Until the mid-nineteenth century, scientists believed that living things were spontaneously generated from the body of decaying corpses; that frogs, snakes and crocodiles were generated from river sludge.
This interpretation of the origin of living beings became known as hypothesis of spontaneous generation or abiogenesis (The= negation prefix, bio = life, genesis = origin; origin of life from raw matter).
Researchers then challenged the spontaneous generation hypothesis, presenting arguments in favor of the other hypothesis, that of biogenesiswhereby all living things originate from other pre-existing living things.
Biogenesis versus Abiogenesis
In 1668, Francesco Redi (1626-1697) investigated the alleged origin of worms in decaying bodies. He noted that flies are attracted to the decaying bodies and lay their eggs on them. From these eggs come the larvae, which turn into adult flies. Because the larvae are wormlike, the "worms" that occur in decaying corpses would be nothing but flyworms. Redi concluded, then, that these larvae do not arise spontaneously from the decomposition of corpses, but are the result of hatching eggs laid by flies attracted to the decomposing body.
To test his hypothesis, Redi performed the following experiment: he placed pieces of raw meat in jars, leaving some covered with gas and others completely open. According to the hypothesis of abiogenesis, worms or even flies born of the decomposition of the flesh itself should appear. This, however, did not happen. In the jars kept open there were eggs, larvae and flies on the meat, but in the gauze-covered jars none of these forms were found on the meat. This experiment confirmed Redi's hypothesis and proved that there was no spontaneous generation of worms from decaying bodies.
Redi's experiments were able to reinforce the hypothesis of biogenesis until the discovery of microscopic beings, when a part of scientists again considered the hypothesis of abiogenesis to explain the origin of these beings.
According to these scientists, microorganisms arise spontaneously everywhere, regardless of the presence of another living being. Another group of researchers did not accept these explanations. For them microorganisms only emerged from “seeds” present in the air, water or soil. These "seeds," finding suitable sites, proliferated (interpretation consistent with the biogenesis hypothesis).
The Needham and Spallanzani Experiments
In 1745, the English scientist John T. Needham (1713-1781) carried out several experiments in which he boiled vials containing nutritive substances. After boiling, I closed the bottles with stoppers and left them for a few days. After examining these solutions under a microscope, Needham observed the presence of microorganisms.
His explanation of his results was that microorganisms appeared by spontaneous generation. He said that the nutrient solution contained a "vital force" responsible for the emergence of living forces.
Later, in 1770, the Italian researcher Lazzaro Spallanzani (1729-1799) repeated Needham's experiments, with some modifications, and obtained different results.
Spallanzani placed nourishing substances in glass balloons, sealing them tightly. These balloons were prepared in boilers with water and boiled for a while. Let it cool for a few days and then he would open the vials and watch the liquid under the microscope. No organism was present.
Spallanzani explained that Needham had not boiled his nutrient solution long enough to kill all the microorganisms in it and thus sterilize it. Needham responded to these criticisms by saying that by long boiling the nutritive substances in hermetically sealed containers Spallanzani had destroyed the "life force" and made the air unfavorable for the appearance of life.
In this controversy, Needham came out strong.