AKA: Living organisms can - and frequently do - arise from non-living matter (e.g. flies can arise from dust).
Theory born: circa 4th century BCE.
Parent(s): Aristotle (formalisation of the ideas of prior philosophers such as Anaximander)
The theory of spontaneous generation was first formally presented in 350BCE by Aristotle in his book 'On the Generation of Animals', though Aristotle had been developing his theories about what distinguished living organisms from non-living objects for many years. He postulated that all organisms contained four elements; earth, air, water and fire. He then went on to theorise that non-living matter contained these elements, as well as pneuma: a 'vital heat' which would act as a 'breath of life', or anima: a soul (depending on the age of the translation of his work being used). If this non-living matter was placed in an appropriate environment, life would emerge from the non-living matter.
By his observations of decaying organisms, Aristotle came to believe that air contained pneuma/anima (as evidenced by the emergence of maggots in a rotting animal exposed to air), that there was air in water (which enabled fish to breathe) which meant that water contained pneuma/anima and that there was water in earth, meaning that earth also contained pneuma/anima. In this way, all things could give rise to life under the right circumstances.
Aristotle, circa 330BCE
Theory death: 1859
Cause of theory's death: Incompatible with the results of Pasteur's experiments.
When Pasteur designed his experiments into spontaneous generation theory in 1859, he had already amassed a significant body of research into the action of microbes. He had spent several years investigating the spoilage of wine and beer by microbial contamination and had developed the technique that became known as pasteurisation to sterilise milk, eventually patenting it in 1865. It was his findings (that sterilised grapes were unable to begin fermentation into wine) that brought him into conflict with a leading proponent of spontaneous generation, Félix-Archimède Pouchet.
Pouchet had earlier attempted unsuccessfully to replicate some of Schwann's experiments with microbes. Schwann had strongly heated broth within a sealed container and discovered that no microbial contamination of the broth occurred following this. Clearly, life could not emerge from the non-living broth. When Pouchet attempted this experiment with a different type of broth, he found that it still gradually became cloudy with bacterial contamination. Clearly then, the living bacteria were emerging from the non-living broth as everything alive inside the container had been killed by heating.
Félix-Archimède Pouchet, date unknown
Pouchet, director of the Rouen Museum of Natural History, became aware of Pasteur's work and the two began an amicable correspondence. However, Pasteur and other members of the French Academy of Sciences criticised Pouchet's experiments, saying that they had been poorly executed and had allowed bacterial contamination from the air. Pouchet did not take the criticism well, fiercely attacking Pasteur's developing 'germ' theory. Their correspondence became considerably less amicable.
To settle things once and for all, the French Academy of Sciences offered a prize to whoever could experimentally demonstrate the validity (or not) of spontaneous generation theory. In response, Pasteur carried out a series of famous experiments. One of his first was to filter air through a piece of gun cotton (cellulose nitrate). When the gun cotton was examined with a microscope, it was found to be full of micro-organisms, suggesting that microbes were already present in the air, rather than arising through some 'life force'. He then went on to design and carry out a series of experiments with swan neck flasks, pictured below.
In the first experiment, Pasteur filled a swan neck flask with nutrient broth and boiled it for a long period. No bacteria grew in this flask, even though air could reach the boiled broth through the swan neck (crucially, the shape of the neck prevented bacteria naturally present in the air from reaching the broth).
The second experiment was similar to the first, except that Pasteur broke the top from the flask once the broth had been sterilised. Over time, the broth became cloudy with bacterial colonies. Clearly, something had entered the flask through the broken top, contaminating the broth.
The third experiment again began with sterilised broth, but then the flask was tipped so that broth flowed down the 'swan neck' and briefly came into contact with the part of the 'neck' where airborne particles would have settled. Again, bacterial contamination ensued. He concluded then bacteria were entering the broth through some particle falling through the air (dust was much discussed as a vector at the time), but that bacteria were not certainly being 'spontaneously generated' from non-living matter.
Louis Pasteur, prior to 1895
As Pasteur himself noted after winning the Alhumbert Prize in 1862:
"Never will the doctrine of spontaneous generation recover from the mortal blow of this simple experiment. There is no known circumstance in which it can be confirmed that microscopic beings came into the world without germs, without parents similar to themselves."
RIP spontaneous generation theory.
Interesting side notes
A Roman architect named Vitruvius (1st century BCE) advised his clients that their libraries not be placed facing the south or the west as the winds from those directions generated bookworms.
Attributions
Mould photograph: RimmaKhaz, Wiki Commons, Creative CommonsAttribution 4.0 International
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