From the Weekly Standard:

...in the third century b.c., Aristophanes of Byzantium, head of the Library of Alexandria, devised the first system of punctuation in order to make reading easier. He created a series of three dots, or distinctiones, to indicate places within a text at which a reader should pause and how long that pause should last. These marks were the komma (·), the kolon (.), and the periodos (·), indicating a short pause, medium pause, and long pause, respectively.

Usefully brief introduction to the subject.