Francesco Redi hypothesized that maggots on rotting meat did not arise from the meat itself but came from eggs laid by flies. In 1668, this idea directly challenged the centuries-old belief in spontaneous generation, the notion that living creatures could spring into existence from nonliving matter. Redi’s hypothesis is often summarized in the Latin phrase “Omne vivum ex vivo,” meaning “All life comes from life.”
The Belief Redi Set Out to Challenge
For most of recorded history, people assumed that rotting organic matter simply produced living organisms on its own. The idea traces back at least to the ancient Greek philosopher Aristotle, who argued that life could arise from nonliving material, and it persisted well into the 1600s. It wasn’t just a folk belief. Educated scholars and physicians accepted it as fact.
The logic seemed obvious from daily life. In a world without refrigeration, a trip to the butcher shop meant seeing carcasses hanging in the open air, swarming with flies. Meat left out at home would soon crawl with maggots. To most observers, the conclusion was straightforward: the decaying meat was generating the maggots. Redi, an Italian physician, suspected the relationship was far simpler. Flies were visiting the meat, laying eggs, and those eggs were hatching into maggots.
How Redi Tested His Idea
Redi designed what is now considered one of the earliest controlled scientific experiments. He placed pieces of meat into multiple jars and divided them into groups with different conditions. Some jars were left completely open to the air. Others were sealed shut. A third group was covered with gauze, a fine mesh that allowed air to reach the meat but physically blocked flies from landing on it.
The setup was elegant because it isolated the one variable that mattered: whether flies could make contact with the meat. If spontaneous generation were real, maggots should appear on the meat regardless of whether the jar was open, sealed, or covered. The meat in all three groups would rot the same way, so the decaying material itself was held constant. Only access by flies changed between groups.
What the Experiment Showed
The results cleanly supported Redi’s hypothesis. Maggots appeared on the meat in the open jars, where flies could freely land and lay eggs. The sealed jars produced no maggots at all. And the gauze-covered jars were the most telling: flies were attracted to the smell of the rotting meat and gathered on top of the gauze, sometimes even depositing eggs on the fabric itself, but no maggots appeared on the meat inside. The meat rotted in every jar, yet maggots only showed up where flies had direct access.
This was powerful evidence that flies are necessary to produce flies. The rotting meat played no creative role. It was simply food, not a source of new life.
Why This Experiment Still Gets Taught
Redi published his findings in 1668 in a work titled Esperienze intorno alla generazione degl’insetti (Experiments on the Generation of Insects). Beyond its conclusion about maggots, the experiment is historically significant because Redi essentially invented the controlled experiment as a scientific method. He kept conditions identical across his groups except for one variable, then observed the outcome. That framework became the backbone of modern experimental science.
A 2025 review in Trends in Parasitology described Redi as the inventor of scientifically controlled experimentation and the father of parasitology, noting that he demonstrated insects arise only from eggs laid by other insects, not spontaneously from decaying matter.
Where Redi’s Own Thinking Fell Short
Interestingly, Redi did not apply his conclusion to all living things. While he correctly showed that maggots come from fly eggs, he still believed spontaneous generation explained other organisms. He accepted that gall flies, the tiny insects found inside abnormal growths on plants, were generated by the plant tissue itself. He also believed intestinal worms arose spontaneously inside the bodies of their hosts.
This inconsistency is worth noting because it shows how deeply embedded the idea of spontaneous generation was, even for the person who delivered one of its strongest experimental refutations. Redi could see the evidence clearly when it came to maggots and meat, but the origin of parasites living inside other organisms remained a mystery that wouldn’t be fully resolved for another two centuries. It took later scientists, most famously Louis Pasteur in the 1860s, to put the theory of spontaneous generation to rest for microorganisms as well.
Still, Redi’s core hypothesis held: maggots on meat are not born from rot. They are the offspring of flies, hatched from eggs too small for most people to notice. That insight, backed by a brilliantly simple experiment, marked one of the first major steps toward understanding how life actually reproduces.