For centuries, people across the world believed that certain animals came from nonliving sources—a theory known as “spontaneous generation.” Sacred documents from India remark on lice coming from sweat, Babylonian inscriptions mention that worms come from canal mud, the ancient Chinese thought that aphids came from bamboo, and other cultures believed that maggots were the product of rotting meat or that mice were created by soiled clothing mixed with wheat grains. Many Greek philosophers also believed that creatures appeared from lifeless matter whenever the conditions were right. 

While today such a thought might seem ludicrous, it is easy to understand how people without recourse to modern science arrived at this point. It was hard to explain why bread wrapped in cloth and put ...