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The Miller-Urey experiment was an experiment that simulated hypothetical conditions present on the early Earth in order to test what kind of environment would be needed to allow life to begin. 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. The experiment used water (H2O), methane (CH4), ammonia (NH3) and hydrogen (H2) - materials which were believed to represent the major components of the early Earth's atmosphere. The chemicals were all sealed and circulated 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 add water vapour to the chemical mixture and the resulting gases were circulated around the apparatus, simulating the Earth's atmosphere. The flask with heated water represents water on the Earth's surface and the recycled water vapor is just as water evaporates from lakes and seas, before going into the atmosphere and forming into rain. Sparks were fired between the electrodes to simulate lightning storms (believed to be common on the early earth) through the water vapors, and then the vapors were cooled again so that the water could condense (simulating the oceans) and trickle back into the first water flask in a continuous cycle. At the end of one week of continuous operation, Miller and Urey observed, by analyzing the cooled water, 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 amino acids, including 13 of the 22 that are used to make proteins in living cells, with glycine as the most abundant. The molecules produced were simple organic molecules, far from a complete living biochemical system, but the experiment established that the hypothetical processes could produce some building blocks of life without requiring life to synthesize them first.