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350 BCE
The spontaneous generation by Aristotle
The theory of spontaneous generation was first comprehensively posited by Aristotle in his book called "On the Generation of Animals". This theory states that organisms do not come from other oganisms necessarily, but can rather be created spontaneously when certain conditions in an environment are fulfilled. -
Redi's experiment
Francesco Redi was an Italian physician and poet. People all around the world believed in spontaneous generation, so he carried out an experiment: he put a piece of meat in three equal jars. He left the first one open, sealed tightly the second one and covered the third one with cloth. He found out that after a few weeks there were maggots in the first piece of meat but not in the second, whereas in the third jar they were on top of the cloth. This came as a surprise to lots of people. -
Needham's experiment
John Needham carried out an experiment in which he boiled meat broth to destroy the pre-existing organisms and placed it in a container that was not properly sealed, since according to his theory, air was needed for this to take place. After some time, he observed colonies in the container, which did not contradict the theory of spontaneous generation. People were more certain about the theory of spontaneous generation. -
Spallazani's experiment
In the same year that John Needham conducted the chicken broth experiment, an Italian priest who did not feel convinced modified Needham’s experiment and vacuumed the air out before sealing the chicken broth and boiling it. During that experiment, no microorganisms grew. However, some believers disagreed with Spallanzani and said he simply just proved that the theory needs air to work. -
Pasteur's experiment
A French chemist Pasteur proved the spontaneous generation theory wrong. He performed a combined experiment of Needham’s and Spallazani’s. He boiled meat in a flask and heated the neck of the flask and bent it to an S shape. This allowed air to go into the flask but does not allow air borne microorganisms because they will settle on the S. Since no microorganisms grew, Pasteur proved the spontaneous generation theory wrong and also demonstrated microorganisms are everywhere.