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What is the Spontaneous Generation Hypothesis?

written by: J.Sace•edited by: Leigh A. Zaykoski•updated: 12/27/2008

Antoni van Leeuwenhoek was the first one to observe protozoans that he called “animalcules" using his simple magnifying lenses. Scientists and philosophers in Antoni’s time used the spontaneous generation hypothesis to explain the origin of the “animalcules". What is this hypothesis?

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    Van Leeuwenhoek's microscopes by Henry Baker 

    Using a simple single lens microscope, the Dutch merchant and amateur scientist Antoni van Leeuwenhoek was able to observe for the first time, living and moving microorganisms that he later called “animalcules". He performed multiple observations of the tiny organisms in materials scraped from his teeth (probably a plaque) and fluids where peppercorns had been soaked. He patiently drew in great detail, the small creatures he viewed in his magnifying lenses. From 1673 to 1723, he sent a series of letters to the Royal Society of London describing his amazing discoveries.

    Leeuwenhoek’s discovery spread so fast in entire Europe and it became a major controversy among many scientists and philosophers at his time. Prior to the discovery of Leeuwenhoek on the protozoans people normally believed that toads, snakes, and mice spontaneously originated from wet soil; that maggots (a.k.a. larvae of flies), could emerge from decomposing human dead body; and that flies could arise from animal feces. Scientists at that time used these “belief system" as the basis of explaining the origin of the “animalcules" that Leeuwenhoek observed. According to them, the little creatures (modern day protozoans) were spontaneously generated from non-living matter like the materials scraped from Leeuwenhoek’s teeth. The spontaneous generation hypothesis had been widely accepted until the later part of the 1850s when Louis Pasteur came up with the best experimental set up demonstrating that microorganisms only arise from preexisting living cells.

    After Leewenhoek’s discovery of the “animalcules", a lot of “erroneous" experimental results and conclusions have been presented by several scientists further strengthening the spontaneous generation hypothesis. One is the experiment of John Needham, an Englishman, who found that after he boiled nutrient solutions (corn and chicken broths) and before pouring them into covered flasks, microorganisms were soon teeming in the cooled solutions. Italian Lazaro Spallanzani, antagonist of the spontaneous generation hypothesis, claimed that microorganisms in the air quickly contaminated the cooled solutions before Needham put them into the covered flasks; this explains the origin of the microbes that Needham saw in his nutrient solutions. Spallanzani was able to prove his point by demonstrating that nutrient solutions heated after being sealed in a flask did not form microbial growth. Needham responded by asserting the “vital force" important for the spontaneous generation of microbes had been destroyed by the heat and the entrance of new “vital life" was prevented by the seals in the flasks.

    Unfortunately, the discovery of chemist Antoine Lavoisier of the element oxygen and its importance to the maintenance of life favored Needham’s experiment. The oxygen, according to Needham, was exactly the “vital life" he was talking about. He added that microbes did not grow in the nutrient solution of Spallanzani’s experiment because his containers were tightly sealed for the “oxygen/vital life" to enter. Because of this ground (not enough oxygen in the flasks), the experiment of Spallanzani received various criticisms from the scientific community. Sadly, Spallanzani did not find any other argument to back up his observations and to disprove the hypothesis that Needham and others were promoting.


    Serafini, Anthony. 1993. The Epic History of Biology. Plenum Press.

    Madigan, Michael. 2006. Brock Biology of Microorganisms. Upper Saddle River, N.J.: Prentice Hall/Pearson Education.

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