Excellent question, dmuldawer. "Sumer is icumen in" is the oldest manuscript we know of, and already shows an expertise presumably based on a century-old tradition. Rounds can be viewed as the most rigid form of imitation, a principle that is much older. The problem is that written sources were ususally restricted to the highbrow culture of the monasteries, where the artistic value of imiatio was often questioned, in favour of varietas. The "Sumer" manuscript seems to be exceptional in many senses.