An Interview with Professor Manuel Varela: There is no such Thing as “spontaneous generation”

Aug 5, 2018 by

Michael F. Shaughnessy –

1) I remember quite clear from high school biology – Drosophila– the fruit fly -and this big debate about “spontaneous generation.” How did this issue first get started?

The notion of spontaneous generation took hold over two millennia ago first with the pre-Socratic school of Ionian thought on the topic by Thales of Miletus (born c.624 B.C. and died c. 546 B.C.) and the later extensive writings of Aristotle (born in 384 B.C. and died in 322 B.C.). Thales and Aristotle were ancient Greek philosophers who contemplated nature. Thales speculated that life emerged from the soil with the help of water and air. Aristotle had further written about many kinds of living beings, ranging from small insects, like flies, and higher forms of life, like worms, fish and salamanders arising spontaneously out of dead matter, like slime, dirt, rainwater, decaying algae, etc.

The basic idea of spontaneous generation, known also as abiogenesis, was that living organisms simply arose out of non-living inanimate material. That is, life arose out of non-life. While Aristotle had acknowledged that higher animals gave rise to offspring directly by sexual means, other living beings were believed to result into existence spontaneously from lifeless ingredients.

Without apparently bothering to test experimentally the spontaneous generation idea directly for themselves, later investigators simply began instead to supplement the abiogenesis concept with new protocols for generating even higher forms of life.

For example, in the early 1600s, Dr. Jean Baptiste van Helmont, physician, scientist, and an avowed proponent of spontaneous generation, had specified a protocol in his writings for the generation of living mice, for instance, by placing inside a wooden box some dirty fabric material and some wheat germ. Next, the van Helmont protocol specified that the box of dead material be allowed to stand for about 3 weeks. At such time, living mice should then emerge spontaneously from the lifeless matter. Spontaneous generation as a practice was all but assured.

In the late 1660s, Dr. Francesco Redi conducted what is considered by many historians of science to be one of the first full-fledged experimental tests of the spontaneous generation theory. He studied the connection between flies (living) and meat (non-living) contained in flasks exposed to the outside elements and in flasks that were sealed with cork or covered with a wire mesh. 

In the open flasks Redi observed flies emerging from the exposed non-living meat, whereas in the other cork-sealed and mesh-covered flasks no flies had appeared. Historians believe that Redi observed flies of the Musca domestica housefly variety rather than the well-studied Drosophila melanogaster species, later known for their famous genetics experiments of the 20th century. Upon further careful observation, Dr. Redi noticed that flies had landed on the meat in the exposed flasks and fly eggs had apparently been deposited.  The eggs were later observed to hatch, releasing fly larvae instead of spontaneously generating, as Aristotle’s theory dictated.  Dr. Redi concluded, therefore, that flies did not undergo spontaneous generation.

After the discovery of the microbes by Antonie Philips van Leeuwenhoek in 1677, the possibility that these microbes might spontaneously generate was also tested experimentally. In 1748, Fr. John Turberville Needham, an Irish priest from the Jesuit order of Catholicism who studied natural history, was reported to have conducted the first experimental testing of the spontaneous generation theory in the microbes. In his publication, Fr. Needham described how he heated flasks containing mutton gravy to the point of boiling and then sealed the heated mutton gravy flasks with cork.  Next, he let the heated flasks set for several days, whereupon Needham examined the now rather cloudy mutton gravy under the microscope and found all sorts of little microbes within the heated material.  Fr. Needham thus concluded that whereas Redi’s flies do not appear spontaneously to generate, the microbes nevertheless do undergo the spontaneous generation process.  Unfortunately, Needham had been incorrect.

It was unanticipated, but Fr. Needham had been in the unfortunate circumstance of being inaccurate in interpreting his heated mutton gravy data. Furthermore, Needham was remarkably unlucky in that his heated mutton gravy had contained heat-resistant endospores that survived the boiling process, allowing the resilient bacterial endospores to germinate and grow in the mutton gravy. Bacterial endospores can survive harsh environmental conditions, like heat. Then, when conditions become favorable, like optimal growing temperatures, the bacteria may germinate into a vegetative metabolizing form and grow. Regrettably, Needham was unsuccessful, therefore, in thoroughly heating the mutton gravy to completion, and his heating did not kill the bacterial endospores.  Thus, the bacteria were able to grow when Needham let his flasks sit at the cooler temperatures. Hence, Needham was led to mistakenly believe that spontaneous generation had occurred in his experiments.

