Spontaneous Generation

What is spontaneous generation? Why did people accept it? Let’s look back at old Greece. By the 4th century B.C., a significant debate regarding the nature of matter arose between the school of Democritus and Aristotle. Democritus argued for a world of atoms: tiny particles that combine to form the variety of matter in our world. In his view, life arose spontaneously through the combination of atoms of soil and fire. Aristotle opposed this atomic hypothesis with its supposed chance mixing of elements. Nevertheless, he too embraced the spontaneous and naturalistic generation of life on Earth. At the core of this belief was the doctrine of vitalism: the idea that every organism is imbued with a life force different from the forces that act on inanimate matter. That’s the way things remained for almost 2000 years.

In the 17th century, the theory of life’s spontaneous generation was accepted knowledge. In fact, the only real challenge to the Aristotelian view came from a few theologians who argued that all living things were formed by God during the first days of creation. A famous quotation by Jan Baptiste van Helmont(1577-1644) exemplify the common views at the time: “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." Van Helmont was describing the spontaneous generation recipe for mice.

Redi’s Experiment

The first experiment to challenge this conventional wisdom was performed by the Italian physician Francesco Redi (1626-1697). Redi tackled the problem of preserving raw meat, which deteriorates rapidly on air. In the first part of his experiment, he used open containers. If you let flies land on the meat, then maggots would invariably appear a few days later. In the second part of the experiment, he covered the containers to protect them from the flies. In that case, no maggots grew. He described all these studies in his influential book “Experiments on the Generation of Insects”. Redi’s discovery that maggots only appear in meat contacted by flies let him to conclude that flies, not spontaneous generation, cause maggots.

Redi’s experiment was brilliantly conceived and executed, and it provides us with one of the earliest examples of the scientific method. Even so, Redi accepted the idea that microscopic organisms arise spontaneously all around us all the time.

For the better part of the next two centuries, well until the 19th century, the doctrine of vitalism was widely accepted. Many naturalists felt that this “life force” was not only different from the well studied forces of the non-living world, but also that it was inherently unknowable. The influential philosopher Immanuel Kant (1724-1804) wrote: “It is absurd for men to hope that another Newton will arise in the future who shall make comprehensible by us the production of a blade of grass according to natural laws which no design has ordered”. In other words, the origin of life is an ongoing supernatural phenomenon.

Why People Believed It

Why so many people accepted this idea of spontaneous generation? Think for a moment about your own experience and you’ll get a feeling for why this idea isn’t such a strange point of view. Worm-like maggots form in meat and every spring new leaves appear and flowers blossom in season of renewal. It is not surprising that many people accepted unquestioningly that life arises trough spontaneous generation.

This view continued well into the 19th century, when there were an increasing number of respected deniers of it. One important factor in the spontaneous generation controversy was the 17th century invention of the microscope and the surprising discovery that microscopic life is everywhere. Microorganisms, however, fail to resolve the controversy. After all, anyone who favors spontaneous generation could say that microbes are just one more manifestation of the life force.

As anyone who does experiments would tell, the interpretation of data is seldom unambiguous. This fact of scientific life was highlighted by a marvelous 18th century exchange. Lazzaro Spallanzani, who was opposed to spontaneous generation, explored the subject by comparing flasks filled with nutrient rich water. He boiled flasks that were sealed, and he observed that those flasks remained sterile indefinitely as long as they remained seal. However, when he boiled unsealed flasks, they rapidly took on a cloudy appearance due to the growth of microbes. Spallanzani concluded that ubiquitous microscopic life forms must have contaminated all of his unsealed flasks.

Spallanzani’s conclusions were challenged by Englishman John Needham. Needham agreed that boiling kills microbes, but he found that microbes soon reappear in abundance when the flask is cooled. That was a result that led him to a very different conclusion. He claimed that this new population of cells arose by spontaneous generation.

Naturally, Spallanzani countered that Needham’s new microbes came from the air contamination. To prove his point, he undertook a new set of experiments in which he pumped air out of his flasks, and then boiled the water. No microbes appeared in these trials. Needham countered with a new argument: It was a property of the air, not the water, that must carry the life force.

Today, we’re likely to react to this historical incident with a rather biased worldview. It seems obvious to us that Spallanzani’s conclusions about microbial contamination were correct. It seems just as obvious that Needham’s support of spontaneous generation was misguided. But put yourself in the place of an impartial observer of the time. You wouldn’t be that familiar with the nature of bacteria, nor with their ability to replicate rapidly. Faced with these conflicting claims you would have had a hard time choosing between invisible microbes on the one hand, an invisible life force on the other. Indeed, both arguments were internally consistent, so doubts remained.

Pasteur Takes Care of the Issue

By the 19th century, experiments had clearly disproved spontaneous generation of larger organisms, such as mice and flies. The origin of microbes was still a matter of much debate. It was the great French chemist Louis Pasteur (1822-1885) who resolved the issue once and for all. Pasteur was a giant of experimental research and he contributed many fundamental ideas to biology.

In the 1850’s and 1860’s, Pasteur helped to abolish belief in vitalism and the theory of spontaneous generation with a brilliant series of experiments on sterilized solutions. Like others, he prepared a nutrient rich sugar solution and poured it into several beakers. As with previous researches, one set of beakers was tightly sealed to prevent any contact with ambient air. Pasteur’s innovation was to prepare other beakers that were left open to the air but with a narrow twisted neck. Thus, the sugar solution was in contact with the ambient air, but microbes were unable to traverse that long glass passage.

Pasteur also ran a serious of controlled experiments by leaving beakers wide open or contaminating them with ordinary dust. Over the course of several years, she showed that boiled water, if isolated from air microbes, remains sterile indefinitely. His conclusion: only microbial contamination causes new growth; cells do not arise by spontaneous generation.

This unambiguous result had more than purely intellectual appeal. His discoveries and subsequent perfection of techniques to sterilize sealed containers of food and beverage proved to have a tremendous practical use. This helped to reduce the incidence of infectious diseases.

In the course of this elegant work, Pasteur also contributed in a significant way to the study of life’s origins. By experimentally verifying the dictum that no cellular life can occur without prior cellular life, he pushed back life’s origins to an inconceivable remote time and place. If life does not arise spontaneously, then where and when did it come from? How could anyone make useful observations and make experiments to study an event so ancient and inaccessible?


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