Spontaneous Generation

Spontaneous generation, also called abiogenesis, is the belief that some living things can arise suddenly, from inanimate matter, without the need for a living progenitor to give them life.

In the fourth century B.C., the Greek philosopher and scientist Aristotle argued that abiogenesis is one of four means of reproduction, the others being budding (asexual), sexual reproduction without copulation, and sexual reproduction with copulation. Indeed, the Greek goddess Gea was said to be able to create life from stones. Even Albertus Magnus (Albert the Great), the great German naturalist of the thirteenth century Middle Ages, believed in spontaneous generation, despite his extensive studies of the biology of plants and animals.

Through the centuries, the notion of spontaneous generation gave rise to a wide variety of exotic beliefs, such as that snakes could arise from horse hairs standing in stagnant water, mice from decomposing fodder, maggots from dead meat, and even mice from cheese and bread wrapped in rags and left in a corner. The appearance of maggots on decaying meat was especially strong evidence, for many people, that spontaneous generation did occur.

Spontaneous generation found further support from the observations of the Dutch merchant Anton van Leewenhoek, the inventor of the first, primitive microscopes. From 1674 to 1723 Leewenhoek corresponded to the Royal Society in London, describing the tiny, rapidly moving, "animacules" he found in rain water, in liquid in which he had soaked peppercorn, and in the scrapings from his teeth (which, to Leeuwenhoek's surprise, had no such animacules after he had drunk hot coffee).

In the seventeenth century, however, some scientists set out to determine whether living organisms could indeed arise through spontaneous generation, or if they arose only from other living organisms (biogenesis).

In 1668, even before Anton van Leeuwenhoek began his study of microscopic organisms with the microscope, the Italian physician Francisco Redi began a series of experiments that showed that dead meat does not give rise spontaneously to maggots.

Redi filled six jars with decaying meat, leaving three open and sealing the other three. The unsealed jars attracted flies, which laid their eggs on the decaying meat, while the meat in the sealed jars was unavailable to flies. When maggots developed on the meat in the open jars, Redi believed he had demonstrated that spontaneous generation did not occur. However, supporters of the notion of spontaneous generation claimed that the lack of fresh air-not the absence of egg-laying flies-had prevented maggots from appearing on the meat.

Therefore, Redi undertook a second experiment, in which he covered the tops of three of the jars with a fine net instead of sealing them. Once again, maggots failed to appear on the meat in the covered jars, but did appear on the meat in the open jars, where flies were able to lay their eggs.

Nevertheless, the tiny "animacules," described by Leeuwenhoek in his observations on microscopic life in drops of water, still held the imagination of many scientists, who continued to believe that such creatures were small and simple enough to be generated from nonliving material.

John Needham, an eighteenth century English naturalist and Roman Catholic theologian, began his study of natural science after reading about Leewenhoek's animacules. Needham became a strong advocate of spontaneous generation, and performed an experiment that he felt supported his belief in biogenesis. In 1745, he heated chicken and corn broths, poured them into covered flasks. Soon after the broths cooled, they teemed with microorganisms, prompting Needham to claim that the organisms arose through spontaneous generation.

Needham's work was contradicted by another religious investigator, the Italian physiologist Lazzaro Spallanzani. Spallanzani, who was educated in the classics and philosophy at a Jesuit college, went on to teach logic, metaphysics, Greek, and physics. About 20 years after Needham announced the results of his own investigation of spontaneous generation, Spallanzani showed that when broth was heated after being sealed in a flask, it did not generate life forms. He suggested that Needham's broths had probably supported growth after being heated because they had been contaminated before being sealed in their containers.

Undeterred, Needham counterclaimed that heat destroyed the "vital force" needed for spontaneous generation, and that, by sealing the flasks, Spallanzani had kept out this vital force.

The argument continued into the nineteenth century, when the German scientist Rudolf Virchow in 1858, introduced the concept of biogenesis; living cells can arise only from preexisting living cells.

But the matter remained unresolved until two years later when the great French scientist Louis Pasteur, in a series of classic experiments demonstrated that (1) microorganisms are present in the air and can contaminate solutions; and (2) the air itself does not create microbes.

Pasteur filled short-necked flasks with beef broth and boiled them, leaving some opened to the air to cool and sealing others. While the sealed flasks remained free of microorganisms, the open flasks were contaminated within a few days.

In a second set of experiments, Pasteur placed 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.

Pasteur not only executed a brilliant set of experiments, he also used his zeal and skill as a promoter of his ideas to strike a decisive blow to spontaneous generation. For example, in a lecture at the Sorbonne in Paris in 1864, Pasteur said that he had water for his experimental liquids to generate life. But, he said, "....it is dumb, dumb since these experiments were begun several years ago; it is dumb because I have kept it sheltered from the only thing man does not know how to produce, from the germs which float in the air, from Life, for Life is a germ and a germ is Life. Never will the doctrine of spontaneous generation recover from the mortal blow of this simple experiment!" Pasteur's work not only disproved abiogenesis, but also offered support to other researchers attempting to show that some diseases were caused by microscopic life forms. Thus, in a simple, but elegant set of experiments, Pasteur not only struck the doctrine of spontaneous generation a "mortal blow," but also helped to establish the germ theory of disease.