To further compound these historically unfortunate matters of Needham, other investigators, like the eminent George-Louis Leclerc, Comte de Buffon, soon fell into the same quagmire and had apparently repeated the entire series of unfortunate microbe experiments as Needham. In obtaining similar findings, Buffon further solidified the incorrect view that spontaneous generation was true for the microbes; however, it was in fact not true for any living organisms, but almost no one knew that in the time of Needham and of his contemporaries. Meanwhile, Needham’s microbe work became widely accepted by members of the scientific establishment, earning membership for him in the prestigious Royal Society of England.

The truth of the matter concerning spontaneous generation would not be resolved definitively until the middle of the 19th century with the elegant work of Louis Pasteur. In the meantime, however, a hint that the spontaneous generation picture was not a true phenomenon was revealed by the experimental work of Lazzaro Spallanzani
 
2) Now Lazzaro Spallanzani -where was he from and what did he have to do with disproving “spontaneous generation”?

Lazzaro Spallanzani was born in Scandiano, an Italian village, on the 12th day (some sources say the 10th) of January in 1729. As a child he received his early education at his local village schools. Later, he attended a Catholic seminary of the Jesuit order in Reggio Emilia, starting at 15 years of age. In 1749, he attended the University of Bologna where he studied law. While in law school, Spallanzani was encouraged by a relative, his cousin Laura Bassa, professor of physics and math, to study math and the natural sciences, known then as the field of natural philosophy.

In the year 1754, Lazzaro Spallanzani took his doctoral degree from the University at Bologna. He then became a professor of metaphysics, logic and Greek at the University of Reggio. Becoming ordained as a priest in 1762, Spallanzani moved to the University of Modena where he became a faculty and held the chair of natural philosophy.  In 1769, Spallanzani moved to the University of Pavia, still in Italy, where he spent approximately 30 years teaching and conducting scientific research as a professor natural history.

Spallanzani’s interest in the spontaneous generation theory began in 1761. At about that time, while on an excursion along the upper Reggian Apennine Mountains and Lake Ventasso, Italy, Spallanzani had heard about the experimental studies of Buffon and Needham through a colleague, Antonio Vallisnieri, who was then a professor of natural history at the University of Padua. Spallanzani was told all about the requirements for the spontaneous generation to come to fruition, of the need for soil, water, etc.

He learned about the various protocols for spontaneously generating numerous living organisms.  Then, fortuitously, Spallanzani learned about the experimental studies of Francesco Redi, whom you will recall had conducted the work with meat and flies, concluding that flies did not spontaneously generate. The fly studies of Redi had contradicted the microbe studies of Needham and Buffon. Redi’s fly work argued against the spontaneous generation theory while those of Needham and Buffon argued in support of it.

Spallanzani began his foray into the experimental studies of the spontaneous generation hypothesis by heating a series of hermetically sealed flasks for different lengths of time. These hermetically sealed flasks had contained water with a mixture of different seed varieties, like those for peas, beans, wheat, corn, etc., an end-result that Spallanzani had called infusions.

The method that Spallanzani used to seal the flasks containing food seeds involved melting the glass flask such that the melted glass came together to make a self-contained vessel, leaving out all outside entry of air. This method of flask sealing by Spallanzani differed from that used by Needham, who simply used cork to seal his mutton gravy flasks.

After the heating, Spallanzani incubated the flasks for several days at ambient temperatures.  He then broke open the variously heated and incubated seed-flasks, and then he examined the seed-water infusions under the microscope.  He found that the flasks that had been heated for short periods killed higher organisms, which historians speculate were probably the protozoa, but which Spallanzani had called animalcula of the higher class.

