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Haldane(1937) and later Oparin (1938) proposed that UV, lightning, heat caused hydrocarbons to be formed in the early atmosphere of the earth. In 1953, Stanley Miller and Harold Urey mixed together water, hydrogen, methane, and ammonia gases in an apparatus which produced electrical sparks to simulate lightning. After a week they found that many organic compounds were present, including some amino acids. This experiment was shown by Carl Sagan, a Professor at Cornell, on the Cosmos TV series. (Urey received the Nobel Prize for chemistry in 1934 for separating isotopes, when Miller, who later worked in his lab, was only 4 years old.)
According to Miller, Urey, and Sagan, hydrogen gradually escaped from the early atmosphere because it is so light and thus the gravitational pull on it is low. Ammonia and methane are unstable in absence of a reducing atmosphere so they decreased in concentration and nitrogen gas rose in concentration.
Prokaryotic cells likely appeared about 3.5 billion years ago. These cells had no nucleus, simple, circular DNA (we suppose), no internal organelles. According to the view of Miller, Urey, and Sagan, they were heterotrophic (other-feeders) and used fermentation (glycolysis + a couple of other steps) to extract energy from the molecules formed as the result of the heat and light in the early atmosphere. Fermentation produced carbon dioxide and so the concentration of this gas rose in the atmosphere.
Photosynthesis probably began about 3.3 billion years ago with autotrophic (self-feeders) prokaryotes. While the earliest autotrophic prokaryotes used hydrogen sulfide gas and produced sulfur, later cells used water and produced oxygen gas, the concentration of which rose in the atmosphere.
Another interesting idea is that life began around thermal vents caused by volcanic activity on the ocean floor. In this scenario, the energy needed by the first prokaryotic cells would have come from chemical processes, probably involving hydrogen sulfide gas, which is abundant near such thermal vents, not from molecules produced by the reductive, pressure-cooker atmosphere.
According to this idea, these first organisms would have been chemoautotrophs, not heterotrophs, and more modern heterotrophs and autotrophs both would have been derived from them. After the first chemoautotrophs developed, the time table would be similar to that discussed above and with the appearance of heterotrophs and the development of photosynthesis (photoautotrophy) about 3.3 billion years ago.
Finally, eukaryotes appeared. Some people, such as Lynn Margulis at Boston University, believe that eukaryotes developed as the result of ingestion of prokaryotes by other prokaryotes (endosymbiont hypothesis). The ingested symbiotic prokaryotes became internal organelles such as mitochondria (if they were heterotrophs) or chloroplasts (if they were autotrophs). The use of oxygen permitted heterotrophic metabolism to break down glucose beyond the point permitted by fermentation. As a result much more energy could be extracted from each molecule of glucose metabolized.
The endosymbiont hypothesis found some powerful support in the results of a recent article in the scientific journal Nature. This article showed striking similarities between the genome of Rickettsia prowazekii, which causes a form of typhus, and that of the mitochondrial DNA from some eukaryotes.