- For many centuries many people believed in the concept of spontaneous generation, the creation of life from organic matter.
- Spontaneous generation is the hypothesis that some vital force contained in or given to organic matter can create living organisms from inanimate objects. Spontaneous generation was a widely held belief throughout the middle ages and into the latter half of the 19th century.
- The Italian Francesco Redi disproved spontaneous generation for large organisms by showing that maggots arose from meat only when flies laid eggs in the meat.
- Spontaneous generation for small organisms again gained favor when John Needham and Lazzaro Spallanzani showed that if a broth was boiled (presumed to kill all life) and then allowed to sit in the open air, it became cloudy.
- Louis Pasteur ended the debate with his famous swan-neck flask experiment, which allowed air to contact the broth. Microbes present in the dust were not able to navigate the tortuous bends in the neck of the flask.
The Louis Pasteur experiment
- The swan neck flask experiment. Pasteur filled a flask with medium, heated it to kill all life, and then drew out the neck of the flask into a long S shape. This prevented microorganisms in the air from easily entering the flask, yet allowed some air interchange. If the swan neck was broken, microbes readily entered the open flask and grew.