On the other hand, Spallanzani found that in the sealed flasks with seed-water infusions that had been heated for longer periods, he found no animalcula of any sort; that is, he found no microbes when examined under the microscope.  In fact, Spallanzani observed that the longer the heating period, the animalcula of the lower class, presumably the bacteria, were seen less often, until at some point, all of these microbes failed to be seen in the scope. The number of bacteria found inversely correlated with longer heating periods until the bacteria were killed. That is, it is likely that the bacteria had been killed at some point during the infusion heating process.

In another set of experiments, Spallanzani repeated his flask heating for various time periods with the seed infusions as before, except this time he had sealed the flasks with cork, just as Needham had. In these flasks, Spallanzani found microbes in virtually all of the flasks with their heated infusions, just as Needham had found. Thus, Spallanzani speculated that Needham had simply failed to effectively heat his mutton gravy to kill microbes that were present in his infusion or that Needham failed to prevent microbes from getting past the cork to the mutton gravy. Spallanzani reported that microbes in Needham’s mutton gravy either survived the heat and grew or entered to the inside of the flask from the outside air to get to the gravy.  Whatever the case, Spallanzani correctly concluded in 1799 that spontaneous generation was not possible, even for the microbes.

Almost immediately upon their reading of Spallanzani’s published report, both Needham and Buffon countered by criticizing the methodology of Spallanzani.

Without conducting any new experiments of their own to support their contention, Needham and Buffon merely replied that Spallanzani’s heat treatment had destroyed the so-called “vegetative force” which they now claimed was necessary for the spontaneous generation to be brought about. Needham and Buffon went on to propose further that Spallanzani’s harsh heating treatment also destroyed the so-called “elasticity” of the required air, and as a result, spontaneous generation could not occur in Spallanzani’s experiment.

Spallanzani replied to these two criticisms of Needham and Buffon by conducting a new set of experiments, each addressing the issues having to do with the “air elasticity” problem and of the “vegetative force” destruction.

First, Spallanzani placed the food seed-water infusions within the flasks but instead of hermetically sealing the flasks with a flame as before, he stretched the heated flasks to make an extremely tiny opening, leaving access to the required air and preserving its elasticity.  He let the flasks equilibrate to maintain the so-called air elasticity.  This modification in Spallanzani’s method was meant to address the air elasticity criticisms of Needham and Buffon. Next, Spallanzani sealed the flasks hermetically as before but this time without disrupting the air elasticity. He then heated the sealed flasks with their air elasticity left intact. The he let the flasks incubate for several days and found that when he opened these flasks, there were no microbes to be seen in the microscope.

Second, to address the criticism of Needham and Buffon regarding the destruction of the newly proposed need for the vegetative force, Spallanzani set out to destroy this vegetative force on purpose, then show that microbes could grow without it. He heated the food seeds by roasting, to destroy the vegetative force. Next, he added water to the roasted seeds and let the infusions incubate.  He examined the infusions with his microscope and found that the microbes were teeming with growth.

Spallanzani had concluded, correctly, that the microbes of Needham and Buffon had survived their heating, presumably with heat-resistant endospores or that the microbes had entered into the sterile infusions from the air outside, contaminating the mutton gravy to produce new growth of microorganisms. Either way, the evidence of Spallanzani pointed to a clear blow against the notion of spontaneous generation. Furthermore, Spallanzani’s studies pointed to the notion that even the microbial offspring have parents.

It remains unclear why Spallanzani’s definitive work was not widely accepted by his fellow scientific investigators and the public in general. In his time, spontaneous generation was almost universally considered a fact, and had been for many centuries, despite what his data had now revealed about the topic. What IS clear is that spontaneous generation was not to be accepted as a falsehood until the classical work of Pasteur.

3) Why is this such a key crucial construct in science, research and experimentation?

There are several primary reasons why Spallanzani’s work was crucial to the scientific research structure. First, his work showed that long-held hypotheses could, nevertheless, be tested in an experimental fashion, complete with proper controls, collection of data, repeating of experiments and addressing criticisms made by others.

Secondly, while Spallanzani’s studies did not definitively disprove the spontaneous generation theory, he nevertheless provided a clear dent it its premise regarding the mode of growth for microbes.  He showed that microbes did not spontaneously generate themselves into existence out of inanimate matter. 

Thirdly, Spallanzani’s work strongly suggested that microbes could be present in the air, probably floating about in it and potentially entering loosely sealed vessels, like those sealed with cork or cotton. 

