In The Paris Review:
The semicolon was born in Venice in 1494.
It was meant to signify a pause of a length somewhere between that of the comma and that of the colon, and this heritage was reflected in its form, which combines half of each of those marks.
It was born into a time period of writerly experimentation and invention, a time when there were no punctuation rules, and readers created and discarded novel punctuation marks regularly.
Texts (both handwritten and printed) record the testing-out and tinkering-with of punctuation by the fifteenth-century literati known as the Italian humanists.
The humanists put a premium on eloquence and excellence in writing, and they called for the study and retranscription of Greek and Roman classical texts as a way to effect a “cultural rebirth” after the gloomy Middle Ages.
In the service of these two goals, humanists published new writing and revised, repunctuated, and reprinted classical texts.