There was, Foolestroupe, quite recently on the BBC a series on the history of sacred music, which , I think, said it was just the chanting of a single-line in churches until the 13th or 14th century, when Italian composers began to employ polyphony. (Must of been 13th, as Jack above mentioned "Summer is a Comin in" as 13th century sacred polyphony, here in England.)
I like a lot of Gaelic music, Stigweard - except the mimicing of Amerindian chants.