It feels like spontaneous generation: the old belief that life could just appear from lifeless matter. For most of history, people thought that living critters—little ones especially—could just materialize into being. It wasn’t until the 17th century that Francesco Redi, an early scientist, began to challenge this idea. He showed that if you kept flies away from rotting meat, no maggots would appear.

Even after Redi’s experiments, belief in spontaneous generation persisted for centuries. In fact, even Redi himself didn’t entirely give up on the idea. While he was disproving spontaneous generation for flies, he still believed in it for other invertebrates. He speculated that plants and animals had some mysterious force within them that could create life. Roundworms might generate inside intestines, or suddenly you’d just be lousy with lice even when there hadn’t been a single other louse in the house. The reason he didn’t just abandon spontaneous generation altogether is that the life cycles of some parasites can be extremely complicated. Without today’s powerful scientific tools, at some point Redi threw up his hands and called it all sorcery.

My big takeaway is that even Redi, one of the people famous for having refuted spontaneous generation, still hedged his bets when reality got too complex. It wasn’t his fault. Settling the question required centuries of additional scientific research, debate, and technological advancements. Only in the 19th century did Louis Pasteur’s work with bacteria finally prove that life doesn’t just hocus-pocus out of thin air.