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A concept that looms large in early ideas about life is spontaneous generation. Aristotle (Figure 2.2), alongside his ideas on hylomorphism, advanced the idea of spontaneous generation, whereby non-living matter could be transformed into living things by acquiring soul. This idea provided a satisfactory explanation of how living things came into existence.

Figure 2.2 Aristotle, ancient Greek philosopher and early natural historian.

Source: Roman copy in marble of a Greek bronze bust of Aristotle by Lysippos, c. 330 BCE with modern alabaster mantle; reproduced with permission of Jastrow, https://commons.wikimedia.org/wiki/File:Aristotle_Altemps_Inv8575.jpg.

By the Middle Ages, this process was thought to occur through the infusion of abiotic material with “vital heat,” or other forms of special energy, through the combination of environmental conditions to which matter was subjected. Spontaneous generation explained how mice emerged from wheat and maggots from meat. Some seventeenth century texts provide examples of strange experiments on spontaneous generation. For example, here is a recipe by Flemish chemist and physician Jan Baptista van Helmont (1580–1644), who in 1620 published a cookbook-like instruction for making mice:

If you press a piece of underwear soiled with sweat together with some wheat in an open mouth jar, after about 21 days the odor changes and the ferment coming out of the underwear and penetrating through the husks of the wheat changes the wheat into mice. But what is more remarkable is that mice of both sexes emerge, and these mice successfully reproduce with mice born naturally from parents. But what is even more remarkable is that the mice which came out of the wheat and underwear are not small mice, not even miniature adults or aborted mice, but adult mice emerge.

Francesco Redi (1626–1697), an Italian physician and naturalist, was one of the first people to challenge spontaneous generation with an elegant experiment using meat. He showed that if meat was covered by a solid lid, no maggots would form in the meat. He also showed that if the meat was covered by gauze that had holes to allow the “vital force” in, but were too small to allow flies in, maggots would still not be generated. His control experiment was meat exposed fully to the atmosphere, in which maggots appeared. His experiment (Figure 2.3) seems absurdly simple today, yet at the time it elegantly showed that a vital force did not account for the appearance of maggots in meat.

Figure 2.3 Disproving spontaneous generation. A schematic diagram showing Francesco Redi's elegant experiment to demonstrate that spontaneous generation was not responsible for the formation of maggots in meat.

This experiment provided some of the first insights into the fact that maggots must be produced by a vector of some sort, determined to be flies.

However, although spontaneous generation was disproven for flies, it wasn't going to disappear that easily for microbial life. John Turbervill Needham (1713–1781), a well-respected scientist of his time, reported his famous mutton gravy experiment. He had taken some of his left over gravy and heated it in a fire. Transferring this into vials with stoppers, he reported how the gravy, after it had been sealed from the outside world, teemed with microscopic life, regardless of whether it had been heated or not. Thus, he surmised, spontaneous generation was proven. The organic matter of the gravy had been infused with a “life-force.”

We now know that it was likely that his gravy either became contaminated with microbes in the air after it was taken from the fire to be poured into the vials or contained spore-forming bacteria (Chapter 7).

Italian scientist Lazzaro Spallanzani (1729–1799), who is better known for his studies on the regeneration of organs in animals such as frogs, continued the investigations. He did the same experiments as Needham using broths, but he was more careful. He put dampened seeds used to make his broths in the vials first, sealed them, and then heated them to kill any life in them. In this way, particles from the air could be stopped from contaminating the infusions. In vials heated for short periods, large organisms died very quickly. We now think these must have been protozoa, which are large amoebae. They are not tolerant of heat. He noticed that organisms of the “lower class,” as he called them, could tolerate heat for many minutes until even they no longer moved. These were almost certainly bacteria. In this important experiment, he showed that microbes are differentially affected by heat, some being more susceptible than others. He also showed that if his vials were heated for long enough then they could be turned, in his words, into “an absolute desert.” He had demonstrated the concept of sterilization.

You might think that this remarkable set of experiments would finally end the idea of spontaneous generation, but not so. By sealing his vials, so his critics said, Spallanzani had denied the organic matter contact with the atmosphere and hence any “life-force.” Remarkably it would be 38 years before a German scientist made an experimental apparatus to take this new criticism on. One way to provide air to a broth without introducing microbes is to put an open tube into the top of the vial. This allows exchange with the atmosphere. If the tube is heated with a flame, then any air coming in is sterilized, or any microbes are killed. However, a vital force or energy should still be able to enter. This allowed Theodore Schwann (1810–1882) to provide clean air to his boiled vials.

