Spontaneous generation refers to the idea that living creatures can arise from nonliving matter. Before microscopes enabled people to view forms of life too small to see with the naked eye, the origins of many living things were a mystery. People often wondered why, for example, maggots would appear in decaying meat. Aristotle first expressed the idea of spontaneous generation in the 4th century b.c., but the scientific community of 17th century Europe raised doubts about Aristotle's idea.
Francesco Redi (1626-1697), an Italian physician and poet, was the first to demonstrate the improbability of spontaneous generation with an experiment involving the reproductive methods of flies. Redi set out to test the possibility that life might arise from tiny seeds or eggs that were too small to be seen. He prepared eight different containers of meat; four were sealed and four left open. After a few days, Redi noticed that maggots appeared only in the meat that was exposed to air, on which flies landed regularly. He repeated the experiment using gauze-covered meat to see whether it was the exposure to air or the fact that flies landed on the meat that caused the maggots. Indeed, the gauze-covered meat did not show any maggot development. His practical experiment demonstrated that maggots were not formed by spontaneous generation, but from eggs laid by flies.
After improvements in the microscope, people could see the microorganisms that preceded visible life. But many still wondered about bacteria, and the mystery of spontaneous generation lingered a while longer, as different experiments showed contradictory results. John Turberville Needham, an English naturalist and Roman Catholic priest, collaborated with Georges Buffon in 1748 to further confuse the issue of spontaneous generation. He boiled mutton broth and sealed it in glass containers for a few days. When the containers were opened, there were numerous microorganisms present. These experiments, he concluded, showed how life could arise from nonliving matter and disputed the fact that boiling water could sterilize tiny life matter. With great acclaim, Needham was elected to the Royal Society, the first Roman Catholic to earn such a distinction. He also became the director of the Academy of Sciences at Brussels.
Twenty years passed before someone noticed the flaw in Needham's experiment. Lazzaro Spallanzani, an Italian biologist, pointed out that Needham had failed to boil the mutton long enough to kill off all the spores. Spallanzani conducted the same experiment after boiling the mutton for close to an hour, and no maggots appeared. But the debate was far from over; many still held on to the theory of spontaneous generation. Some thought that Spallanzani had destroyed a suspected vital component in the air and that the boiling time was not significant.
It was another century before Louis Pasteur would prove conclusively that sterilization could hinder the growth of tiny life forms. In the mid-1800s, John Tyndall, an Irish physicist, showed that some of the dust in air consisted of microorganisms. This explained why broths so easily became affected by life forms. Louis Pasteur made the greatest breakthroughs in refuting spontaneous generation. The French chemist made many of his observations by using the microscope and working with heat. For example, he discovered two types of yeast in fermented wine, one of which could cause the wine to spoil. In order to ensure that only the other, harmless type of yeast remained in the wine, the wine was heated; this idea was initially strongly opposed by French winemakers. After Pasteur illustrated his theory, the process of pasteurization became a widely used method of killing undesirable microscopic organisms in many liquids. Like Tyndall, Pasteur showed that the dust in air included spores of living organisms that could cause the spreading of living organisms. In 1864, he conducted an important experiment that showed that dust could spread organisms that would cause meat to spoil. He reaffirmed these results in other experiments, in which he showed that no bacteria appeared in boiled sugar solutions that were in contact with air, as long as bacteria were first removed from the air by filters. Once and for all, Pasteur had disproved the theory of spontaneous generation that had occupied scientists for previous centuries.
Rudolph Carl Virchow, a German pathologist, challenged Pasteur's ideas, because Virchow thought that all cells arose from cells. He refused to accept the possibility that disease could travel in microscopic form, but thought that it had to be passed by direct contact from one cell to another.
Today, a revised version of the theory of spontaneous generation exists in the scientific community; this question of spontaneous generation deals with the origins of life on Earth. These origins are a mystery, because the elements that currently comprise the Earth and other planets were made in the interior of stars--environments where the existence of life is impossible. Yet, somehow life formed. The most popular theory at present is that life did indeed erupt spontaneously from nonliving matter, but this process happened over a period of perhaps a billion years. Some experiments have demonstrated the strong possibility of spontaneous generation. Before our planet was filled with living material, it was called prebiotic Earth. When gas samples are struck by lightning in simulated electric storms, most of the fundamental building-block molecules of present-day life appear. These experiments led to the notion that life would have developed inevitably and spontaneously, given the compounds and conditions of matter on the prebiotic Earth.
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