It was once widely accepted that the planet was flat and the sun orbited the Earth. It was also commonly believed you could make a mouse by placing a dirty shirt in a container of wheat.
"If a soiled shirt is placed in the opening of a vessel containing grains of wheat, the reaction of the leaven in the shirt with fumes from the wheat will, after approximately 21 days, transform the wheat into mice," wrote 17th-century Flemish chemist Jean Baptiste van Helmont.
This "recipe" for mice was a product of the body of thinking known as spontaneous generation.
People didn't have an explanation for how maggots materialized on old meat or how mollusks seemed to appear in the sea, so they surmised that such creatures simply arose from inanimate matter.
The idea wasn't unique to Europeans like Helmont. The Babylonians, Indians and Chinese drew similar conclusions based on their own observations.
Although people across the globe came up with a variety of ways in which animals were created, Helmont's "recipes" are perhaps the most creative.
For example, take his method of scorpion manufacture: Carve an indentation in a brick, fill it with basil, cover the hole with another brick and place it in the sun.
In a matter of days, "fumes from the basil, acting as a leavening agent, will have transformed the vegetable matter into veritable scorpions," he wrote.
How did other animals supposedly spontaneously generate?
Bees were once thought to swarm from the corpses of partially buried dead bulls.
Salamanders were born from fire. It's likely that people sitting around fires saw the unfortunate animals — which often hide in logs — crawling out of the flames.
Because cities lacked sewage systems and proper waste disposal methods, they threw trash into the streets, which attracted rats and led people to believe rats were generated from garbage.
After observing flooded rivers leaving behind both mud and frogs, it was assumed that wet soil created the amphibians.
He said they hung from their beaks along the sea until they grew feathers and either fell into the water or flew away.
"I have frequently seen, with my own eyes, more than a thousand of these small bodies of birds, hanging down on the sea-shore from one piece of timber, enclosed in their shells, and already formed," he wrote.
How it was disproved
Aristotle first put forth the concept of spontaneous generation, and his ideas were widely believed for centuries.
Italian physician Francesco Redi, who suspected maggots developed on old meat from flies' eggs instead of simply appearing out of nowhere, was the first scientist to attempt to disprove spontaneous generation.
In 1688, he put meat in several flasks, leaving some open to the air, others sealed and others covered with gauze.
As he'd hypothesized, maggots appeared only in the open flasks and atop the gauze on the ones that were still able to let air enter.
Despite his experiment, belief in spontaneous generation remained popular until French chemist Louis Pasteur proved the existence of microorganisms in 1859.
Pasteur boiled meat broth in a flask and then heated the neck of the flask and twisted it into an S shape.
The flask's design allowed air to enter the flask; however, airborne microorganisms couldn't — they settled in the flask's neck. As a result, no microorganisms grew in the broth.
But when Pasteur tilted the flask so the broth reached the neck, it came in contact with the microorganisms and they flourished, leaving the broth cloudy.
With one experiment, Pasteur had both disproved spontaneous generation and confirmed that microorganisms are everywhere.
Related on MNN: