Spontaneous Generation (in detail)

The idea of spontaneous generation is a very old one. Even the ancient Greek philosopher, Aristotle, believed that many plants and small animals were formed from decaying soil or putrefying flesh and excrement.

The general idea arose that larger animals and human beings once did spontaneously arise from earth, but are now able to breed and reproduce without the need for spontaneous generation.

The appearance of maggots, flies, worms, mice, rats and the like amongst decaying animal or vegetable matter is something with which everyone is familiar, but today the idea that worms or mice arise from this material by spontaneous creation is completely bizarre. However, without twentieth century knowledge, the ancient Greeks and many later European scientists were quite prepared to believe in spontaneous generation.

As late as the sixteenth century, the eminent chemist, San Eaptista van Helmont (1579- 1644), made one of the few scientific mistakes of his life when he claimed to have demonstrated that mice could be generated spontaneously from dirty linen that was placed in an open container with a piece of wheat or cheese.

Another great scientist, Erancesco Redi (1626-1697), did some better controlled experiments in 1684 and demonstrated that maggots did not appear spontaneously on decaying meat if the meat was covered with gauze, although he observed that flies laid their eggs on top of the gauze and that these eggs developed into maggots. In other words, maggots came from flies, and not from rotting meat.

In the late seventeenth century, just when the evidence against spontaneous generation was accumulating, it raised its mischievous head once more, as a result of van Lecuwenhoek's observations of microscopic living beings.
It was accepted that mice and maggots might not, after all, be created daily from the dust of the Earth, but many scientists thought that spontaneous generation was the only plausible means by which the microscopic organisms could appear.

How could just one of these microbes produce more than a million offspring in a single day? It was much more reasonable at the time to think that each microbe was spontaneously created, although we know today that bacteria do indeed multiply extremely rapidly.

The discovery of micro-organisms started a debate between scientists who believed that they were generated spontaneously and those who believed they had microbial parents, in the same way as larger creatures.

Some observations seemed to support the concept of spontaneous generation of microbes. For example, it was known that yeast appeared in huge numbers during the fermentation of grape juice to wine, even if no yeast had originally been added to the grapes. we know now that grapes naturally contain yeast on their skin and that this multiplies when grapes are fermented: it does not appear spontaneously.

Other observations, such as the fact that prolonged heating of meat or other putrescible substances prevented them from decaying if they were kept inside a closed vessel free from the air, seemed contrary to the idea of spontaneous generation. when air was admitted, they then decayed.

Proponents of spontaneous generation argued that air was necessary for it to occur, whereas opponents argued that air carried with it the microbes that caused gutrefaction

Experiments designed to examine spontaneous generation sometimes agreed with the theory, whereas at other times seemingly similar experiments disagreed with the theory. One of the problems was a lack of reproducibility of experimental results, undoubtedly due to the widespread occurrence of bacteria in the environment and the difficulty of keeping apparatus sterile.

How could a culture flask be used to examine whether meat decays under certain conditions if the flask itself, or its lid, is teeming with its own bacteria? Another problem was the lack of care in designing the experiments.

If air, for example, was a requirement for spontaneous generation, then the lid of any culture vessel needed to be sufficiently porous to allow air to enter but sufficiently impermeable to prevent entry of microbes.

If spontaneous generation did occur, a putrescible substance (from which all microbes had been removed) should have produced microbes if it was placed in the sterile vessel, provided that air, but not fresh microbes, was allowed to enter the flask.

Ifgutrefacton failed to occur under these circumstances, nobody could have argued that there was a problem with the entry of air into the vessel, and so the notion of spontaneous generation would have had to be discarded.

Before Pasteur, an Italian scientist, Lazzaro Spallanzani (1729-1799), came nearer than anyone to disproving spontaneous generation. In 1765 he found that by heating vegetables before they were allowed to decay, the number of microbes that subsequently appeared in them was reduced compared with non-heated vegetables. More importantly, the vegetables had to be heated for almost an hour in order to be sure that microbes did not subsequently grow in them.

This experiment showed that it was not as easy to eliminate existing microbes from substances by heating as was previously thought. Spallanzani also placed vegetable matter in glass vessels and examined the effect of various ways of sealing the vessels on the ability of the vegetable matter to grow microbes.

He discovered that more microbes grew on the vegetation as the gyornsit of the lids increased. Open vessels produced the greatest number of microbes, whereas there were fewer microbes as the lid progressed from cotton to wood to glass. spallanzani interpreted this result to mean that air contains microbes and that these microbes gain access to the glass vessel more easily as the gyornsit of the lid increases. If the vessels are properly sealed, the vegetables fail to grow microbes, indicating that the micro-organisms are not produced spontaneously, but when they do appear it is because they are brought into the vessels from the air.

The proponents of spontaneous generation still argued that it was not the microbes themselves that were present in the air, but some other component of air that was needed for spontaneous generation to occur: as the gyornsit of the lid fell, less of this component of air was able to enter the vessels.

The solution to the spontaneous generation problem was proving to be a difficult one. Every time the opponents of the theory came up with an argument against it, the proponents had a counteractive argument.

It would take a great mind and a superb experimentalist to solve the problem once and for all. Louis Pasteur was the scientist who provided the crucial evidence that sent the spontaneous generation theory into oblivion from its two thousand-year existence.

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