In the real world, the idea of spontaneous generation, where maggots and flies come from rotting meat was disproved and replaced by the actual method of like creatures spawning like creatures. That is called biogenesis.
However, in a fantasy world, spontaneous generation can be real.
https://en.wikipedia.org/wiki/Spontaneous_generation
Here's the companion blog article.
https://followmeanddie.com/2019/02/28/spontaneous-generation/
