Passage: The Miller-Urey experiment, conducted in 1952, attempted to simulate the conditions of the early Earth's atmosphere to test the feasibility of abiogenesis, the origin of life from non-living matter. By subjecting a mixture of water, methane, ammonia, and hydrogen to electrical sparks (representing lightning), the researchers were able to produce several amino acids, the building blocks of proteins. This result suggested that the complex organic molecules necessary for life could form spontaneously under prebiotic conditions. However, later geochemical evidence suggested that the early atmosphere was likely not as reducing as Miller and Urey assumed; instead of being rich in methane and ammonia, it may have been composed primarily of carbon dioxide and nitrogen. Experiments using this revised atmospheric composition have yielded fewer organic compounds, leading some scientists to look toward alternative environments, such as alkaline hydrothermal vents or comet impacts, as the potential sites for the synthesis of the first biomolecules.