This is what I remember from my reading on the origins of the ballad form. I cannot vouch for its veracity and it may be oversymplistic.
The form itself evolved from French carols in the medieval period, usually with the couplet and refrain form which Child claims precedes the quatrain form. This is certainly borne out in the examples he gives but may not be the full picture. The few ballads that came down to us in tradition that predate print can be counted on the fingers of one hand, and even these are from manuscripts which were simply the forerunners of print, i.e., copied down by scribes from one manuscript to another for the use of the wealthy. We have very contentious clues as to what was actually sung (other than religious material) before print. The Complaynt of Scotland for instance gives us a few titles that MIGHT relate to songs still sung today, the most likely one being 'The Frog Cam to the Mil Dur'. My own opinion on the 'Tomlin' mentioned there is that it is not 'Tam Lyn' but the early Scots version of 'Tom-a lin' aka 'Brian O Lynn', for which we have very early versions, much earlier than any version of 'Tam Lyn'.