Spontaneous Generation

That's right, kids. Pasteur Was Wrong.

Funny thing, science. For ages, there was no certain way of determining that maggots grew from eggs laid by flies, so it was generally assumed they arose naturally from putrefying meat and other sources. It was a different time then—heavier rocks fell faster; bloodletting cured illness; we didn't know Soylent Green was people!

Funny aside: Spontaneous Generation is an ancient example of You Fail Biology Forever. It makes it possible for living things to spring up from non-living—or at least non-fecund—things through a vague and inexplicable process. It's most noticeable in myths—creation myths generally have no choice but to resort to this explanation for the First Mover that comes into existence after chaos. However, some scientists continued to believe in spontaneous generation of small creatures (e.g., flies appearing on carrion) up until the 18th century or so, when experiments with sterile, vacuum-sealed containers disproved it for good.

It should be noted that the extremely simple chemical processes scientists propose as the origins of life are not the same as maggots forming from rotting flesh, let alone a chicken popping out of a pile of primordial sludge. So don't even go there. Just don't.

See also Love Imbues Life.

Non-Myth Beliefs
"The first thing Tak did: he wrote himself."
 * Greek philosopher Aristotle, and by extension, his massive slavish medieval following, believed that insects had no parents, but were born right out of mud, corpses, etc.
 * In the Zombie Apocalypse novel series Autumn, the survivors seem to be under the impression that decomposing human bodies spontaneously generate all manner of disease.
 * Fictional example. In Babylon 5 Lorien claims that his entire super advanced species was created spontaneously by the universe. He might have just been metaphorical about it, though.
 * Given the way mushrooms seem to pop up overnight and their mysterious, invisible reproductive process (spores, which are so tiny they cannot be seen without a microscope), it's easy to see why people used to think mushrooms generated spontaneously and circles of fungi were magical "fairy rings."
 * Journey to the West gives us Sun Wukong, who hatched full-grown from a giant stone egg. Not much detail is given as to how or why this happens.
 * An episode of History Bites explained spontaneous generation to the audience of a kids' show hosted by Peter Oldring and Squeakus the Mouse.
 * The aspect of the Big Bang theory that draws the most questions is where the original particle came from. Since the possibility of eternal recurrences became widely acknowledged, consensus among proponents is that it came from the Big Crunch at the end of a previous universe. That's not the only explanation out there, however.
 * Fictional example from the Discworld is the Dwarf god Tak.

Egyptian Mythology

 * Older Than Dirt: All Ancient Egyptian creation myths start with spontaneous generation.
 * In one creation myth, one version has Hathor, a bovine sky goddess, created from a cosmic imbalance of primal forces. Then the sun god Re hatches from an egg laid on top of Hathor by a big heron (itself a sun-god) from, um, somewhere.
 * Alternatively, Re arises, weeping, out of a lotus bloom whose bud was formed from said prior imbalance, rather than Hathor. Or else the bud grew out of the waters of primordial chaos.
 * Another myth has the universe being masturbated and/or spat out by Atum, starting with the gods Tefnut and Shu. Atum created himself from nowhere first.
 * In the Ogdoad myth, the first eight gods of air/invisibility, darkness, eternal time, and water/chaos grew from the first mound of earth that spontaneously rose from the primordial chaos. The mound itself was deified as a god, and depending on interpretation Nun, god of the primordial chaos, could be his own father or grandfather.

Norse Mythology

 * In Norse Mythology, life began when the warm air of Muspelheim hit the cold ice of Niflheim, creating the giant Ymir and the cow Audhumla. Other giants grew from Ymir's sweat and Bori, the first of the gods, appeared from Audhumla's salt lick. Fun times.

Classical Mythology

 * Gaia, the goddess and personification of the earth, just sort of... forms from the dark, shapeless nothing that was primordial Chaos. (Except in versions where she hatches from a World Egg laid by even older gods.)
 * Interestingly, quantum physicists now consider it possible for matter and energy to spontaneously generate in the quantum foam that encompasses what we usually consider empty void (actually filled to the brim with low-grade energy) over the span of trillions of years. Even in the more conventional astronomy Earth was formed out of tiny particles colliding with each other over time. This could almost be considered a myth with a scientific backing, if taken as a metaphor.
 * Aphrodite, among others, sprung into existence when sky-god Uranus's newly liberated ballsack hit sea foam. Ouch. (Thanks to his son/brother Cronus.)
 * Some of his semen also fell on the earth, i.e. his wife/mother Gaia, and made yet more offspring, which is somewhat more borderline Spontaneous Generation, as godly gametes were at work, and wombs weren't considered more than passive vessels for male-contributed genes.
 * A few drops of blood were also spilled in the process, from which sprouted the Giants, Ash-nymphs, and Furies. You'd be pissed too.
 * Deucalion and his wife repopulate the human race after The Great Flood by tossing rocks over their shoulders, causing whole intelligent human beings to spring up.
 * As Perseus flew over the desert on his winged sandals, blood from Medusa's severed head fell on the sand and formed "poisonous asps and adders," as a Just-So Story element.
 * Pegasus and his less popular, human (!) (or winged bull) brother Chrysaor were born from Medusa post-mortem, and depending on the version, are either her children by Poseidon or an example of Spontaneous Generation. In one, her blood falls in the sea and spawns them, Aphrodite-style.
 * The teeth of Ares's water-dragon that Cadmus killed, once sown in the ground, grow into sentient warriors. In Cadmus's story, five of them help to found Thebes and produce offspring. In Jason's far more famous adventure, the remaining teeth's newborn warriors are all killed.

Film

 * In Tron: Legacy, the Isos spontaneously generated themselves from the Sea of Simulation and became an unknown, uncontrollable factor on the Grid. Flynn was thrilled, but his administrator / counterpart Clu viewed them as a threat, setting into motion the events of the film.

Video Games

 * Okami: According to his backstory, sub-boss Red Helm spontaneously sprung from the spilled blood of Orochi, a bigger bad.
 * In the original Sims, all the Sims had to do in order to make a baby (before expansion packs and later editions with the "Woo Hoo" function came out) was kiss passionately, and suddenly a baby bassinet would arrive in a shower of daisies.