One could become somewhat exasperated because still the theory of spontaneous generation would persist. Others had difficulty reproducing Schwann's experiments, and after boiling and allowing heated air through, they still found life emerging in their vials. This inconsistency in the results, probably caused by contamination, encouraged continuity of the debate.

Schwann's observations were confirmed by Frenchman Charles Cagniard de la Tour (1777–1859), who also performed experiments to find out how yeast grew. He reported on some fundamental discoveries of importance to astrobiology and the limits of life.

In his work, he made the first observations that yeast could survive freezing – a notable difference from many “higher” animals and a fact that would later become important for understanding how life can survive in permafrost and polar environments. This observation still looms large in considerations of how life, whether contamination on spacecraft or hypothetical indigenous life, might survive in freezing extraterrestrial environments. He deserves the recognition of almost certainly being the first person to show that microbes can grow without oxygen. He showed that yeasts could cause fermentation in an atmosphere of carbon dioxide. But his observations did not stop there. He noticed that yeast does not die in the absence of water. Thus, he had documented freezing tolerance, desiccation tolerance, and oxygen-free growth of microbes.

Despite the wonderful observations that were being made with yeast, which suggested that microbes could be behind many transformations in the natural world, this germ theory was not going to be accepted without a little bit of further struggle. German scientist Justus von Liebig (1803–1873) was not impressed. As a chemist, he was convinced that all reactions attributed to these microbes were chemical and you could explain them without having to invoke biology. His insistence that chemistry was all that there was to fermentation would muddy the waters. However, as can often be the case with “disproved” scientific hypotheses, he was not entirely wrong.

In the late 1890s, Eduard Buchner (1860–1917) did in fact show that if you took the extract from yeast, you could get sugar to ferment. We now know that he had liberated the enzymes needed for fermentation, even if the yeast cells themselves were dead. In some ways he confirmed Liebig's ideas: he had shown that chemical reactions were sufficient for fermentation; however they were chemical reactions from living things. Buchner might be described as having contributed to founding the science of biochemistry – the science of studying chemical reactions that occur within life. However, Liebig's focus purely on chemistry as the cause of fermentation would weaken the argument for those seeking to get acceptance of germ theory for a while.

It is at this stage that Frenchman Louis Pasteur (1822–1895) comes onto the stage (Figure 2.4). He had a mind of wonderful clarity when it came to planning experiments. He began to publish observations on the growth of yeast. He would later pioneer the process of pasteurization, where the rapid and short-lived heating of milk could kill off microbes without changing its taste, and so help preserve it. Among scientific challenges, there was no greater irritant to him and his fellow scientists than spontaneous generation.

Figure 2.4 Louis Pasteur, microbiologist. He took on the idea of spontaneous generation with an ingenious experiment.

Source: Reproduced with permission of Pal Nadar, https://commons.wikimedia.org/wiki/File:Louis_Pasteur_(1822_-_1895),_microbiologist_and_chemist_Wellcome_V0026980.jpg.

His response to this centuries-old question was an experiment of ingenious simplicity. He invented his famous swan neck flasks (Figure 2.5). These flasks contained a variety of liquids including yeast extracts, pepper water, and urine. He heated them until they boiled for several minutes. Then he cooled them. The swan neck flasks were a very simple alteration to the type of flasks used by Schwann. The swan necks prevented aerially transported entities from entering the broth. In contrast, the flasks with the swan neck removed were rapidly contaminated and microbes grew. This experiment showed that broths were contaminated by organisms in the air and that life could not spontaneously be generated in a sterile medium. He concluded his remarkable paper in which he reported the results with a sentence of extraordinary clarity and importance: “There exist continually in the air organized bodies which cannot be distinguished from true germs.” With this sentence, microbiology had truly been propelled to center stage in medicine and our understanding of the environment. Pasteur's experiment was a response to a French Academy of Sciences prize in 1859 that challenged scientists to disprove spontaneous generation.

Figure 2.5 Louis Pasteur's swan neck flasks and his experiment to disprove spontaneous generation. The swan necks prevent microbes from dropping directly into the flasks after the broth has been heat-sterilized. Flasks with open necks become colonized, and microbial growth occurs.

Despite the end of spontaneous generation as an idea for the emergence of life as a continuous process on Earth, we know that it must have happened once. When life first arose, there was a transition from abiotic chemistry to the first replicating molecules and cellular life. So, the problem of spontaneous generation still has not gone away. However, to avoid confusing it with the old obsolete ideas of spontaneous generation, the origin of life is often referred to today as abiogenesis. How did simple molecules come together to form more complex ones and ultimately a self-replicating organism? We will investigate this question in Chapter 12.