What Were the Conditions?
Life may have been present on Earth around 3.8 billion years ago, right after the presumed end of Late Heavy Bombardment. (The duration and end of the Late Heavy Bombardment is currently in controversy.) It probably was present before that, but we have not found the evidence. The bombardment must have destroyed much of it. (We can see the effects of bombardment when we look at the ancient cratered face of the Moon.)
Courtesy: Don Dixon
By modern standards, Earth was a very nasty place 4 billion years ago. The first 100 million years or so were marked by severe melting and re-melting of the rocks on the surface of the planet. After this process slowed, a solid crust began to form; but the continued bombardment from comets and asteroids kept on tearing up its face for another several hundred million years. The changes on the surface that began to take place in this time led to an environment capable of supporting organic compounds. Water was being released from the hot rocks below the crust (a process referred to as "outgassing"). This process went on for some time, and may still be active to some extent. However, the water emitted today in volcanoes, geysers and undersea vents is mostly recycled surface water.
A portion of the early water was brought in from asteroids, according to the experts studying the isotopic composition of water). Liquid water, as far as we know, was a crucial requirement for the formation of life. Oceans formed, as well as lagoons, lakes, puddles and groundwater pools. All became enriched with a variety of organic compounds. Perhaps, in the absence of organisms, such enrichment could proceed to a point where the label "prebiotic soup" is appropriate.
The continued supply of organic molecules depended in part on the composition of the early atmosphere. Taking a clue from Earth's sibling planets, we might expect high concentrations of carbon dioxide (CO2),with small amounts of methane (CH4), and ammonia (NH3). Hydrogen and nitrogen probably also were present, as H2 and N2, as was hydrogen sulfide (H2S). Hydrogen has a tendency to leave the planet. (Can you tell why?) Thus, it would have quickly reached low levels. The rise of molecular oxygen was yet to come. It had to wait for photosynthetic activity, to split the oxygen from carbon in CO2.
Mixtures of the gases mentioned, when stimulated with electric discharge, can produce a large diversity of molecules familiar from organic chemistry: amino acids, purines, pyrimidines and sugars. This was shown by experiment, in 1952, by the American chemist Stanley Lloyd Miller (b. 1930) (now a professor at UCSD). Miller was then a graduate student working under the tutelage of Harold Urey (1893-1981). Miller used a mixture of hydrogen, ammonia and methane in these first experiments. Subsequent experiments by others, using different mixtures and varying energy sources, established that many of the familiar building blocks of living organisms could have been produced quite readily from a primitive reducing atmosphere. Several lines of research, however, indicate that early atmosphere was not reducing, but high in CO2. In such an oxidizing atmosphere, the Miller-Urey experiment does not generate a supply of prebiotic building blocks.
Clearly, we need to have organic molecules before we can begin to build organisms. Thus, the main point of the Urey-Miller experiment is that we should set the appropriate conditions for the early atmosphere, for our thought experiments on the origin of Life.
A pile of bricks does not make a cathedral, and a collection of organic molecules does not make a living cell. There is presently no such thing as a "primitive" cell. There is no experiment that produces anything resembling living things. Imagine a junk yard with bits and pieces of metal of various shapes. Then think of a modern automobile with GPS and onboard computer, and a voice telling you to fasten your seat belt. That is roughly the distance between the organic matter seen in experiments simulating early-Earth conditions and the life forms now extant.