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.
In fact, some people still believe in it today. The idea was attractive because it meshed
nicely with the prevailing religious views of how God created the universe. There was a
strong bias to legitimize the idea because this vital force was considered a strong proof of
God’s presence in the world. Many recipes and experiments were offered in proof. To create
mice, a recipe called for dirty underwear and wheat grain to be mixed in a bucket and left
open outside. In 21 days or less, you would have mice. The real cause may seem obvious from a
modern perspective, but to the proponents of this idea, the mice spontaneously arose from the
Another often-used example was the generation of maggots from meat that was left in the open.
The failing here was revealed by Francesco Redi in 1668 with a classic experiment. Redi
suspected that flies landing on the meat laid eggs that eventually grew into maggots. To test
this idea he devised the experiment shown in Figure 1-20. Here he used three pieces of meat.
One piece of meat was placed under a piece of paper. The flies could not lay eggs onto the
meat and no maggots developed. The second piece was left in the open air, resulting in
maggots. In the final test, a third piece of meat was overlaid with cheesecloth. The flies
were able to lay the eggs into the cheesecloth and when this was removed no maggots
developed. However, if the cheesecloth containing the eggs was placed on a fresh piece of
meat, maggots developed, showing it was the eggs that “caused” flies and not spontaneous
generation. This helped to end the debate about spontaneous generation for large organisms.
However, spontaneous generation was so seductive a concept that even Redi believed it was
possible in other circumstances.
Figure 1-20 The Redi experiment
Using several pieces of meat, paper and cheesecloth, Francesco Redi produced compelling
evidence against the theory of spontaneous generation. One of the strong points of this
experiment was its simplicity, which allowed others to easily reproduce it for themselves.
See the text for details of the experiment.
The concept and the debate were revived in 1745 by the experiments of John Needham. It was
known at the time that heat was lethal to living organisms. Needham theorized that if he took
chicken broth and heated it, all living things in it would die. After heating some broth, he
let a flask cool and sit at a constant temperature. The development of a thick turbid
solution of microorganisms in the flask was strong proof to Needham of the existence of
spontaneous generation. Lazzaro Spallanzani later repeated the experiments of Needham, but
removed air from the flask, suspecting that the air was providing a source of contamination.
No growth occurred in Spallanzani’s flasks and he took this as evidence that Needham was
wrong. Proponents of spontaneous generation discounted the experiment by asserting that air
was required for the vital force to work.
It was not until almost 100 years later that the great French chemist Louis Pasteur, pictured
in Figure 1-5, put the debate to rest. He first showed that the air is full of microorganisms
by passing air through gun cotton filters. The filter trapped tiny particles floating in the
air. By dissolving the cotton with a mixture of ether and alcohol, the particles were
released and then settled to the bottom of the liquid. Inspection of this material revealed
numerous microbes that resembled the types of bacteria often found in putrefying media.
Pasteur realized that if these bacteria were present in the air then they would likely land
on and contaminate any material exposed to it.
Figure 1-5 Louis Pasteur
The French microbiologist Louis Pasteur. Drawing by Tammi Henke
Pasteur then entered a contest sponsored by The French Academy of Sciences to disprove the
theory of spontaneous generation. Similar to Spallanzani’s experiments, Pasteur experiment,
pictured in Figure 1-6, used heat to kill the microbes, but left the end of the flask open to
the air. In a simple, but brilliant modification, the neck of the flask was heated to melting
and drawn out into a long S-shaped curve, preventing the dust particles and their load of
microbes from ever reaching the flask. After prolonged incubation the flasks remained free of
life and ended the debate for most scientists.
Figure 1-6 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 flask and grew.
A final footnote on the topic was added when John Tyndall showed the existence of
heat-resistant spores in many materials. Boiling does not kill these spores and their
presence in chicken broth, as well as many other materials, explains the results of Needham’s
While this debate may seem silly from a modern perspective, remember that the scientists of
the time had little knowledge of microorganisms. Koch would not isolate microbes until 1881.
The proponents of spontaneous generation were neither sloppy experimenters nor stupid. They
did careful experiments and interpreted them with their own biases. Detractors of the theory
of spontaneous generation were just as guilty of bias, but in the opposite direction. In
fact, it is somewhat surprising that Pasteur and Spallanzoni did not get growth in their
cultures, since the sterilization conditions they used would often not kill endospores. Luck
certainly played a role. It is important keep in mind that the discipline of science is
performed by humans with all the fallibility and bias inherent in the species. Only the
self-correcting nature of the practice reduces the impact of these biases on generally held
theories. Spontaneous generation was a severe test of scientific experimentation, because it
was such a seductive and widely held belief. Yet, even spontaneous generation was overthrown
when the weight of careful experimentation argued against it. Figure 1-21 lists important
events in the spontaneous generation debate.