Spontaneous generation

This article is about historical theories on the ongoing emergence of life. For the modern hypothesis of the origin of life, see Abiogenesis.

Spontaneous generation or Equivocal generation is an obsolete principle regarding the origin of life from inanimate matter, which held that this process was a commonplace and everyday occurrence, as distinguished from univocal generation, or reproduction from parent(s). The hypothesis was synthesized by Aristotle, who compiled and expanded the work of prior natural philosophers and the various ancient explanations of the appearance of organisms; it held sway for two millennia. It is generally accepted to have been ultimately disproven in the 19th century by the experiments of Louis Pasteur, expanding upon the experiments of other scientists before him (such as Francesco Redi who had performed similar experiments in the 17th century). Ultimately, it was succeeded by germ theory and cell theory.

The disproof of ongoing spontaneous generation is no longer controversial, now that the life cycles of various life forms have been well documented. However, the question of abiogenesis, how living things originally arose from non-living material, remains relevant today.

Description

Spontaneous generation refers to both the supposed process by which life would systematically emerge from sources other than seeds, eggs or parents and to the theories which explained the apparent phenomenon. The first form is abiogenesis, in which life emerges from non-living matter. This should not be confused for the modern hypothesis of abiogenesis, in which life emerged once and diversified. The second version is heterogenesis (sometimes called xenogenesis), in which one form of life emerges from a different form.

Pre-Aristotelian philosophers

As part of his overall attempt to give natural explanations of things that had previously been ascribed to the agency of the gods, Anaximander believed that everything arose out of the elemental nature of the universe, which he called the "apeiron" or "unbounded". According to Hippolytus in the third century CE, Anaximander claimed that living creatures were first formed in the "wet" when acted on by the Sun, and that they were different then than they are now. For example, he claimed humans, in a different form, must have earlier been born mature like other animals, or they would not have survived. Anaximander also claimed that spontaneous generation continued to this day, with aquatic forms being produced directly from lifeless matter.

Anaximenes, a pupil of Anaximander, thought that air was the element that imparted life, motion and thought, and speculated that there was a primordial terrestrial slime, a mixture of earth and water, which when combined with the sun's heat formed plants, animals and human beings directly.

Xenophanes traced the origin of man back to the transitional period between the fluid stage of the earth and the formation of land. He too held to a spontaneous generation of fully formed plants and animals under the influence of the sun.

Empedocles accepted the spontaneous generation of life, but held that there had to be trials of combinations of parts of animals that spontaneously arose. Successful combinations formed the species we now see, unsuccessful forms failed to reproduce.

Anaxagoras also adopted a terrestrial slime account, although he thought that the seeds of plants existed in the air from the beginning, and of animals in the aether.

Aristotle

Aristotle lay the foundations of Western natural philosophy. In his book, The History of Animals, he stated in no uncertain terms:

Now there is one property that animals are found to have in common with plants. For some plants are generated from the seed of plants, whilst other plants are self-generated through the formation of some elemental principle similar to a seed; and of these latter plants some derive their nutriment from the ground, whilst others grow inside other plants, as is mentioned, by the way, in my treatise on Botany. So with animals, some spring from parent animals according to their kind, whilst others grow spontaneously and not from kindred stock; and of these instances of spontaneous generation some come from putrefying earth or vegetable matter, as is the case with a number of insects, while others are spontaneously generated in the inside of animals out of the secretions of their several organs.

—Aristotle, History of Animals, Book V, Part 1

According to this theory, living things came forth from nonliving things because the nonliving material contained pneuma, or "vital heat". The creature generated was dependent on the proportions of this pneuma and the five elements he believed comprised all matter. While Aristotle recognized that many living things emerged from putrefying matter, he pointed out that the putrefaction was not the source of life, but the byproduct of the action of the "sweet" element of water.

Animals and plants come into being in earth and in liquid because there is water in earth, and air in water, and in all air is vital heat so that in a sense all things are full of soul. Therefore living things form quickly whenever this air and vital heat are enclosed in anything. When they are so enclosed, the corporeal liquids being heated, there arises as it were a frothy bubble.

—Aristotle, On the Generation of Animals, Book III, Part 11

Scallop

Numerous forms were attributed to various sources. The testaceans (shelled molluscs) are characterized by forming by spontaneous generation in mud, but differ based upon the material they grow in — for example, clams and scallops in sand, oysters in slime, and the barnacle and the limpet in the hollows of rocks. Some reddish worms form from long-standing snow which has turned reddish. Another grub was said to grow out of fire.

Concerning sexual reproduction, Aristotle argued that the male parent provided the "form," or soul, that guided development through semen, and the female parent contributed unorganized matter, allowing the embryo to grow.

Classical writers after Aristotle

Vitruvius, a Roman architect and writer of the 1st century BCE, advised that libraries be placed facing eastwards to benefit from morning light, but not towards the south or the west as those winds generate bookworms.

Aristotle claimed that eels were lacking in sex and lacking milt, spawn and the passages for either. Rather, he asserted eels emerged from earthworms. Later philosophers dissented. Pliny the Elder did not argue against the anatomic limits of eels, but stated that eels reproduce by budding, scraping themselves against rocks, liberating particles that become eels. Athenaeus described eels as entwining and discharging a fluid which would settle on mud and generate life. On the other hand, Athenaeus also dissented towards spontaneous generation, claiming that a variety of anchovy did not generate from roe, as Aristotle stated, but rather, from sea foam.

