Here is "Worldes Blis Ne Last", a 13th century tune:
Ick.
Now I reckon ye olde music transcribers may have gotten a few notes wrong. Probably got the sequence of ups and down right, and I'll make a rather unjustified leap and guess that the same written note always refers to the same sung note, but the actual notes may differ from what they should be. So here is a version with the note values automagically remapped to be on average more consonant with the final note (which is presumably the tonic):
Well, I like it better. It may or may not be closer to the original...
It's even nicer with a few extra voices...