Spontaneous generation
Until the middle of the seventeenth century it was common belief that God had created man and other higher organisms, while amphibians, worms, insects and in general smaller animals would have spontaneously generated from mud or substances in decomposition.
This belief has very distant origins. In ancient China, for example, people thought that aphids spawned spontaneously from bamboos, and sacred documents from India testify to the spontaneous birth of flies from sweat and dirt.
According to the Babylonians, the mud of the canals generated worms.
For the Greek philosophers, life was inherent in matter itself and emerged spontaneously from it when conditions became favorable: they believed, for example, that fish and frogs were generated from the putrid mud of the Nile.
Aristotle collected the ideas relating to spontaneous generation formulated by the philosophers who lived before him and synthesized them in a theory whose effects have been felt until very recently. According to the great thinker of antiquity, living organisms are generally born from other similar organisms, but sometimes they can also arise from inert matter.
In fact, in all things, there is a "passive principle" represented by matter and an "active principle" represented by form. This would be a kind of internal force that guides and directs the matter itself, giving it its shape. Mud, for example, is inert matter, but it contains an active principle that is nothing material, but simply an inclination, a predisposition to organize matter into something alive, such as a worm or a frog.
From the ideas of Aristotle the biblical account of Genesis would have taken inspiration according to which God in six days created the world from primitive chaos: first by separating the earth from the water and the sky then creating the herbs, the fish, the birds and the rest of the world. He created finally the man, whom he fashioned from clay and then infused it with the breath of life.
The spontaneous generation theory passed unscathed through the Middle Ages and the Renaissance and was supported by illustrious thinkers such as Newton, Descartes and Bacon. In the sixteenth century there was still some willing to believe that geese were born from some trees that were in contact with the waters of the ocean or that lambs were formed inside melons, as some travelers were saying on their return from long journeys. in the East.
In the seventeenth century the era of legends ended and the first experiments began to support the theory of spontaneous generation. The Flemish doctor Jean Baptiste Van Helmont, announced (seriously) that he had conducted an experiment by bringing together wheat grains and a dirty shirt, following which mice would be born after 21 days.
According to Van Helmont, human sweat would have represented the active principle necessary to push inert matter to transform into living matter.
The first experiments
It is evident that Van Helmont's experiment was a badly conducted, however the right path, that of the experimental verification of ideas, had been opened. In 1668 the Tuscan doctor and poet Francesco Redi illustrated a series of experiments which should have shown that spontaneous generation does not exist.
He put some veal and fish in some containers which he hermetically sealed, leaving others open. After some time he could notice the presence of worms (in reality they were insect larvae) on the rotting meat inside the open containers in which flies and other insects entered and exited freely, while there was no trace of organisms living inside closed containers.
Around the same time that Redi was carrying out his experiments, a Dutch naturalist, named Anton Van Leeuwenhoek (1632–1723), observed, for the first time, the presence of microorganisms through a rudimentary microscope he built himself. Microscopic observations soon multiplied and the presence of such an abundant number of microorganisms within all the substances examined revived the idea of spontaneous generation, which Redi's experiments seemed to have pushed away.
Leeuwenhoek's observations stimulated new research in that direction and the dispute between the theory of biogenesis (life derives from life) and the theory of abiogenesis (life originates from non-living substances) moved from the macroscopic world of worms and flies to that microscopic view of protozoa and bacteria.
In 1745, the English naturalist John Needham conducted a series of experiments that gave new vigor to the thesis of abiogenesis. He heated various nutritional liquids such as chicken broth or herbal infusions with which he filled some test tubes which he then capped with gauze.
Well, despite all the precautions taken to ensure that nothing entered the test tubes that contained the nutritional solutions made sterile by the heat, after a few days it could be seen that these were teeming with living organisms. The results of his experiments convinced him that spontaneous generation was indeed possible.
The same results did not convince the Italian abbot Lazzaro Spallanzani who, a few years later, repeated the same experiments as Needham but heating the nutrient liquid much longer and at much higher temperatures, until it boiled for a few minutes.
Well the result was that even after many days the liquids contained in the tubes, this time hermetically sealed, remained clear and did not show the presence of microorganisms inside them.