Lastly, this implication by Spallanzani that microbes exist in the air provided impetus years later for Prof. Louis Pasteur to evaluate this very possibility directly (microbes in air) with his famous Swan-necked flask experiments in the mid-19th century. 

4) Obviously, there are microbes that are not visible to the naked eye. What challenges does this present to researchers?

Indeed, because the vast majority of microbes, such as the bacteria and the viruses, are perhaps too tiny to be seen with the unaided eye, they are, thus, essentially invisible. This presents several problems that need to be overcome in order to study these tiny living creatures. First, microscopes are clearly needed in order to visualize the microbes. Powerful microscopes, like those used in electron microscopy, are required to see the smaller viruses, which are considered to be amongst the smallest of the many types of microbes.

However, simply observing what the microbes look like, as important as it is, may still not be sufficient for scientific investigators to study closely and to learn about the microbes. This is because the microscope offers us only a few key details. For instance, we may learn about the shapes of the microbes, or how they may associate with each other to make certain bacterial cell arrangements, or whether the microbes have certain outward-facing features, or how the microbes may move about.

Yet, with these types of visual information at hand, we will still be unable, for example, to identify the species of the microbes, and this may hamper our attempts to study clearly the microbes in depth. Therefore, other methodological approaches are needed to study the microbes in greater and comprehensive detail.

One such approach uses the field of biochemistry in order to do so. For example, knowledge gained about the certain types of enzymes that a microbe may harbor tells us something about its identity. A particular species of microbe possesses a certain array of enzymes, biochemical pathways, and end-products of which are biosynthesized as a prime result of their biochemical characteristics. Investigators can readily exploit these specific microbial biochemical properties to help learn about their species identity. Knowledge of a microbe’s biochemistry can also facilitate efforts to coax them to make products that are useful to us, such as medicines, hormones, food, diagnostic kits, etc.

Another methodological approach uses the field of genetics to study the microbes. In this arena, investigators use key information contained within the genomes of the microbes to study their behavior, evolution, regulation, relationships with other groups of living organisms, etc. One new field that has emerged recently is called metagenomics, in which an entire group of microorganisms in an ecological niche can be identified, to the species level.  For example, one may learn what microbiome exists in the human gut, or in a body of water, or in a sample of food, etc. The possibilities in this genetic realm are virtually endless. 

A serological method can be used to study the microbes. This technological approach takes advantage of the fact that microbes are detected by our immune system. As a result, humans and any other animals with immune systems can produce highly specific proteins called antibodies that can specifically recognize and bind to just about any microbe that is introduced to a human or an animal host. These highly specific antibodies are then collected, purified and can be effectively used to detect certain potentially pathogenic microbes in human patients, or certain microbes present in environmental specimens, etc. 

5) What other things was Lazzaro Spallanzani know for?

Despite being famous amongst the microbiologists for his studies of microbial growth and spontaneous generation, Spallanzani was perhaps more famous amongst the physiologists for his body of work dealing with blood circulation. He studied how blood filled and emptied in the chambers of the heart during its contraction. He identified the various effects of gravity and wound trauma on the flow of blood through the circulatory system.

Spallanzani is known also for his studies on the digestion system. He identified the relationship between the types of foods that are digested versus the various types of chemicals contained within gastric juice. It is reported that he conducted studies on himself in which he obtained gastric juice samples by inducing vomiting in order to do so. 

Spallanzani conducted studies pertaining to the regeneration of tissues in various test animals. He was said to have surgically excised limbs from various organisms to examine their potential to grow back to their original forms.

He also performed tissue transplantation experiments, attempting to graft particular tissues from one part of a laboratory organism to another location in the same individual.

Another area of study by Spallanzani is that of sexual reproduction. He is reported to have determined that sexual reproduction required both sperm and eggs.

Additionally, Spallanzani was known to have studied the navigational flight of bats while in the dark. Toward this, his work paved the way for future investigators to continue their efforts along this line to establish modern modes of operation during bat flight in the dark.

Spallanzani studied marine fossils contained within landlocked mountainous regions, tying ancient marine life to these distant locations, devoid of more modern marine environments. 

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