A NATURALISTIC idea is testable, as opposed to a supernaturalistic idea, and therefore resides within the realm of science. There have been only three naturalistic theories of the origin of life since the time of Aristotle. These are spontaneous generation, panspermia and chemical evolution.
The current scientific consensus as to the origin of life is the theory of chemical evolution culminating in the spontaneous eruption of life. I have written of this on several occasions in the past. The theory of panspermia (that pre-formed life was seeded on Earth from elsewhere in the universe) remains a respectable, if a small minority, opinion. Although the general theory of spontaneous generation was finally disproved over 100 years ago, it had an interesting history which I will outline in this article.
Spontaneous generation is the name that was given to the general idea that life can spontaneously and continually arise from non-living matter - from mud, refuse, etc. The idea also claimed that one form of life could directly turn into another form, e.g. grain could turn into mice, etc. Spontaneous generation of plants and animals was commonly accepted as fact until the middle of the 17th century.
The theory lived on until the middle of the 19th century in a slightly modified form, with microbes replacing higher forms of life as the products of spontaneous generation.
The medieval church accepted the doctrine of spontaneous generation. St Thomas Aquinas (1225-74) pronounced on the exact mechanism of spontaneous generation it was all drone by angels acting through the agency of the sun.
The 16th century was particularly enthusiastic about spontaneous generation. Jean Baptiste van Helmont (1577-1644), a disciple of Paracelsus (the Father of Modern Medicine) published a well known method for breeding mice - place grains of wheat in a jar, add an old vest, and wait. Two hundred years later Pasteur commented on van Helmont's recipe: "This proves it is easy to do experiments, but hard to do them flawlessly."
The doctrine of the spontaneous generation of animals finally died during the Renaissance, at the hands of Francesco Redi (1626-97). Redi was one of the first modern biologists. He was also a physician and a popular poet - a formidable Renaissance man. Insects were his main scientific interest, and his clever and careful experiments brushed aside the accumulated lore of 20 centuries regarding spontaneous generation.
Redi first of all observed what happened when dead eels were allowed to decay in an open box. He noted that the flesh quickly became covered with worms, which gradually increased in size and numbers. Redi described how these worms changed into pupae and finally into adult flies.
He carried out many such experiments using the flesh of a wide variety of animals and found that the same thing happened in every case. Redi also noted that, in addition to the worms, the decaying flesh was also peppered with eggs from which the worms hatched. He reasoned that it was most likely that these eggs were dropped on to the meat by flies, and that all the worms found on decaying meat came directly from fly droppings.
Redi tested his hypothesis with an experiment. He divided fresh flesh into two lots. He placed one lot in a flask, which he then sealed. He placed the other lot in a separate flask, which was left open to the air. After a short while the flesh in the open flask became wormy, and flies were seen readily entering and leaving the flask. Worms were not seen in the closed flask. In a further refinement of this experiment Redi allowed air to enter both flasks, but he protected one flask with a fine veil that prevented flies from entering, while still allowing air to circulate. The other flask was not protected with any veil. No worms developed in the flask protected against the flies, while worms appeared in the unprotected flask.
Redi's account of his experiments was published widely and belief in the spontaneous generation of animals died out among educated people. However, the doctrine of spontaneous generation was revived at another level. About 1675 micro organisms were discovered by the Dutch microscopist, Anton Leeuwenhoek (1632-1723).
Microbes were so small and simple that it was widely believed they existed in a twilight zone between life and non life, and it seemed intuitively "logical" that they could arise spontaneously as the products of decay. An English clergyman, J.T. Needham (1713-81), claimed that, for example, mutton gravy, heated to kill all existing micro organisms, and sealed in a vessel with some air, would, after a few days, spontaneously generate new micro organisms and decompose.
These experiments were repeated by an Italian physiologist Spallanzani, who showed that, if the flasks were heated after sealing, the contents did not putrefy and nothing grew in the flasks no matter how long they were kept. Need ham replied that heating the sealed flasks destroyed some vital element in the air which was necessary for spontaneous generation.
There the matter lay until Louis Pasteur (1822-95) eventually solved the problem. In a series of technically brilliant experiments, Pasteur eventually closed the long history of the spontaneous generation doctrine. He showed clearly that the "vital principle" in air responsible for the appearance of microbial life in sterilised broths is microbial life itself, carried on dust particles. Pasteur proved that life can only arise from pre existing life. The modern theory of the spontaneous origin of life on Earth about 3 1/2 billion years ago is considered to be a unique exception to the principle established by Pasteur.
In one of Pasteur's simplest experiments, he placed a yeast extract plus sugar into a glass flask and, using a flame, pulled out the neck of the flask to form a long narrow tube which he bent in various ways, for example into a U shape. The bore of the tube was very narrow but open. He next heated the medium in the flask to the boil for a few minutes and then let the flask cool down. The yeast extract remained sterile indefinitely, even though it was in contact with air.
Pasteur explained that the air in the long narrow neck acts as a buffer, damping down rapid movement of air, so that any dust attempting to enter the flask falls out and is held on the walls of the neck before it gets into the liquid contents of the flask. To show that the boiled yeast extract would actually support micro organism growth in the presence of dusty air, Pasteur cut the necks off some flasks and they soon sprouted microbial growths.
At that time, between a quarter and a half of patients receiving amputations in British hospitals died, mainly from infections. Joseph Lister (1827-1912) was familiar with the work of Pasteur. He reasoned that if he prevented micro organisms in the air from reaching incisions in his patients he would lower the incidence of infection. Up until then no special precautions were taken to exclude germs from wounds because they were believed to be generated spontaneously.
Lister decided to kill bacteria before they reached patients' incisions and he used carbolic acid as an anti bacterial agent. He sprayed the surgery and his instruments, and he applied carbolic acid to his patients' dressings. These measures were quite successful. Antiseptic surgery was born,