This is one of my favorite experiments, quite possibly because it's one of the few that I can remember easily. The purpose of this experiment was to figure out how the macromolecules came to be on this earth. There are four macromolecules that life needs: carbohydrates, lipids, proteins, and nucleic acids. From research scientists found that before life, none of these existed in the environment. So the Urey-Miller experiment simulated hypothetical conditions that were thought to be in existence during the time of early earth (before life): water (H2O), methane (CH4), ammonia (NH3), and hydrogen (H2). Using an electric shock as a catalyst (which represented lightening), they were able to synthesize carbohydrates, lipids, and proteins from inorganic materials. After Miller's death the experiment was reexamined and found to contain the 20 amino acids needed for life (precursors to nucleic acids!).
There are some scientists that believe earth's atmosphere had a much different composition than what was used in this experiment. Much evidence shows that there were many volcanic eruptions during early earth times. This means that hydrogen sulfide and sulfur dioxide would also be in the atmosphere, which results in many different molecules becoming synthesized.
Do you think that live was formed from the composition of the early earth atmosphere?