Why (and how) did humans evolve the ability to use papain if it's only found in papaya

Papaya is a fruit that is limited to specific types of environments - environments that most human beings didn't evolve in. So how is it that papain is compatible with our digestive processes?

(By the way, this isn't a question aimed at dismissing evolution. I accept evolution.)

Papain has nothing to do with humans.
Papain is simply a proteoytic enzyme. It will catalyze the breakdown of any protein.
Various proteolytic enzymes are found in all kinds of cells.

Papain is able to readily break certain peptide bonds that some people's own proteolytic enzymes have difficulty with. And it happens to be present in large enough quantities in papaya and pineapple that we can harvest it and use it.