What an interesting question Cathy. What’s left of my memory tells me that using lines to indicate pitch started with Gregorian Chant. So maybe we’re talking 5th century. It started with one line. As time went on more lines were added until there were so many it was like looking at striped wallpaper. So the original line was eliminated. This was the C which is the ledger line below the treble staff and the ledger line above the bass staff, thus separating all these lines into two staves. Where the flat, natural, and sharp symbols came from I have no idea.