biogenesis

Biogenesis is the principle that living things come only from other living things — and, at the cell level, that every cell arises from a pre-existing cell. It sounds obvious today, but for most of history people believed the opposite: that maggots simply appeared in rotting meat, mice grew from stored grain, and microbes sprang from broth on their own. That idea was called spontaneous generation.

Biogenesis is the rejection of spontaneous generation, settled by careful experiments. In the 1660s Francesco Redi showed that maggots come from fly eggs, not from meat, by keeping flies away with gauze. The decisive blow came in the 1860s when Louis Pasteur boiled broth in swan-neck flasks: the curved necks let air in but trapped dust and microbes, and the broth stayed clear forever — yet spoiled within days once the neck was broken. Life had to come from life. At the cellular level, the same idea is captured in Rudolf Virchow's phrase omnis cellula e cellula, 'every cell from a cell,' which became cell theory's third pillar.

Biogenesis is why a cut heals, a baby grows, and a yeast colony spreads — always by existing cells dividing, never by new cells condensing out of nonliving matter. One honest subtlety: biogenesis describes how life continues now, not how the very first life began. The deep-time origin of life from chemistry (abiogenesis) is a separate, open scientific question; biogenesis simply says that ever since, life has only ever come from life.