Francesco Redi, an Italian physician and naturalist, conducted a landmark experiment in 1668 that challenged the long-held scientific belief in spontaneous generation (abiogenesis). This theory proposed that living organisms could arise from non-living matter, such as maggots emerging from putrefying meat. Redi’s work aimed to test this ancient idea by focusing specifically on the appearance of maggots on decaying flesh. His rigorous methodology helped lay the groundwork for the scientific method and ushered in a more evidence-based understanding of the origin of life.
Redi’s Central Hypothesis
Redi hypothesized that the maggots observed on rotting meat were not a product of the meat itself, but the larval offspring of flies. This meant that life arose from pre-existing life (biogenesis), rather than spontaneously from decaying material. Specifically, Redi suggested that flies must first land on the meat and lay eggs, which then hatch into the visible maggots. He believed that if flies were physically prevented from accessing the meat, no maggots would appear, thereby disproving the common assumption that decay caused life.
Designing the Controlled Experiment
To test his hypothesis, Redi designed an experiment using wide-mouthed glass jars containing pieces of meat. He established three distinct conditions to isolate the variable of fly access.
Condition 1: Open Jars
The first set of jars served as the control group, left completely open to the environment. The meat in these jars was fully accessible to flies, mirroring natural conditions.
Condition 2: Sealed Jars
A second set of jars was tightly sealed, preventing both the entry of flies and the circulation of air. This condition tested spontaneous generation directly, but critics later argued that sealing prevented the entry of a supposed “life force” in the air.
Condition 3: Gauze-Covered Jars
To address this criticism, Redi introduced a third set of jars, covering them with a fine mesh or gauze. This porous covering allowed air to flow freely over the meat, satisfying the requirement for the atmospheric “vital force,” while simultaneously acting as a physical barrier to keep out the adult flies.
Interpreting the Experimental Results
After several days, Redi observed the contents of the jars and recorded his findings across the three conditions. In the open jars, flies were seen landing on the meat, and shortly after, maggots appeared directly on the decaying flesh. The sealed jars, which were completely inaccessible to flies, remained entirely free of maggots, supporting his initial suspicion.
The most telling results came from the gauze-covered jars, which addressed the critics’ concern about air. Maggots did not appear on the meat in these jars, but Redi observed fly eggs and sometimes maggots on the mesh itself, just above the meat. This observation demonstrated that flies were attracted to the meat and tried to lay their eggs on it, but the physical barrier prevented the eggs from reaching the flesh. The findings conclusively showed that the maggots arose only when flies could access the meat, providing strong evidence that maggots were the result of reproduction by flies, not spontaneous generation.