Adoption in Christianity

The goose barnacle:
Pollicipes cornucopia

The Barnacle Goose:
Branta leucopsis

Augustine of Hippo discussed spontaneous generation in The City of God and The Literal Meaning of Genesis, citing Biblical passages such as "Let the waters bring forth abundantly the moving creature that hath life" (Genesis 1:20) as decrees that would enable ongoing creation. Augustine emphasized that appearing ignorant of the natural world would not cast Christians in a good light. Other writers pointed to the generation of animals at various times in the narrative of the Exodus, and the incident of the beehive generated in the carcass of the Lion in the story of Samson in Judges 14.

From the fall of the Roman Empire in 5th century to the East-West Schism in 1054, the influence of Greek science declined, although spontaneous generation generally went unchallenged. New descriptions were made. Of the numerous beliefs, some had doctrinal implications outside of the Book of Genesis. For example, the idea that a variety of bird known as the Barnacle Goose emerged from a crustacean known as the Goose Barnacle, had implications on the practice of fasting during Lent. In 1188, Gerald of Wales, after having traveled in Ireland, argued that the "unnatural" generation of barnacle geese was evidence for the Immaculate Conception. Where the practice of fasting during Lent allowed fish, but prohibited fowl, the idea that the goose was in fact a fish suggested that its consumption be permitted during Lent. The practice was eventually prohibited by decree of Pope Innocent III in 1215.

Aristotle, in Arabic translation, was reintroduced to Western Europe. During the 13th century, Aristotle reached his greatest acceptance. With the availability of Latin translations Saint Albertus Magnus and his student, Saint Thomas Aquinas, raised Aristotelianism to its greatest prominence. Albert wrote a paraphrase of Aristotle, De causis et processu universitatis, in which he removed some and incorporated other commentaries by Arabic scholars. The influential writings of Aquinas, on both the physical and metaphysical, are predominantly Aristotelian, but show numerous other influences.

Spontaneous generation is discussed as a fact in literature well into the Renaissance. Where, in passing, Shakespeare discusses snakes and crocodiles forming from the mud of the Nile (Ant 2.7 F1), Izaak Walton again raises the question of the origin of eels "as rats and mice, and many other living creatures, are bred in Egypt, by the sun's heat when it shines upon the overflowing of the river...". While the ancient question of the origin of eels remained unanswered and the additional idea that eels reproduced from corruption of age was mentioned, the spontaneous generation of rats and mice engendered no debate.

Scientific method

The last great proponent, as experimentation began to transform science, was Jan Baptist van Helmont (1580–1644). He used experimental techniques, such as growing a willow for five years and showing it increased mass while the soil showed a trivial decrease in comparison. As the process of photosynthesis was not understood, he attributed the increase of mass to the absorption of water. His notes also describe a recipe for mice (a piece of soiled cloth plus wheat for 21 days) and scorpions (basil, placed between two bricks and left in sunlight). His notes suggest he may even have done these things.

Where Aristotle held that the embryo was formed by a coagulation in the uterus, William Harvey's dissection of deer showed that there was no visible embryo during the first month. Although his work predated the microscope, this led him to suggest that life came from invisible eggs. In the frontispiece of his book Exercitationes de Generatione Animalium (Essays on the Generation of Animals), he made an expression of biogenesis: "omnia ex ovo" (everything from eggs).

Illustration of the Swan-necked bottle used in Pasteur's experiments to disprove spontaneous generation

The ancient beliefs were subjected to testing. In 1668, Francesco Redi challenged the idea that maggots arose spontaneously from rotting meat. In the first major experiment to challenge spontaneous generation, he placed meat in a variety of sealed, open, and partially covered containers. Realizing that the sealed containers were deprived of air, he used "fine Naples veil", and observed no worm on the meat, but they appeared on the cloth.

In 1745, John Needham performed a series of experiments on boiled broths. Believing that boiling would kill all living things, he showed that when sealed right after boiling, the broths would cloud, allowing the belief in spontaneous generation to persist. His studies were rigorously scrutinized by his peers and many of them agreed.

Lazzaro Spallanzani modified the Needham experiment in 1768, attempting to exclude the possibility of introducing a contaminating factor between boiling and sealing. His technique involved boiling the broth in a sealed container with the air partially evacuated to prevent explosions. Although he did not see growth, the exclusion of air left the question of whether air was an essential factor in spontaneous generation.

In 1837, Charles Cagniard de la Tour, a physicist, and Theodor Schwann, one of the founders of cell theory, published their independent discovery of yeast in alcoholic fermentation. They used the microscope to examine foam left over from the process of brewing beer. Where Leeuwenhoek described "small spheroid globules", they observed yeast cells undergo cell division. Fermentation would not occur when sterile air or pure oxygen was introduced if yeast were not present. This suggested that airborne microorganisms, not spontaneous generation, was responsible.

Louis Pasteur's 1859 experiment put the question to rest. He boiled a meat broth in a flask that had a long neck which curved downward, like a goose. The idea being that the bend in the neck prevented any particles from reaching the broth, while still allowing the free flow of air. The flask remained free of growth for an extended period. When the flask was turned so that particles could fall down the bends, the broth became quickly clouded.