thumb|upright=1.35|Spontaneous generation of seashells, according to [[Aristotle, varied with the nature of the seabed. Slime gave rise to oysters; sand, to scallops; and the hollows of rocks, to limpets and barnacles. People kept on wondering, though, whether the eggs of these animals might not be central to the generation process.]]
Spontaneous generation is a superseded scientific theory that held that living creatures could arise from non-living matter and that such processes were commonplace and regular. It was hypothesized that certain forms, such as fleas, could arise from inanimate matter such as dust, or that maggots could arise from dead flesh. The doctrine of spontaneous generation was coherently synthesized by the Greek philosopher and naturalist Aristotle, who compiled and expanded the work of earlier natural philosophers and the various ancient explanations for the appearance of organisms. Spontaneous generation was taken as scientific fact for two millennia. Though challenged in the 17th and 18th centuries by the experiments of the Italian biologists Francesco Redi and Lazzaro Spallanzani, it was not discredited until the work of the French chemist Louis Pasteur and the Irish physicist John Tyndall in the mid-19th century.
Among biologists, rejecting spontaneous genesis is no longer controversial. Experiments conducted by Pasteur and others were thought to have refuted the conventional notion of spontaneous generation by the mid-1800s. Since all life appears to have evolved from a single form approximately four billion years ago, attention has instead turned to the origin of life.
Description
"Spontaneous generation" means both the supposed processes by which different types of life might repeatedly emerge from specific sources other than seeds, eggs, or parents, and the theoretical principles presented in support of any such phenomena. Crucial to this doctrine are the ideas that life comes from non-life and that no causal agent, such as a parent, is needed. Supposed examples included the seasonal generation of mice and other animals from the mud of the Nile, the emergence of fleas from inanimate matter such as dust, or the appearance of maggots in dead flesh. Such ideas have something in common with the modern hypothesis of the origin of life, which asserts that life emerged some four billion years ago from non-living materials, over a time span of millions of years, and subsequently diversified into all the forms that now exist.
The term equivocal generation, sometimes known as heterogenesis or xenogenesis, describes the supposed process by which one form of life arises from a different, unrelated form, such as tapeworms from the bodies of their hosts.
Antiquity
Pre-Socratic philosophers
Active in the 6th and 5th centuries BCE, early Greek philosophers, called physiologoi in antiquity (Greek: φυσιολόγοι; in English, physical or natural philosophers), attempted to give natural explanations of phenomena that had previously been ascribed to the agency of the gods. The physiologoi sought the material principle or arche (Greek: ἀρχή) of things, emphasizing the rational unity of the external world and rejecting theological or mythological explanations.
Anaximander, who believed that all things arose from the elemental nature of the universe, the apeiron (ἄπειρον) or the "unbounded" or "infinite", was likely the first western thinker to propose that life developed spontaneously from nonliving matter. The primal chaos of the apeiron, eternally in motion, served as a platform on which elemental opposites (e.g., wet and dry, hot and cold) generated and shaped the many and varied things in the world. According to Hippolytus of Rome in the third century CE, Anaximander claimed that fish or fish-like creatures were first formed in the "wet" when acted on by the heat of the sun and that these aquatic creatures gave rise to human beings. The Roman author Censorinus, writing in the 3rd century, reported:
The Greek philosopher Anaximenes, a pupil of Anaximander, thought that air was the element that imparted life and endowed creatures with motion and thought. He proposed that plants and animals, including human beings, arose from a primordial terrestrial slime, a mixture of earth and water, combined with the sun's heat. The philosopher Anaxagoras, too, believed that life emerged from a terrestrial slime. However, Anaximenes held that the seeds of plants existed in the air from the beginning, and those of animals in the aether. Another philosopher, Xenophanes, traced the origin of man back to the transitional period between the fluid stage of the Earth and the formation of land, under the influence of the Sun.
In what has occasionally been seen as a prefiguration of a concept of natural selection, Empedocles accepted the spontaneous generation of life, but held that different forms, made up of differing combinations of parts, spontaneously arose as though by trial and error: successful combinations formed the individuals present in the observer's lifetime, whereas unsuccessful forms failed to reproduce.
Aristotle
In his biological works, the natural philosopher Aristotle theorized extensively the reproduction of various animals, whether by sexual, parthenogenetic, or spontaneous generation. In accordance with his fundamental theory of hylomorphism, which held that every physical entity was a compound of matter and form, Aristotle's basic theory of sexual reproduction contended that the male's seed imposed form, the set of characteristics passed down to offspring on the "matter" (menstrual blood) supplied by the female. Thus female matter is the material cause of generation—it supplies the matter that will constitute the offspring—while the male semen is the efficient cause, the factor that instigates and delineates the thing's existence. Yet, Aristotle proposed in the History of Animals, many creatures form not through sexual processes but by spontaneous generation:
According to this theory, living things may come forth from nonliving things in a manner roughly analogous to the "enformation of the female matter by the agency of the male seed" seen in sexual reproduction. Nonliving materials, like the seminal fluid present in sexual generation, contain pneuma (πνεῦμα, "breath"), or "vital heat". According to Aristotle, pneuma had more "heat" than regular air did, and this heat endowed the substance with certain vital properties:
Aristotle drew an analogy between the "foamy matter" (τὸ ἀφρῶδες, to aphrodes) found in nature and the "seed" of an animal, which he viewed as being a kind of foam itself (composed, as it was, from a mixture of water and pneuma). For Aristotle, the generative materials of male and female animals (semen and menstrual fluid) were essentially refinements, made by male and female bodies according to their respective proportions of heat, of ingested food, which was, in turn, a byproduct of the elements earth and water. Thus any creature, whether generated sexually from parents or spontaneously through the interaction of vital heat and elemental matter, was dependent on the proportions of pneuma and the various elements which Aristotle believed comprised all things. 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.
With varying degrees of observational confidence, Aristotle theorized the spontaneous generation of a range of creatures from different sorts of inanimate matter. The testaceans (a genus which for Aristotle included bivalves and snails), for instance, were characterized by spontaneous generation from mud, but differed based upon the precise material they grew in—for example, clams and scallops in sand, oysters in slime, and the barnacle and the limpet in the hollows of rocks.
As the dominant view of philosophers and thinkers continued to be in favour of spontaneous generation, some Christian theologians accepted the view. The Berber theologian and philosopher 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" () as decrees that would enable ongoing creation.