Life on Earth will have appeared about 3400 M.a., as the prokaryote fossils found in South Africa seem to demonstrate.
Eukaryotic cells appeared about 2000 to 1400 M.a., followed by multicellular organisms about 700 M.a. In this time the fossils are abundant, indicating a rapid evolutionary process.
Until the nineteenth century all living beings were considered to present themselves as they had always been. All Life was the work of an all-powerful entity, a fact that served to mask the lack of sufficient knowledge to create a rational explanation.
This theory, the CreationismHowever, as early as ancient Greece was not satisfactory. In order to circumvent the need for divine intervention in the creation of species, several alternative theories arise, based on the observation of natural phenomena, as far as the knowledge of the time allowed.
Aristotle He elaborated one of these theories, which he accepted for centuries with the help of the Catholic Church, which adopted it. This theory considered that Life was the result of the action of a active principle about inanimate matter, which then became animated. Thus, there would be no supernatural intervention in the emergence of living organisms, just a natural phenomenon, the spontaneous generation.
These ideas lasted until the modern era, because Van Helmont (1577 - 1644) still considered that "swamp smells spawned frogs and that dirty clothes spawned fully formed rats." It was also considered correct by naturalists that the intestines spontaneously produce worms and that the rotten flesh generated flies. All these theories considered possible the emergence of Life from inanimate matter, whatever the catalyst for such a transformation, and hence they fall under the general designation of Life. Abiogenesis.
In the seventeenth century Francisco Redi, naturalist and poet, he opposed Aristotle's ideas, denying the existence of the active principle and arguing that all living organisms sprang from egg insemination and never by spontaneous generation.
To demonstrate the truth of his theory, Redi conducted an experiment that became famous for being the first recorded to use a control in his experiments. Put meat in 8 jars. Sealed 4 of them and left the remaining 4 open, in contact with air.
Within a few days he found that the opened vials were full of flies and other worms, while the sealed vials were free of contamination.
This experience seemed to unequivocally deny the abiogenesis of macroscopic organisms, having been accepted by the naturalists of the time.
However, the discovery of the microscope came to raise the question again. The theory of abiogenesis was partially rehabilitated because it seemed the only one capable of explaining the development of microorganisms visible only under the microscope.
This situation continued until the late eighteenth century, when the subject was again debated by two famous scientists of the time, Needham and Spallanzani.
Needham used several infusions, which he put in vials. These vials were heated and left for a few days. He noted that infusions were rapidly invaded by a multitude of microorganisms. Interpreted these results by spontaneous generation of microorganisms, by action of the active principle of Aristotle.
Spallanzani used in his experiments 16 vials. It boiled several infusions for one hour and put them in vials. Of the 16 bottles, 4 were sealed, 4 tightly capped, 4 capped with cotton and 4 left open. It found that the proliferation of microorganisms was proportional to the contact with air. He interpreted these results as air containing eggs from these organisms, so that all Life would come from a pre-existing one.
However, Needham did not accept these results, claiming that excessive boiling would have destroyed the active principle present in the infusions.
The controversy continued until 1862, when the French Louis Pasteur, It definitely ended the idea of spontaneous generation with a series of experiments retained for posterity by the French museums. Pasteur put several infusions into glass balloons in contact with the air. He stretched the necks of the balloons to flame so that they made several turns. It boiled the liquids until steam escaped freely from the narrow ends of the balloons. He found that after cooling the liquids remained unchanged in both odor and taste. However, they were not contaminated by microorganisms.
To eliminate Needham's argument, he broke a few balloon necks, verifying that the liquids were immediately infested with organisms. It thus concluded that all microorganisms were formed from any kind of airborne solid particle. In intact balloons, the slow entry of air through the narrow, curved necks caused these particles to deposit, preventing contamination of infusions.
It has definitely been proved that under current conditions life always arises from another, pre-existing life.