The **Miller–Urey experiment** (or **Urey–Miller experiment**) was an experiment that made organic compounds out of Inorganic compounds by applying a form of energy.

The idea was to simulate hypothetical conditions thought to be present on the early Earth (Hadean or early Archaean). It was a test of  the chemical origins of life. Specifically, the experiment tested Alexander Oparin's and J.B.S. Haldane's hypothesis that conditions on the primitive Earth favored chemical reactions that synthesized organic compounds from inorganic precursors. Considered to be the classic experiment on the origin of life, it was conducted in 1952 and published in 1953 by Stanley Miller and Harold Urey at the University of Chicago.

The experiment
After Miller's death in 2007, scientists examined sealed vials preserved from the original experiments. They were able to show that there were well over 20 different amino acids produced in Miller's original experiments. That is considerably more than those Miller originally reported, and more than the 20 that naturally occur in life.

History
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Miller graduated from the University of California at Berkeley in 1951 with a Bachelor of Science in chemistry. He then went to the University of Chicago to do graduate work. In University of Chicago, he got registered for a PhD program. Initially, he started work with the theoretical physicist Edward Teller on synthesis of elements. During this time, Miller attended a lecture given by Harold Urey on the subject of the origin of the solar system. Urey presented the idea of organic molecules being synthesized in a early Earth atmosphere. It inspired Miller very much. However, he kept working with Teller. After a year of fruitless work with Teller, and the prospect of Teller leaving Chicago to work on the Hydrogen bomb, Miller approached to Urey in September 1952 for a fresh research project. At first, Urey was reluctant to put a graduate student on such a risky project but finally he agreed.

When Miller showed his results to Urey, he suggested Miller to immediate publish the results. Urey refused to be the co-author as he felt that Miller did all the work and lest Miller receive no credit. Miller submitted the manuscript to Science on 10 February 1953. After weeks of silence, Urey inquired and wrote to the chair of the editorial board on 27 February about this. Then a month passed, but still there was no decision. As a reasult, on 10 March the infuriated Urey withdrew the manuscript and he himself submitted it to the Journal of the American Chemical Society on 13 March. By then, the editor of Science, apparently annoyed by Urey's insinuation, wrote directly to Miller that the manuscript was to be published. Miller accepted it and withdrew the manuscript from the Journal of the American Chemical Society.

The experiment
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For this experiment, Miller designed a sealed glass apparatus and used it to simulate the conditions on Earth before life appeared. It had two two glass flasks. One was of 500ml with 200 ml of water poured in it. The other was of 5 liter with a pair of electrodes. This one was filled with 100 mmHg of Hydrogen (H2, 200 mmHg of Methane (CH4), and 200 mmHg of Ammonia (NH3). To simulate rain, the water in the smaller flask was heated to create evaporation, and the water vapour was allowed to enter the larger flask and it mixed with the other gases. To simulate lightning in the water vapour and gaseous mixture, electrical sparks were fired between the electrodes. Then the water was cooled again and the water created rain. This process continued for a week.

After a day, the solution turned pink in colour, and after a week, the solution became deep red and turbid. Then, mercuric chloride was added to prevent microbial contamination. The reaction was stopped by adding barium hydroxide and sulfuric acid, and evaporated to remove impurities. When Miller analysed the solution, he found a lot of amino acids; one of the building blocks of life.

Recent related studies
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Moreover, some evidence suggests that Earth's original atmosphere might have had a different composition than the gas used in the Miller–Urey experiment. There is abundant evidence of major volcanic eruptions 4 billion years ago, which would have released carbon dioxide, nitrogen, hydrogen sulfide (H2S), and sulfur dioxide (SO2) into the atmosphere. Experiments using these gases in addition to the ones in the original Miller–Urey experiment have produced more diverse molecules.

Note that organic compounds, including amino acids, are present in the meteorite dust which rains down on Earth. Therefore, interest in abiogenesis and the origin of life has shifted to the question of how cells and large macromolecules formed.