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In the Ancient Egyptian Civilization, people believed that everytime the Nile River flooded, it gave rise to frogs and other insects.
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In around 400 BC, Aristotle wrote a book "The History of Animals", in which he discussed spontaneous generation. He was considered the first person to put this theory into words, by explaining that there were some animals who came from parent animals, while others grew spontaneously from inanimate objects.
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In some European cities which didn't have proper waste disposal systems, people believed that the sewage and garbage turned into rats. Farmers presumed that mice kept growing out of moldy grains and other crops, and that rotting meat led way to flies.
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Francesco Redi was an Italian Physician who believed that maggots developed from eggs laid by flies. This went against what others thought: maggots arose from rotting meat. Redi tested his hypothesis by placing different meats in 3 types of jars and looked at them through a microscope. He found that in the gauze-covered jar, no flies were inside, but were seen around the rim. In the sealed jar, no maggots or flies were seen, but in the open jar, and the number of maggots grew by the day.
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1745 John Needham, an English clergyman, proposed to test whether or not microorganisms appeared spontaneously after boiling boiled chicken broth, put it in a flask, sealed it and waited. He saw that microorganisms grew. He claimed that “life force” present in the molecules of all inorganic matter, including air, could cause spontaneous generation.
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Lazzaro Spallanzani, an Italian priest, was not convinced by Needham's theory and thought his experiment had a lot of flaws in them. To test it out himself, he modified Needham’s experiment by placing the chicken broth in a flask, sealing it, then drawing out the air to create a partial vacuum, then boiling it. He found that no microorganisms grew in the broth, thus proving that spontaneous generation does not occur.
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A French chemist, Louis Pasteur, used both Needham and Spallanzani’s experiments to come to his own conclusion. He boiled meat broth in a flask, heated the neck of the flask in a flame until it became pliable, then bent it into the shape of an S. This way, air could enter, but airborne microorganisms could not as they would settle in the neck by gravity. He saw that no microorganisms grew, but there were a lot settled in the neck, proving microorganisms are everywhere.
