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Famous Greek philosopher Aristotle laid down theories for spontaneous generation. In his book, "The History of Animals," he stated that "so with animals, some spring from parent animals according to their kind, whilst others grow spontaneously...." According to this theory, living things came forth from nonliving things because the nonliving material contained "vital heat".
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Anaximander, a pre-Socratic Greek philosopher, believed that everything arose out of the elemental nature of the universe. According to "Hippolytus of Rome," Anaximander claimed that spontaneous generation continued to this day, with aquatic forms being produced directly from lifeless matter.
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St Thomas Aquinas declared that spontaneous generation occurred as angels took action as the Sun.
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Francesco Redi, an Italian physician, conducted the first major experiment to challenge spontaneous generation. He placed meat in a variety of sealed, open, and partially covered containers. Realizing that the sealed containers were deprived of air, he used "fine Naples veil," and observed that maggots were not on the meat, but appeared on the cloth. Redi used his experiments to support the preexistent theory: that living things originated from its parents.
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As precise microscopes were invented in the late 1600's, scientists were able to discover smaller organisms. People began to realize that just because these living things (tied to the "spontaneous generation" of maggots in meat) could not be seen with the naked eye, it didn't mean that they don't exist.
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John Needham, an English biologist, performed a series of experiments on boiled broths. Believing that boiling would kill all living things, he showed that when sealed right after boiling, the broths would cloud, allowing the belief in spontaneous generation to persist.
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Lazzaro Spallanzani modified John Needham's experiment, which defended spontaneous generation, and attempted to exclude the possibility of introducing a contaminating factor between boiling and sealing. Spallanzani's technique involved boiling the broth in a sealed container with the air partially evacuated (to prevent explosions). As he did not see growth, he concluded that spontaneous generation was indeed a false belief, but the general public was still not convinced.
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Louis Pasteur's experiment is seen as the last in disproving spontaneous generation. He boiled broth in a flask that had a long neck, curved downward like a goose. The bend prevented falling particles from reaching the broth without breaking air flow. In the end, nothing grew in the broths until broken open, proving that organisms came from outside rather than spontaneously generated within the broth. This was one of the most important experiments disproving the theory of spontaneous generation.
