BIOLOGY Volume 3 - A Guide to General Biology - 2004

26. EVOLUTION, OR THE HISTORY OF LIFE ON EARTH

26.1. Theories of the Origin of Life

26.1.2. Spontaneous Generation

This theory was widespread in ancient China, Babylon, and Egypt as an alternative to creationism, alongside which it coexisted. Aristotle (384–322 BC), often hailed as the father of biology, adhered to The Theory of spontaneous generation. Based on his own observations, he developed this theory further, linking all organisms in a continuous sequence — the "ladder of nature" (scala natura).

"For nature makes the transition from lifeless objects to animals with such a smooth sequence, placing between them creatures that live without being animals, that between adjacent groups, thanks to their close proximity, the differences can scarcely be perceived" (Aristotle).

With this statement, Aristotle reinforced Empedocles' earlier assertions regarding organic evolution. According to Aristotle's hypothesis of spontaneous generation, certain "particles" of matter contain a specific "active principle" that, under suitable conditions, can give rise to a living Organism. Aristotle was right in believing that this active principle is contained in a fertilized egg, but mistaken in thinking it is also present in sunlight, mud, and rotting meat.

"Such are the facts — living things can originate not only from the mating of animals, but also from the decomposition of soil... The same holds true for plants: some develop from seeds, while others seem to self-generate under the action of natural forces, emerging from decomposing earth or from certain parts of plants" (Aristotle).

With the spread of Christianity, the theory of the spontaneous generation of life fell out of favor; it was recognized only by those who believed in witchcraft and worshipped evil spirits, yet the idea persisted in the Background for many centuries to come.

Jan Baptista van Helmont (1579–1644), a highly renowned and successful scientist, described an experiment in which he allegedly created mice in three weeks. For this, he used a dirty shirt, a dark closet, and a handful of wheat. He considered human sweat to be the active principle in the Generation of the mice.

26.1. What omission did van Helmont make in his experiment?

In 1688, the Italian biologist and physician Francesco Redi, living in Florence, approached Structure/149.html">The problem of THE ORIGIN OF life more rigorously and challenged the theory of spontaneous generation. Redi established that the small white worms appearing on rotting meat were fly larvae. Having conducted a series of experiments, he obtained data supporting the concept that life can arise only from preexisting life (METABOLISM/2.html">THE CONCEPT OF biogenesis).

"Belief would remain in vain if it could not be confirmed experimentally. Therefore, in mid-July, I took four vessels, placed a snake in one of them, several fish in another, eels from the Arno in the third, and a piece of milk-fed veal in the fourth, and tightly closed and sealed them. Then I laid out the exact same things in four other vessels, but left them open" (Redi).

Redi described his findings as follows:

"Soon the meat and fish in the unsealed vessels became wormy; one could see flies freely flying in and out of the vessels. But in the sealed vessels, I did not see a single worm, although many days had passed since the dead fish was placed inside them" (Redi).

26.2. What, in your opinion, was Redi's initial assumption?

These experiments, however, did not lead to the abandonment of the idea of spontaneous generation, and although it receded somewhat into the background, spontaneous generation continued to be the leading theory in non-religious circles.

While Redi's experiments seemed to refute the spontaneous generation of flies, Anton van Leeuwenhoek's early microscopic studies reinforced this theory with regard to microorganisms. Leeuwenhoek himself did not enter into the disputes between the proponents of biogenesis and spontaneous generation, but his microscopic observations provided fuel for both theories and eventually prompted other scientists to conduct experiments to resolve the question of the origin of life via spontaneous generation.

In 1765, Lazzaro Spallanzani conducted the following experiment: after boiling meat and vegetable broths for several hours, he sealed them immediately, then removed them from the heat. Examining the Contents of the vessels a few days later, Spallanzani found no Signs of Life in them. From this, he concluded that high temperatures destroyed all forms of living things and that without them, no living thing could arise.

26.3. What other reason could have prevented the growth of microorganisms in Spallanzani's experiments?

In 1860, Louis Pasteur took up the problem of the origin of life. By that time, he had already achieved much in the field of microbiology and managed to solve problems that threatened the silk and wine industries. Pasteur also demonstrated that Bacteria are ubiquitous and that non-living Materials can easily be contaminated by living creatures if not properly sterilized.

26.4. What were Pasteur's main assumptions regarding the origin of life?

Data from a series of experiments based on Spallanzani's Methods allowed Pasteur to prove the validity of the theory of biogenesis and finally refute the theory of spontaneous generation.

However, the confirmation of the theory of biogenesis gave rise to another problem. Since the appearance of every new living organism requires the presence of another living organism, where did the very first living organism come from? Only the steady-state theory avoids answering this question; all other theories imply that at some stage in The history of life, a transition from non-living to living occurred. Was this primary spontaneous generation?