Spontaneous Generation

In the 6th and 5th centuries BCE, Anaximander looked for an origin of life that did not rely on the gods. He theorized that life arose from the "wet" when acted on by the sun. The way that small life forms appeared in puddles gave him the idea that life—including human life—started as an aquatic form and progressed. This has caused some to claim he was a "prophet" of Darwin and others.

His student, Anaximenes, thought there was a primordial slime consisting of earth and water on which the sun worked to form plants and animals—including human beings.

"Spontaneous Generation" was accepted in the Middle Ages, especially after Aristotle became widely available. His History of Animals explained that animals were generated directly from the elements. Here are some translations of his thinking:

...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.

 ...

...all testaceans [creatures having a shell] grow by spontaneous generation in mud, differing from one another according to the differences of the material; oysters growing in slime, and cockles and the other testaceans above mentioned on sandy bottoms;

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Other insects ... are generated spontaneously: some out of dew falling on leaves, ordinarily in spring-time...; others grow in decaying mud or dung; others in timber, green or dry; some in the hair of animals; some in the flesh of animals; some in excrements: and some from excrement after it has been voided, and some from excrement yet within the living animal, like the helminthes or intestinal worms. 

(See the illustration, where flies spring from the corpse of a quadruped.)

Some writers actually aver that mullet all grow spontaneously. In this assertion they are mistaken, for the female of the fish is found provided with spawn, and the male with milt. However, there is a species of mullet that grows spontaneously out of mud and sand.

Shakespeare refers to spontaneous generation as if it were still a known and accepted occurrence in the Elizabethan Era. "Your serpent of Egypt is bred now of your mud by the/operation of your sun. So is your crocodile." [Antony and Cleopatra]

Spontaneous Generation was eventually supplanted by the understanding that animals could be extremely tiny (and cause infection, leading to germ theory) and that small eggs or larvae could explain creatures arising from corpses or puddles or slime.

There was one anecdote about the generation of life that required the intervention of a pope to help set people straight. Next time I'll tell you about the barnacle goose myth.