While there’s plenty of literature for beginners on existing symbiotic relationships, less discussed is the origin. Humans and gut bacteria are a good example – the bacteria needed to be the sort that wouldn’t kill us, and our stomach needed to be a suitable environment for them to thrive within. Here’s the thoughts of someone:

While I’m not knee deep in literature about the subject, it seems to me to be an evolutionary symbiotic relationship: bacteria we ingested that could survive in our GI tract and didn’t cause us to poo poo ourselves to death and didn’t cause any other negative side-effects that would prevent it from being passed down from mother to child would be readily able to live in a generation-to-generation symbiotic relationship inside our bodies. Of course there are some bacteria like Clostridium difficile that live in us but are kept in check by the immune system, and when the immune system fails or is artificially inhibited, they can grow uncontrolled and cause all sorts of havoc, so not every bacteria inside us is totally harmless.

Source: Something Awful