It's more the probabilities, I think[1] - even in one generation, (say) 26 trillion[0] bacterium are going to experience a lot of mutations.

[0] Which is an extremely low ball number for worldwide bacterium, even billions of years ago, I think, given your average human has around 30 trillion inside them.

[1] Like "anomalies in Minecraft" series - given how many seeds there are, how many people play it, and how big the worlds are, eventually even the rarest things generate naturally.