The theory of spontaneous generation proposes that life can arise spontaneously from nonliving matter. One of the first scientists to challenge this theory was the Italian physician Francesco Redi (1626–1698). In 1668, Redi performed an experiment to show that meat placed in covered containers (either glass-covered or gauze-covered) remained free of maggots, while meat left in an uncovered container eventually became infested with maggots from flies laying their eggs on the meat.

The controversy over spontaneous generation was finally solved in 1861 by French chemist Louis Pasteur (1822–1895). He showed that the microorganisms found in spoiled food were similar to those found in the air—and thus concluded that the microorganisms on spoiled food were from the air and did not spontaneously arise.