Spontaneous generation is an obsolete body of thought on the ordinary formation of living organisms without descent from similar organisms. Typically, the idea was that certain forms such as fleas could arise from inanimate matter such as dust or that maggots could arise from dead flesh.
A variant idea was that of equivocal generation, in which species such as tapeworms arose from unrelated living organisms, now understood to be their hosts. Doctrines held that these processes were commonplace and regular. Such ideas were in contradiction to that of univocal generation: effectively exclusive reproduction from genetically related parent s , generally of the same species. The doctrine of spontaneous generation was coherently synthesized by Aristotle, who compiled and expanded the work of prior natural philosophers and the various ancient explanations of the appearance of organisms; it held sway for two millennia.
Today spontaneous generation is generally accepted to have been decisively dispelled during the 19 th century by the experiments of Louis Pasteur. The debate over spontaneous generation continued for centuries. In , John Needham, an English clergyman, proposed what he considered the definitive experiment. Everyone knew that boiling killed microorganisms, so he proposed to test whether or not microorganisms appeared spontaneously after boiling. He boiled chicken broth, put it into a flask, sealed it, and waited - sure enough, microorganisms grew.
Needham claimed victory for spontaneous generation. An Italian priest, Lazzaro Spallanzani, was not convinced, and he suggested that perhaps the microorganisms had entered the broth from the air after the broth was boiled, but before it was sealed. To test his theory, he modified Needham's experiment - he placed the chicken broth in a flask, sealed the flask, drew off the air to create a partial vacuum, then boiled the broth.
No microorganisms grew. It was once believed that life could come from nonliving things, such as mice from corn, flies from bovine manure, maggots from rotting meat, and fish from the mud of previously dry lakes. Spontaneous generation is the incorrect hypothesis that nonliving things are capable of producing life. Several experiments have been conducted to disprove spontaneous generation; a few of them are covered in the sections that follow. In , Francesco Redi, an Italian scientist, designed a scientific experiment to test the spontaneous creation of maggots by placing fresh meat in each of two different jars.
One jar was left open; the other was covered with a cloth. Days later, the open jar contained maggots, whereas the covered jar contained no maggots. He did note that maggots were found on the exterior surface of the cloth that covered the jar. Redi successfully demonstrated that the maggots came from fly eggs and thereby helped to disprove spontaneous generation. Or so he thought. In England, John Needham challenged Redi's findings by conducting an experiment in which he placed a broth, or gravy, into a bottle, heated the bottle to kill anything inside, then sealed it.
When the roof leaked and the grain molded, mice appeared. Jan Baptista van Helmont , a seventeenth century Flemish scientist, proposed that mice could arise from rags and wheat kernels left in an open container for 3 weeks. In reality, such habitats provided ideal food sources and shelter for mouse populations to flourish. He predicted that preventing flies from having direct contact with the meat would also prevent the appearance of maggots.
Redi left meat in each of six containers Figure 1. Two were open to the air, two were covered with gauze, and two were tightly sealed. His hypothesis was supported when maggots developed in the uncovered jars, but no maggots appeared in either the gauze-covered or the tightly sealed jars. He concluded that maggots could only form when flies were allowed to lay eggs in the meat, and that the maggots were the offspring of flies, not the product of spontaneous generation. Figure 1.
Maggots only appeared on the meat in the open container. However, maggots were also found on the gauze of the gauze-covered container. In , John Needham — published a report of his own experiments, in which he briefly boiled broth infused with plant or animal matter, hoping to kill all preexisting microbes. After a few days, Needham observed that the broth had become cloudy and a single drop contained numerous microscopic creatures.
He argued that the new microbes must have arisen spontaneously. In reality, however, he likely did not boil the broth enough to kill all preexisting microbes. This suggested that microbes were introduced into these flasks from the air. Any subsequent sealing of the flasks then prevented new life force from entering and causing spontaneous generation Figure 2.
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