The Theory of Spontaneous Generation has been around since the Ancient Roman times, and was ultimately disproven in the 19th Century, specifically in 1859.
The original theory was hypothesized by Aristotle, who put together the work of natural philosophers preceding him such as Anaximenes and Empedocles. Aristotle believed that animals spontaneously generated from mud, decaying matter, or waste.
Charles Darwin also theorized that chemical objects developed without real cause into the complex single-cell organisms that then evolved into multi-cellular organisms.