The criticisms now came from the English researcher who accused Spallanzani of having heated the nutrient liquids to too high temperatures, unnecessarily "torturing" the substances present to the point of destroying the active principle contained in them; and to have also sealed the test tubes to the point of preventing the passage of air essential for life.
The controversy continued for many years and ended definitively in the mid-nineteenth century when the French biologist Louis Pasteur devised an experiment that would put an end to a question that seemed unsolvable.
The experiment was conducted inside a very simple, but ingenious apparatus, built in such a way that it could no longer give rise to doubts about the possibility that the active principle could be destroyed by heat or suffocated due to lack of air.
Pasteur personally built glass containers with a long curved neck (called, due to their shape, "swan neck bottles"), inside which the nutrient solution was placed and boiled for more than an hour, leaving that the steam came out freely from the terminal orifice of the curved neck. When the flame was extinguished, the liquid contained in the container began to cool slowly after drawing from the outside air contaminated by bacteria and other microorganisms from the outside due to the depression resulting from the heating. These, however, in contact with the still boiling liquid that they found inside, were killed. After a few months the infusion was kept clear, demonstrating that there were no germs of any kind, while on the outermost part of the neck the presence of dust and microorganisms evidently entered through the terminal opening could be seen.
Pasteur's experiment was simple and complete just as experiments should be to appear convincing, and it responded clearly and unambiguously to the objections advanced by proponents of abiogenesis. In fact, they argued that if the nutrient liquid was brought to a boil for a long time, the active principle was destroyed and life could no longer develop.
Pasteur instead showed that any active principle present in the matter was not damaged by the long and intense heating. In fact, if the twisted neck had been broken with a sharp stroke of a file, placing the nutritional liquid in direct contact with the air, after a few hours it would have become cloudy due to the presence of spores and germs which would then have continued to develop. Moreover Pasteur by leaving his container open with the curved neck and allowing the air to enter and exit freely, albeit through a long and tortuous path, extinguished in the bud the objections of those who argued that the active principle, without air, was prevented. in its function as generator of life.
The theory of biogenesis
According to Arrhenius, life would then move from one planet to another in the form of spores or germs that would be pushed by the radiation pressure of the stars until they meet another planet on which to glide and then evolve towards more complex life forms.
Biologists, however, are well aware that a journey in the interstellar medium, especially if it is very long, is by no means without risks: there are radiations of all kinds (ultraviolet rays, X rays, gamma rays), temperatures close to absolute zero, or vice versa very high temperatures in the vicinity of the stars, capable of destroying any form of life, even if placed inside thick protective casings.
To overcome the risks associated with the bombardment of cosmic radiation it was then thought that the seeds of life could have traveled inside meteorites where in fact the presence of some organic compound was found. In this case, however, the difficulties would have arisen at the moment of entry into the atmosphere, when the heating due to friction and the subsequent crash to the ground could have caused the killing of the germs previously protected by the solid crust of the vehicle. The finer interplanetary dust would instead have a soft impact with the atmosphere and the ground and would therefore seem more suitable than meteorites for the transport of life germs on the planets, but in this case it would be the very severe conditions encountered in cosmic spaces that make it extremely the journey is dangerous.
There are even those who have thought, in order to overcome the difficulties connected with the crossing of cosmic spaces, of a kind of artificial panspermia, that is, forms of life sown by the hands of intelligent beings. The idea came to Francis Crick, Nobel Prize for Medicine in 1962, who imagines that some members of an extraterrestrial civilization visiting the planets of the galaxy have landed on ours and have left, perhaps accidentally, some residue of their excursion.
The real flaw of the panspermia theory lies in the failure to explain the real origin of life: if living beings necessarily derive from other living beings, then life would never have been born, but it would have always existed.
However, most biologists don't like the idea that life originates outside our planet, just as most astronomers don't like the idea of a universe that has always existed. Of course, both of them have good reasons to oppose these hypotheses. If life and the Universe that contains it had always existed, some fundamental laws of physics would be violated, such as the second law of thermodynamics which provides for a continuous and incessant increase in entropy, that is, in general disorder. Now, if the Universe had always existed, this general disorder would have been reached for an infinite time and today there would be no ordered structures that we can observe in it, starting precisely from life.
