Is Spontaneous Generation Real?

For several centuries it was believed that living organisms could spontaneously come from nonliving matter. This idea, known as spontaneous generation, is now known to be false. Proponents of at least some aspects of spontaneous generation included well-respected philosophers and scientists such as Aristotle, Rene Descartes, William Harvey, and Isaac Newton. Spontaneous generation was a popular notion due to the fact that it seemed to be consistent with observations that a number of animal organisms would apparently arise from nonliving sources. Spontaneous generation was disproved through the performance of several significant scientific experiments.

Do Animals Spontaneously Generate?

Prior to the mid-19th century, it was commonly believed that the origin of certain animals was from nonliving sources. Lice were thought to come from dirt or sweat. Worms, salamanders, and frogs were thought to be birthed from the mud. Maggots were derived from rotting meat, aphids and beetles supposedly sprang from wheat, and mice were generated from soiled clothing mixed with wheat grains. While these theories seem quite ludicrous, at the time they were thought to be reasonable explanations for how certain bugs and other animals seemed to appear from no other living matter.

Spontaneous Generation Debate

While a popular theory throughout history, spontaneous generation was not without its critics. Several scientists set out to refute this theory through scientific experimentation. At the same time, other scientists tried to find evidence in support of spontaneous generation. This debate would last for centuries.

Redi Experiment

In 1668, the Italian scientist and physician Francesco Redi set out to disprove the hypothesis that maggots were spontaneously generated from rotting meat. He contended that the maggots were the result of flies laying eggs on exposed meat. In his experiment, Redi placed meat in several jars. Some jars were left uncovered, some were covered with gauze, and some were sealed with a lid. Over time, the meat in the uncovered jars and the jars covered with gauze became infested with maggots. However, the meat in the sealed jars did not have maggots. Since only the meat that was accessible to flies had maggots, Redi concluded that maggots do not spontaneously arise from meat.

Needham Experiment

In 1745, English biologist and priest John Needham set out to demonstrate that microbes, such as bacteria, were the result of spontaneous generation. Thanks to the invention of the microscope in the 1600s and increased improvements to its usage, scientists were able to view microscopic organisms such as fungi, bacteria, and protists. In his experiment, Needham heated chicken broth in a flask in order to kill any living organisms within the broth. He allowed the broth to cool and placed it in a sealed flask. Needham also placed unheated broth in another container. Over time, both the heated broth and unheated broth contained microbes. Needham was convinced that his experiment had proven spontaneous generation in microbes.

Spallanzani Experiment

In 1765, Italian biologist and priest Lazzaro Spallanzani, set out to demonstrate that microbes do not spontaneously generate. He contended that microbes are capable of moving through the air. Spallanzani believed that microbes appeared in Needham's experiment because the broth had been exposed to air after boiling but before the flask had been sealed. Spallanzani devised an experiment where he placed the broth in a flask, sealed the flask, and removed the air from the flask before boiling. The results of his experiment showed that no microbes appeared in the broth as long as it remained in its sealed condition. While it appeared that the results of this experiment had dealt a devastating blow to the idea of spontaneous generation in microbes, Needham argued that it was the removal of air from the flask that made spontaneous generation impossible.

Pasteur Experiment

In 1861, Louis Pasteur presented evidence that would virtually put an end to the debate. He designed an experiment similar to Spallanzani's, however, Pasteur's experiment implemented a way to filter out microorganisms. Pasteur used a flask with a long, curved tube called a swan-necked flask. This flask allowed air to have access to the heated broth while trapping dust containing bacterial spores in the curved neck of the tube. The results of this experiment were that no microbes grew in the broth. When Pasteur tilted the flask on its side allowing the broth access to the curved neck of the tube and then set the flask upright again, the broth became contaminated and ​bacteria reproduced in the broth. Bacteria also appeared in the broth if the flask was broken near the neck allowing the broth to be exposed to non-filtered air. This experiment demonstrated that bacteria appearing in broth are not the result of spontaneous generation. The majority of the scientific community considered this conclusive evidence against spontaneous generation and proof that living organisms only arise from living organisms.

Sources

• Microscope, Through the. “Spontaneous Generation Was an Attractive Theory to Many People, but Was Ultimately Disproven.” Through the Microscope Main News, www.microbiologytext.com/5th_ed/book/displayarticle/aid/27.