Introduction
Theories on the Origin of Life
Since the emergence of science as a subject and even before then, many philosophers that we currently refer to as scientists today have inquired into how life came into existence. Many theories (hypotheses rather) - ranging from the reasonables, to the less reasonables, and to the absurds - have been formed and disproved.
Many religious folks believe that the question on the origin of life is absolutely not necessary and that there is a supernatural being somewhere that created life. This idea originated in the 19th century and is today known as the theory of creationism.
Away from creationism, some beliefs were also previously held that life must have arose from another planet. The seeds of life must have followed the interplnetary transfer of non-living materials such as comets to the earth. The seeds eventually germinated and gave rise to life. This theory is referred to as the panspermia theory.
Many of the ancient philosophers-turned-scientists that hypothesized about the origin of life were Greeks. Some of the most prominent among them were Thales, Anaximander, Anaximenes, and Aristotle. Thales believed that life, and of course, everything the earth contains arose from water. Anaximander also supported Thales idea and went as far as saying that man use to be some sort of fishes at some points in their lives. This idea by Anaximander formed the basis of evolution theory on which Darwin later built on.
Anaximenes in his own wisdome beleieved that life arose from air. According to him, natural forces have the tendency to act on air and transform it into virtually anything, including life. He went as far as holding the air in divinity.
One of the most ridiculous idea about the origin of life was perhaps, the idea by Aristotle that life arose from nothing - spontaneous generation. Aristotle did not stop at just hypothesizing about the spontaniety of life but tried to prove it by coundcting several experiments. One of such experiments is the jar-meat experiment.
Making use of two glass jars, Aristotle proved the veracity of spontaneous generation by observing the emergence of maggots from the lump of meats kept in the glass jar after some days. He was convinced beyond reasonable doubts that life could arose from non-living objects. If otherwise, how could mice have arose from a stack of corn? How about the emergence of fish from the muds of dry lakes?
Aristotle must have been feeling fly based on his discovery until another scientist from Italy, Francesco Redi, performed an experiment to disprove the theory of spontaneous generation. Making use of glass jars similar to the ones used by Aristotle, Redi placed a lump of meat in the jars but this time, unlike Aristotle, he covered one of the jars and left the other opened. Observation after a few days showed that maggots only emerged from the jar left uncovered. Redi then hypothesized that something must have brought the maggots into the meat - they never spontaneously emerged from it.
The news from Italy did not sit well with Needham, a scientist in England. He conducted an experiment by heating a broth (to kill off microbes) and then sealing off the mouth of the vessel used for boiling. The presence of microbes in the broth a few days after convinced Needham that Redi was wrong with his observation and Aristotle was probably right with his theory of spontaneous generation.
The claim by Needham was investigated by Lazzaro Spallanzani. He repeated Needham's experiment but this time, he boiled two vesseles containing brough, sealed one off and left the other uncovered. Spallanzi observed teeming growth of microbes in the broth left uncovered but witnessed no sign of life in the covered one. He pointed out that Needham's brought was simply not heated enough to kill off the microbes before sealing the container.
Scientists were still not totally convinced by the report of Spallanzani enough to jettison the idea of spontaneous generation. They claimed that the sealing off of the vessel prevented air from getting access into the brough and it might just be that air is necessary for spontaneous generation. Spontaneous generation was eventually put to rest by the work of Lous Pasteur.
Pasteur was able to repeat the experiment of Spallazani but modified it by by using Swan neck flasks. He observed no growth for a long time after boiling the broth in the flask. He then decided to cut off the Swan neck of the flask and repeated the experiment. With this, he observed the growth of microbes in the broth just after a few days. The Swan neck, even though permitted the entry of air into the broth, was able to trap dust and other air particles and prevent them from getting to the broth. He then concluded that the air contains microbes and this put spontaneous generation to perpetual rest.
Resources
https://iep.utm.edu/anaximen/
https://iep.utm.edu/anaximan/
https://iep.utm.edu/thales/
https://en.wikipedia.org/wiki/History_of_creationism
https://www.infoplease.com/math-science/biology/genetics-evolution/origin-of-life-spontaneous-generation