Two explanations dominated prescientific thinking about the origin of life: special creation and spontaneous generation. According to the former view, supernatural intervention was essential for the creation of life; according to the latter, living organisms could form spontaneously—for example, from the mud of the Nile. Not surprisingly, special creation was usually favored as an explanation of the origin of humans and the higher animals, whereas spontaneous generation seemed adequate to explain the origin of insects, frogs, and even mice.

The theory of spontaneous generation came under attack in the seventeenth century when the Italian scientist Francesco Redi showed that maggots do not arise spontaneously in rotting meat but develop from eggs laid by flies. The spontaneous generation controversy persisted for another two hundred years or so until the classic experiments of Louis Pasteur convinced almost everyone that even microorganisms appear only as the...