The Evolution of Spontaneous Generation Theory
Long ago it was believed that living creatures originate spontaneously from non-living matter. Although there was no scientific theory supporting spontaneous generation, people have developed this concept through their routine observations that occur around them. Aristotle was the first one to record his observations on spontaneous generation. In his writings, Aristotle proposed that certain life forms emerge spontaneously from non-living matter under specific conditions. He described about the formation of maggots from decaying meat and, the sudden appearance of rats from stored grains, etc. However, his concepts have remained unchallenged for more than two thousand years.
Francesco Redi’s experiment
Francesco Redi, an Italian physician, is often credited with one of the earliest experiments challenging spontaneous generation. In 1668, Redi conducted experiments involving meat and maggots. He demonstrated that maggots arise from flies, not from the decaying matter. Redi’s work provided experiential evidence against spontaneous generation and laid the groundwork for later experiments by other scientists.
In this experiment, three bottles were taken with a piece of meat placed in each bottle. The first bottle was kept open and exposed to air. The second bottle was completely sealed. The third bottle’s neck was tied with porous mesh (gauze).
After some time, the following observations were made:
The meat in the first bottle attracted files and later maggots were found on the meat. There were no files, and no maggots were observed in the second bottle as the bottle was completely sealed. As the third bottle was tied with porous mesh, the flies were attracted to the smell of the meat. But the flies could not enter the bottle and laid eggs on the mesh. Later, the maggots were found on the mesh.
With this experiment, Redi concluded that maggots were not raised spontaneously from the meat. The maggots were raised from the eggs of flies. Life is necessary to produce another life.
Spontaneous generation for Microorganisms
Then the real challenge was faced when Leeuwenhoek discovered tiny microorganisms under the microscope in 1676. After the discovery of Microorganisms by Leeuwenhoek, the arguments on spontaneous generation have taken a new turn. Many scientists had proposed theories that supported spontaneous generation for microorganisms, but not for the larger living animals. Later, there were many experiments and arguments on spontaneous generation.
In 1748, John Needham observed microorganisms in heated meat broth that were not present at the beginning of the experiment. He concluded that the bacteria originated from the meat.
At the same time, Lazaro Spallanzani further improved Needham’s experiment. He boiled the meat broth, and completely sealed the flask, and the microorganisms did not grow. He concluded that air carried the germs to the broth in Needham’s experiment.
Theodore Schwann passed the air into a flask containing broth through a red-hot tube. The broth in the flask had shown no growth.
Simultaneously in 1850, George Friedrich Schroder and Theodor von Dusch allowed air into the flask by passing it through a sterile cotton wool. There was no growth observed. They said the cotton wool has trapped the microbes from the external environment. The usage of cotton plugs to avoid microbial contamination exists even today.
Later, in 1861, Louis Pasteur completely disproved the spontaneous generation concept with his swan neck experiment.
Disproval of Spontaneous Generation Theory (Pasteur’s Experiment)
The turning point in the discourse on spontaneous generation came with the meticulous and groundbreaking experiments conducted by Louis Pasteur in the 19th century. Pasteur designed experiments that involved specially designed swan-necked flasks filled with nutrient-rich broth, heated to eliminate existing microorganisms. By preventing dust and external particles from entering the flasks while allowing air to reach the broth, Pasteur aimed to test the prevailing idea of spontaneous generation. Over the course of these experiments, he observed that no microbial growth occurred in the sealed flasks, disproving the concept that microorganisms could spontaneously generate. Pasteur’s experiments marked a revolutionary departure from previous thought, introducing a methodical and empirical approach to microbiology and challenging centuries-old beliefs.
Swan neck flask experiment
In the step-1 of 1 of the experiment, the swan neck flask was filled with meat broth, and it was boiled to kill the microorganisms that were already present. Then the broth was allowed to cool and waited for some time. The broth in the flask did not show any color change as there was no microbial growth. He explained, that although the air particles could enter the flask, the germs could not enter the flask and they were trapped at the curved neck (S-shaped bend).
In step 2, the neck of the flask was broken. After some time, it was observed that the color of the broth was changed due to microbial growth. He explained that certain germs from the air caused the contamination of the broth, not the air alone as claimed previously.
If Spontaneous generation was real, the curved neck flask would have generated bacteria spontaneously. But it did not happen, which indicates germs could come from other germs.
Pasteur’s experiment has become a benchmark in Microbiology and led to the “Germ theory of Disease”.
