Wikipedia has:

It was not until the 14th century that something like the present system of fixed note lengths arose. Starting in the 15th century, vertical bar lines were used to divide the staff into sections.These did not initially divide the music into measures (bars) of equal length (as most music then featured far fewer regular rhythmic patterns than in later periods), but appear to have been introduced as an aid to the eye for "lining up" notes on different staves that were to be played or sung at the same time.The use of regular measures (bars) became commonplace by the end of the 17th century.