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Pasteur and the The Origin of Life In the 19th century, Pasteur questioned Aristotle’s long-held assertion on spontaneous generation as he proposed an alternative hypothesis, later leading to more research concerning the origin of life. Logical and methodical scientists sometimes, ironically, blindly adhere to spuriously hypothesized assertions. For instance, Aristotle hypothesized that flies were the result of spontaneous generation in rotting materials and that microorganisms could appear naturally. Scientists believed these theories for nearly two millennia. These errant theories led them to believe that the bacteria required for fermentation were naturally generated without any external stimuli. Scientist Louis Pasteur, however, skeptical of spontaneous generation, successfully falsified the long-held theory. Pasteur proposed the new theory of biogenesis, claiming that the natural generation of bacteria was preposterous, and that fermentation required the introduction of the necessary bacteria. In order to prove this, he conducted two experiments. In the first, a glass filled with meat broth, which would have fermented in open air, was placed in a sealed box that was completely sterile and had a filter over its opening. This did not allow anything to enter the glass and as a result there were no signs of fermentation in the glass. This, however, did not convince scientists at the time, because they thought that even if organisms were generated in the glass, they would have expired due to lack of oxygen. Therefore, he conducted a second experiment, the swan-neck duct’ experiment. In this experiment, he put the broth into a spherical glass with a long curved tube atop it. With this structure, nothing but weightless air could enter. This also yielded no fermentation. In 1864, Pasteur finally announced the results of these and further experiments, demonstrating that life cannot spontaneously arise in areas that have not been exposed to existing life. Unsurprisingly, Pasteur’s experiments were eye- openers, having definitively disproved the antiquated theory of spontaneous generation and giving birth to biogenesis. Even though he was the first to empirically prove this, he was not the first scientist to propose this type of theory. Scientists Girolamo Fracastoro, Agostino Bassi, and Friedrich Henle had previously suggested various theories which influenced Pasteur's work later. The development of biogenesis, however, left a vacuum in the understanding of how life first arose. As it requires previous forms of life, biogenesis alone could not ultimately explain the origin of life on the early, barren Earth. No notable related theories, however, appeared until 1924, when Alexander Oparin surmised that the presence of oxygen in the atmosphere prevents the synthesis of certain organic compounds that are necessary for the evolution of life. In ‘The Origin of Life’， he proffered that the type of abiogenesis that Pasteur attacked had in fact occurred once, but no longer could because the conditions found on the early Earth had changed. Oparin felt that an organic ‘primeval soup’ could be created in an oxygen-free atmosphere due to the effects of sunlight. He hypothesized that 4 billion years ago, the earth’s environment met these criteria after volcanic eruptions filled the environment with carbon dioxide, nitrogen, and other gases. Originally he thought that the early atmosphere, in contrast to the Earth’s current atmosphere, contained mostly ammonia and methane, but it is likely that most of the atmospheric carbon was carbon dioxide with perhaps some carbon monoxide and nitrogen. In practice these mixtures have many of the same properties as those containing ammonia and methane as long as free oxygen molecules are not present. These theories were tested in the Miller and Urey experiment which attempted to recreate these hypothetical conditions, and tested for the occurrence of the chemical origins of life. In order to test for the generation of the amino acids, required for protein generation in living cells under primitive Earth’s conditions, they required a less hydrogen-rich mixture than the current environment. Their experiments used water, methane, ammonia, and hydrogen, sealed inside interconnected sterile glass tubes and beakers, with one half filled with water and another holding two electrodes. The water was boiled to produce evaporation, then the electrodes were fired to simulate lightning through the atmosphere and water vapor, and then it was cooled again to condense the water back into the first beaker in a repetitive cycle. After one week, they observed that 10-15 percent of the carbon was in the form of organic compounds. Two percent of the carbon had formed amino acids, with glycine being the most plentiful. Although nucleic acids were not formed, the 20 common amino acids were formed in various concentrations. This was interesting because amino acids can potentially create the most basic component of most living creatures, namely DNA.