Spontaneous generation (abiogenesis), a long-held theory that life springs up from non-living or decaying organic matter, was based on observations of rotting food seemingly producing living organisms. Francesco Redi, a respected philosopher at the court of the Medici Grand Duke in Tuscany, was the first scientist to question the idea of spontaneous generation. By setting up a simple experiment in which decaying meat was placed in three jars, one uncovered, one sealed, and one covered by mesh, allowing air to circulate, he demonstrated that only the open jar which flies could access produced maggots. Thus, decaying meat does not spontaneously produce maggots. Partially due to the simplicity of Redi’s experiment (anyone could reproduce it), people began to doubt spontaneous generation.
It is important to note that what Redi and others demonstrated is that life does not currently spontaneously arise in complex form from nonlife in nature; they did not demonstrate the impossibility of life arising in simple form from nonlife by way of a long and propitious series of chemical steps/selections under conditions that do not exist on Earth today. In particular, they did not show that life cannot arise once, and then evolve. Neither Pasteur, who put to rest the notion of spontaneous generation for microorganisms, nor any other post-Darwin researcher in this field denied the age of planet Earth or the fact of evolution.