordial Earth produce organic compounds—particularly amino acids—thereby demonstrating the chemical evolution believed to have contributed to the origin of life.
Premises:
(1) The giant planets that are now distant from the Sun and may have undergone relatively little change over geological history—such as Jupiter and Saturn—all possess reducing atmospheres devoid of free oxygen (O₂). Their principal components are hydrogen (H₂), helium (He), methane (CH₄), and ammonia (NH₃). By analogy, the atmosphere of the primordial Earth was likely a similar reducing atmosphere.
(2) Measurements indicate that the energy sources currently capable of acting upon the Earth's atmosphere are primarily ultraviolet radiation from the Sun, lightning, and cosmic rays, among others. Of these, cosmic rays are insufficient to synthesize organic matter. Reducing gases absorb only short-wavelength ultraviolet radiation, and short-wavelength ultraviolet (wavelength < 1500 Å) constitutes only a minuscule fraction of the total solar ultraviolet output, leaving very little energy available for organic synthesis. Lightning, by contrast, occurs many times each year and provides a comparatively large reservoir of energy for organic synthesis. Moreover, lightning discharges near the ocean surface, meaning that products synthesized in the reducing atmosphere of the primordial Earth would dissolve readily into the primordial ocean.
Guided by these considerations, Miller carried out a laboratory experiment simulating thunder and lightning in the reducing atmosphere of the primordial Earth, to test whether organic compounds could be produced—particularly the small biological molecules that make up proteins and nucleic acids, such as amino acids, ribose, pyrimidines, and purines.
Experimental Results
The experiment yielded a total of twenty organic compounds (as shown in Table 1). Of the eleven amino acids produced, four—glycine, alanine, aspartic acid, and glutamic acid—are those found in the proteins of living organisms.
Subsequently, Miller concluded that a gas mixture of CH₄, N₂, trace amounts of NH₃, and H₂O would be a more reasonable approximation of the primordial Earth's reducing atmosphere, since ammonia could not have persisted in large quantities in the atmosphere—it would dissolve into seawater.
Accordingly, he and his collaborators performed spark-discharge experiments in 1972 using this revised gas mixture and obtained thirty-five organic compounds, including ten proteinogenic amino acids: glycine (440 μmol, same units throughout), alanine (790), valine (19.5), leucine (11.3), isoleucine (4.8), proline (1.5), aspartic acid (34), glutamic acid (7.7), serine (5.0), and threonine (~0.8).
If hydrolysis was carried out prior to analysis, asparagine and glutamine could also be generated. When hydrogen sulfide (H₂S) was added, methionine could be produced. Photolysis of a gas mixture containing CH₄, NH₃, H₂O, and H₂S yielded cysteine. Pyrolysis of CH₄ and other hydrocarbons at high temperatures produced phenylalanine, tyrosine, and tryptophan. To date, the Miller-Urey simulation experiment and other analogous experiments have been able to synthesize seventeen of the twenty naturally occurring amino acids. The remaining three—lysine, arginine, and histidine—are believed to be synthesizable in the near future with improved techniques.
From this experiment it can be demonstrated: the synthesis of small organic molecules from inorganic substances is entirely possible.