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Miller experiment

The Urey-Miller experiment was an experiment that simulated hypothetical conditions present on the early Earth and tested for the occurrence of chemical evolution (the Oparin[?] and Haldane[?] hypothesis stated that conditions on the primitive Earth favored chemical reactions that synthesized organic compounds from inorganic precursors; the Urey-Miller tested this hypothesis). The experiment is considered to be the classic experiment on the origin of life. It was conducted in 1953 by Stanley L. Miller[?] and Harold C. Urey[?] at the University of Chicago.

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The experiment

the experiment used water (H2O), methane (CH4), ammonia (NH3) and hydrogen (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 induce evaporation, sparks were fired through the atmosphere and water vapor to simulate lightning, 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, Urey and Miller 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 formed some of the amino acids which are used to make proteins in living cells.


The molecues produced were relatively simple organic molecules, far from a complete living biochemical system, but the experiment established that natural processes could produce the building blocks of life without requiring life to synthesize them in the first place.

Other experiments

This experiment inspired many similar experiments in a similar vein. In 1961, Juan Oro[?] found that amino acids could be made from hydrogen cyanide (HCN) and ammonia in a water solution. He also found that his experiment produced a large amount of the nucleotide base adenine. Experiments conducted later showed that the other RNA and DNA bases could be obtained through simulated prebiotic chemistry with a reducing atmosphere.


There have been a number of objections to the implications derived from these experiments. It is now believed that Earth's original atmosphere did not contain as large a quantity of reducing molecules as were thought at the time (the composition of the early atmosphere is now thought to be a mixture of H2O, CO, CO2, N2, H2S, CH4, NH3 and possible traces of H2). Also, although lightning storms are thought to have been common in the primordial atmosphere, they are not thought to have been as common as the amount of electricity used by the Urey-Miller experiment implies. These factors suggest that much lower concentrations of biochemicals would have been produced on Earth than was originally predicted. Similar experiments, both with different sources of energy and with different mixtures of gases, have resulted in amino and hydroxy acids[?] being produced; it is likely that at least some organic compounds would have been generated on the early Earth. However, as soon as oxygen gas is added to the mixture, no organic molecules are formed.

Conditions similar to those of the Urey-Miller experiments are present in other regions of the solar system, often substituting ultraviolet light for lightning as the driving force for chemical reactions. On September 28, 1969, a meteorite that fell over Murchison[?], Australia 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 formed by these processes, in some cases so much so that the surfaces of these bodies are turned dark red or as black as asphalt. 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.

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