Although today we understand that living things arise from other living things, the idea of spontaneous generation was entrenched in the minds of man throughout most of history. Spontaneous generation is the belief that, on a daily basis, living things arise from nonliving material. This debunked belief is not the same as abiogenesis, the study of how life on earth could have arisen from inanimate matter billions of years ago.
Aristotle and Spontaneous Generation
Aristotle was one of the first to record his conclusions on the possible routes to life. He saw beings as arising in one of three ways, from sexual reproduction, asexual reproduction or nonliving matter.
According to Aristotle, it was readily observable that aphids arise from the dew on plants, fleas from putrid matter, and mice from dirty hay; and this belief remained unchallenged for more than two thousand years.
Francesco Redi’s Experiments (late 1600s)
Redi was and Italian physician and one of the first to formally challenge the doctrine of spontaneous generation. Redi's question was simple, “Where do maggots come from?”
According to spontaneous generation, one would conclude that maggots came from rotting food. Redi hypothesized that maggots came from flies and designed an experiment, elegant in its simplicity, to challenge spontaneous generation.
Redi put meat into three separate jars:
- Jar #1 he left open. He observed flies laying eggs on the meat and the eventual development of maggots.
- Jar #2 he covered with netting. Flies laid their eggs on the netting and maggots soon appeared.
- Jar #3 he sealed. Flies were not attracted to this jar and no maggots developed on the meat.
This seems to be a clear demonstration of life giving rise to life. Yet it took another two hundred years for people to accept spontaneous generation as a fallacy.
Anthony van Leeuwenhoek’s “Animalcules” (1600-1700s)
Leeuwenhoek was a Dutch cloth merchant, and due to his trade, frequently used lenses to examine cloth. Rather than employing lenses made by others, he ground his own, and the expertise that he gained through lens crafting combined with a curious mind eventually led to an interest in microscopy.
During his life, Leeuwenhoek assembled more than 250 microscopes, some of which magnified objects 270 times. Through magnification, he discovered presence of “micro” organisms--organisms so tiny that they were invisible to the naked eye.
He called these tiny living things “animalcules,” and was the first to describe many microbes and microscopic structures, including bacteria, protozoans and human cells.
John Needham & Lazzaro Spallanzani (1700s)
The debate over spontaneous generation was reignited with Leeuwenhoek’s discovery of animalcules and the observation that these tiny organisms would appear in collected rainwater within a matter of days. John Needham and Lazzaro Spallazani both set out to examine Leeuwenhoek's animalcules.
Needham’s Experiment
John Needham was a proponent of spontaneous generation, and his beliefs were confirmed when, after boiling beef broth to kill all microbes, within the span of a few days, cloudiness of the broth indicated the respawning of microscopic life.
Spallazani’s Experiment
Lazzaro Spallazani noted a flaw in Needham’s experiment. The containers holding Needham’s beef broths had not been sealed upon boiling. So Spallazani modified Needham’s experiment, boiling infusions, but immediately upon boiling he melted the necks of his glass containers so that they were not open to the atmosphere. The microbes were killed and did not reappear unless he broke the seal and again exposed the infusion to air.
Louis Pasteur Settles It (1800s)
Pasteur, a French scientist who made great contributions to our understanding of microbiology and for whom the process of “pasteurization” is named, repeated experiments similar to those of Spallazani’s and brought to light strong evidence that microbes arise from other microbes, not spontaneously.
Pasteur’s Swan-Necked Flasks
Pasteur created unique glass flasks with unusual long, thin necks that pointed downward. These “swan-necked” flasks allowed air into the container but did not allow particles from the air to drift down into the body of the flask.
The End of Spontaneous Generation
After boiling his nutrient broths, Pasteur found that these swan-necked containers would remain free of microbes until he either broke the necks of the flasks, allowing particles from the air to drift in, or until he tilted the flask so that the liquid came in contact with dust that had accumulated at the opening of the flask. It was these carefully controlled experiments of Pasteur that finally put to rest the debate over spontaneous generation.
More Resources on Microbiology
For additional information on micribiology, see Science Prof Online or Suite101 articles, including Early Germ Theory of Disease and Late Germ Theory of Disease.
Sources
Bauman, R. (2005) Microbiology.
Park Talaro, K. (2008) Foundations in Microbiology.
