According to the hypothesis of spontaneous generation , or abiogenesis , some life forms could originate from inanimate (non-living) gross matter, and life force, a kind of active principle, would allow brute matter to become alive. In this perspective, defended by Aristotle (384-322 BC), from the decomposing flesh would arise, by means of the vital force, insect larvae.
Similarly, the tadpoles would appear in puddles of water with mud, for the mud could be transformed, under the effect of the vital force, in being alive.
In the late Middle Ages, the idea of spontaneous generation was widely advocated and discussed by leading scientists such as William Harvey (1578-1657), René Descartes (1596-1650), and Isaac Newton (1643-1727).
Descriptions of experiments of that time have already been considered as recipes for the spontaneous generation of living beings, such as the emergence of mice from wheat grains, advocated by Jan Baptista van Helmont (1579-1644) in Brussels.
Helmont defended the idea that by putting wheat grains in contact with a sweaty shirt in a container, mice would appear after about 21 days, as the “leaven” of the shirt would be modified by the odor of the grain, and wheat would suffer a “Transmutation”, transforming into mice.
According to him, the grains of wheat became mice because of the active principle contained in human sweat. Certainly, if Helmont’s “recipe” were subjected to rigorous experimentation, as in the current science, it would be possible to observe that the mice come from another environment, attracted by the grains of wheat.
During the seventeenth century, the idea of spontaneous generation began to face contrary evidence. One well-known case was that of the Italian physician Francesco Redi (1626-1691), a scholar of the life cycle of flies that arose from vermiform organisms in the decaying flesh of dead animals.
After much observation, Redi realized that these beings were transformed into several species of flies. To test the idea that the vermiform animals came from the eggs deposited by the flies in the decaying flesh, he conducted a series of experiments.
The most famous was to put pieces of decaying meat into jars. Some bottles were left open, and others were closed with gauze. In this experiment, Redi verified the appearance of larvae in the open flasks; in the closed bottles, larvae did not appear.
Thus, the physician showed that the larvae found on putrefying flesh and adult flies originated from eggs laid by other flies. Even so, Francesco Redi was adept at spontaneous generation. He believed, for example, in the spontaneous appearance of intestinal worms. For him, the non-spontaneous generation of flies was just a specific case.
Redi’s experiment, however, allowed other scientists to check the biogenesis hypothesis that life must come from another preexisting life form, not gross matter.
An example well known and favorable to the hypothesis of spontaneous generation was some experiments conducted by John Turberville Needham (1713-1781).
He prepared, inside jars, culture media containing chicken broth and fruit juices. These bottles were boiled and then sealed with cork stoppers. After a few days, the vials were cloudy, indicating the presence of microorganisms.
Needham was investigating heterogeny, that is, the emergence of a living being based on the matter of another living being. The results of his experiments were considered favorable to the hypothesis of spontaneous generation, since, in this perspective, the
The Italian Lazzaro Spallanzani (1729-1799), one of the adherents of biogenesis, repeated Needham’s experience, but with some differences. The same type of culture medium was used, which was submitted to prolonged boiling, but the bottles were hermetically sealed, ie the bottlenecks were melted and sealed, avoiding the contact of the culture medium with air.
Spallanzani obtained different results from those of Needham: the vials remained clear, without indication of contamination by microorganisms. Spallanzani said that Needham had left the culture media contaminated with microorganisms, and these would give rise to new populations.
Needham claimed that Spallanzani, with his procedure of heating the jars a lot, had destroyed the active principle and a new active principle, present in the air, could not come in contact with the broth, since the tubes were closed, the formation of microorganisms.
This discussion lasted until the nineteenth century, when, around 1860, Pasteur’s experiments began to clarify it.