Any number of tunes we think of as folk tunes saw service as hymn tunes, before the songs we know that use them came into being. And, very likely, they may have been folk tunes used for for earlier folk songs before the hymns were written to them. And before that...

And so ad infinitum. Because sooner or later someone is going to use the folk tune in the first line of this post to write another hymn or similar, andn teh whole cycle begins again.

Now that needs to be fleshed out with examples. Maybe someone will do it.