Biogenesis is the production of life. In Latin, bio means life, and genesis means the beginning or origin of. For most of history, mankind thought that biogenesis often occurred through spontaneous generation from earth or vegetable matter, alongside reproduction, which we now know is the only way that biogenesis ever happens. Anaximenes and Anaxagoras, pre-Aristotle Greek natural philosophers, believed that biogenesis could occur from the action of the Sun on primordial terrestrial slime, a combination of water and earth. A related idea is xenogenesis, which argues that one type of life form can arise from another, completely different life form.
Around 343 BCE, Aristotle wrote the book History of Animals which laid out the spontaneous generation theory of biogenesis which would remain dominant for over 2000 years. Besides including lengthy descriptions of countless species of fish, shellfish, and other animals, the book also presented Aristotle's theory of how animals come to be in the first place. Aristotle believed that different animals could spontaneously arise from different forms of inanimate matter -- clams and scallops in sand, oysters in slime, and the barnacle and the limpet in the hollows of rocks. However, no one seemed to claim that humans could emerge from spontaneous generation, being higher creatures that can apparently only be produced through direct reproduction by other humans.
As early as 1668, the Italian physician Francesco Redi proposed that higher forms of life (microbes) did not arise spontaneously, and the idea got more popular, but the proponents of spontaneous generation still maintained that microbes arose via these means. In 1745, John Needham, an English biologist and Roman Catholic priest, added chicken broth to a sealed flask, boiled it, waited, then observed microbial growth, pointing to this as an example of spontaneous generation. In 1768, Lazzaro Spallanzani repeated this same experiment, but removed all air from the jar, and microbes did not grow within it. This must have been one of the earliest experiments to conclusively disprove spontaneous generation, but the idea that spontaneous generation was false did not spread at the time.
Moving on to 1859, French biologist Louis Pasteur finally disproved spontaneous generation for good. He boiled meat broth in a goose-necked flask. The goose neck allowed in air but not, as the reasoning went, tiny particles from the air. The experiment showed that microbial growth did not occur in the flask until the flask was turned so that particles could fall down the bends, at which point the water quickly became cloudy, showing the presence of microorganisms. After 2000 years, the spontaneous generation theory of biogenesis was finally brought to rest. Today, it has been replaced by cell biology and the biology of reproduction.
