The University of Chicago scientists Stanley L. Miller and Harold C. Urey performed an experiment in 1953 that had far-reaching implications for the study of life's origins. Three important outcomes of the experiment were (1) the discovery of new chemical processes, (2) the instigation of further research and (3) the beginning of the science of exobiology -- the study of extraterrestrial life.
In the early 1950s, scientific interest in the origins of life was prominent and led to an investigation by Miller and Urey into how the chemical building blocks of life might have formed on early Earth. In the experiment, Miller boiled methane, ammonia, hydrogen, nitrogen, carbon dioxide and water -- chemicals he believe to be present on early Earth -- in a flask attached to spark chamber that was hooked to an electrical power supply. The sparks simulated lightning storms at what the experimenters believed to be the proper intensity to match early conditions. Miller and Urey ran the closed experiment for a week.
The experimenters ran the liquid solution through a chromatograph -- an apparatus for separating mixtures -- and found that 10 to 15 percent of the carbon in the flask had taken the form of organic compounds not present at the start of the experiment. They found that 2 percent of the carbon had formed amino acids, the molecules that make up proteins. The importance of this finding was in demonstrating that the chemical components of life could be made in the environment assumed to exist on early Earth. It was, in short, a new theory about the chemical origins of life.
Other experimenters expanded on the work of Miller and Urey. For example, in 1961, the University of Barcelona biochemist Juan Oro synthesized the nucleotide base adenine, a component of DNA, from a solution of hydrogen cyanide, ammonia and water. However, excitement gave way to a measure of skepticism as scientists questioned some of Miller and Urey's assumptions about the composition of the early atmosphere and the frequency of electrical storms during that period. This is an important example of how the scientific method tests, refines and corrects theories based upon new information.
Dawn of Exobiology
The results from the Miller-Urey experiment took an important turn in 1969, when a meteorite landed in Murchison, Australia, containing a rich supply of over 90 amino acids, of which 19 are present on Earth. Miller, and others, realized that even if the conditions set up in his experiment didn't accurately reflect early Earth's environment, they could have mimicked conditions on another world from which the meteorite originated. This was evidence for hypothesizing that meteors brought the building blocks of life to Earth from outer space. Furthermore, exobiologists ask whether the same process may have sparked life throughout the universe. This remains one of the most important unanswered questions in current scientific research.