In 1953 (the year that Watson and Crick published their famous paper on DNA structure), Stanley Miller (1930–2007), a graduate student in the lab of Harold Urey (1893–1981), built an apparatus that mimicked what was then thought to be the atmosphere of early Earth, a reducing atmosphere containing methane, ammonia, and hydrogen. In the closed chamber, Miller boiled water, then exposed it to electric shocks and then cooled it. After the apparatus ran for a period of days, Miller tested the water and found several amino acids, which are the building blocks of proteins. Eventually, scientists replicating the Miller-Urey experiment were able to generate other types of amino acids as well as nucleotides (basic units of DNA) and sugars.