April 7, 1864 (a Thursday)

On this date, Louis Pasteur uttered his famous statement, “The doctrine of spontaneous generation will never recover from the mortal blow inflicted by this experiment,” during an address he delivered at the Sorbonne in Paris, France. The experiment he was referring to involved swan-necked flasks.

Pasteur filled a flask with medium, heated it to kill all life, and then drew out the neck of the flask into a long S. This prevented microorganisms in the air from entering the flask, yet allowed air to flow freely. If the swan neck was broken, microbes could enter the flask and grow.

Pasteur had placed beef broth in flasks that had open-ended, long necks. After bending the necks of the flasks into S-shaped curves that dipped downward, then swept sharply upward, he boiled the contents. The contents of these uncapped flasks remained uncontaminated even months later.

Pasteur explained that the S-shaped curve allowed air to pass into the flask; however, the curved neck trapped airborne microorganisms at the bottom of the curve, preventing them from traveling into the broth.  If the swan neck was broken, microbes readily entered the flask and grew. Thus, Pasteur demonstrated that (1) microorganisms are present in the air and can contaminate solutions; and (2) the air itself does not create microbes.

Interestingly, despite Pasteur’s successful demonstrations against spontaneous generation, attempts to repeat his experiments occasionally failed because, after some time, microbial growth occurred in some broths of swan-necked flasks. This created doubt in the minds of many. But, this problem was solved by John Tyndall, an English physicist, in the year 1877. He explained that bacteria exist in two forms: Heat-labile forms which could be killed by exposure to high temperatures, and heat-resistant forms (known as spores) which could not be killed by continuous boiling of the broth.  After the broth has cooled, the latter resulted in microbial growth in such broths.

Tyndall further stated that if such broths are subjected to intermittent boiling (that is, discontinuous boiling) on successive occasions, a process now popular as tyndallization, the heat-resistant forms of bacteria will be killed and the broths become completely free of them, and do not show any microbial growth. It so happens because the first boiling kills vegetative cells of bacteria but spores remain as such. The spores now germinate in cooled broth and produce new bacteria cells which are killed during further boiling and so on. In this way, Tyndall validated Pasteur’s results and helped end the debate on spontaneous generation.

Many creationists maintain that, as a result of the above experiment, “Louis Pasteur disproved Darwin’s theory.” In fact, Pasteur did no such thing. What he and the others who denied spontaneous generation demonstrated is that life does not currently spontaneously arise in complex form from non-life in nature; he did not demonstrate the impossibility of life arising in simple form from non-life by way of a long and propitious series of chemical steps/selections. Pasteur’s experiments were limited to a smaller system, and for a shorter time, than the open surface of the planet over millions or billions of years. In particular, they did not show that life cannot arise once, and then evolve. Neither Pasteur, nor any other post-Darwin researcher in this field, denied the age of the earth or the fact of evolution.

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