Before the 1860s and 1870s&mdash;when Pasteur published his work on this theory&mdash;it was believed that microorganisms and even some small animals such as frogs would spontaneously generate. Spontaneous generation was historically explained in a variety of ways. Aristotle, an ancient Greek philosopher, theorized that creatures appeared out of certain concoctions of earthly elements, such as clay or mud mixing with water and sunlight. Later on, Felix Pouchet argued for the existence of plastic forces within plant and animal debris capable of spontaneously generating eggs, and new organisms were born from these eggs. On top of this, a common piece of evidence that seemed to corroborate the theory was the appearance of maggots on raw meat after it was left exposed to open air.
In the 1860s and 1870s, Pasteurs interest in spontaneous generation led him to criticize Pouchets theories and conduct experiments of his own. In his first experiment, he took boiled sugared yeast-water and sealed it in an airtight contraption. Feeding hot, sterile air into the mixture left it unaltered, while introducing atmospheric dust resulted in microbes and mold appearing within the mixture. This result was also strengthened by the fact that Pasteur used asbestos, a form of totally inorganic matter, to carry the atmospheric dust. In a second experiment, Pasteur used the same flasks and sugar-yeast mixture, but left it idle in swan-neck flasks instead of introducing any extraneous matter. Some flasks were kept open to the common air as the control group, and these exhibited mold and microbial growths within a day or two. When the swan-neck flasks failed to show these same microbial growths, Pasteur concluded that the structure of the necks blocked the passage of atmospheric dust into the solution. From the two experiments, Pasteur concluded that the atmospheric dust carried germs responsible for the spontaneous generation in his broths. Thus, Pasteur's work provided proof that the emergent growth of bacteria in nutrient broths is caused by biogenesis rather than some form of spontaneous generation.