Russian scientist and academecian A.I. Oparin, in 1922, hypothesized that cellular life was preceeded by a period of chemical evolution. These chemicals, he argued, must have arisen spontaneously under conditions exisitng billions of years ago (and quite unlike current conditions).
In 1950, then-graduate student Stanley Miller designed an experimental test for Oparin's
hypothesis. Oparin's original hypothesis called for : 1) little or no free oxygen (oxygen
not bonded to other elements); and 2) C H O and N in abundance. Studies of modern volcanic
eruptions support inference of the existence of such an atmosphere. Miller discharged an
electric spark into a mixture thought to resemble the primordial composition of the
atmosphere. From the water receptacle, designed to model an ancient ocean, Miller
recovered amino acids. Subsequent
modifications of the atmosphere have produced representatives or precursors of all four
organic macromolecular classes.
Ingredients used in Miller's experiments, simple molecules thought at the time to have existed on the Earth billions of years ago.
A diagrammatic representation of Miller's experimental
Molecules recovered from Miller's and similar experiments.
The primordial Earth was a very different place than today, with greater amounts of energy, stronger storms, etc. The oceans were a "soup" of organic compounds that formed by inorganic processes (although this soup would not taste umm ummm good). Miller's (and subsequent) experiments have not proven life originated in this way, only that conditions thought to have existed over 3 billion years ago were such that the spontaneous (inorganic) formation of organic macromolecules could have taken place. The simple inorganic molecules that Miller placed into his apparatus, produced a variety of complex molecules:
The interactions of these molecules would have increased as their concentrations increased. Reactions would have led to the building of larger, more complex molecules. A pre-cellular life would have began with the formation of nucleic acids. Chemicals made by these nucleic acids would have remained in proximity to the nucleic acids. Eventually the pre-cells would have been enclosed in a lipid-protein membrane, which would have resulted in the first cells.