During the dark ages, people speculated that rats and flies arose spontaneously from garbage because they mysteriously appeared when garbage was left out.
Louis Pasteur entered the debate in 1862 when he published the results of his experiments on the spontaneous generation of microorganisms in broths.
He proved that microorganisms were transported through the air to the broth and not generated from the broth itself.
The work of Pasteur seemingly ended the debate on the question of the sudden, spontaneous origin of life.
Ernst Haeckel, one of the chief proponents of Darwinism, stated in 1876: "If we do not accept the hypothesis of spontaneous generation, then at this one point in the history of evolution we must have recourse to the miracle of a supernatural creation." The spontaneous generation debate heated up again in 1924 when Russian biochemist, I. Oparin proposed that life had arisen from simpler molecules on the lifeless earth under much different atmospheric conditions than exist today. In the nineteenth century Ernst Haeckel argued that although spontaneous generation was not observable under the current conditions on earth, it did take place in the past under different chemical conditions.
However, instead of life arising suddenly, as previous spontaneous generation theories proposed, Oparin believed that it occurred over a very long period of time. Oparin and Haldane made the first serious proposals regarding those conditions.