According to Keith Houston, the “germ” of the quotation mark is to be found in the “diple (>)” placed by first-century scribes in the margin to indicate a line which contained “some noteworthy text”. Christian scholars used the diple to reveal the presence of that most noteworthy of texts, the Bible, but as their theological disputes became more and more involved they started using it to distinguish their own words from those of their opponents. With the invention of the printing press, compositors began reaching for a pair of commas (“,,”) to indicate quotations, hanging doubled commas in the margin of passages containing quoted text. But in the 18th century, Houston explains, the impetus to standardise the use of quotation marks came from the “drive for realism” shown by authors such as Daniel Defoe, Henry Fielding and Samuel Richardson as they experimented with the newest form of literature, the novel.
Eschewing paraphrased, reported speech filtered through a narrator, these new novelists presented readers with their characters’ unvarnished words, and with this new directness came a need to separate speech from narration.
The 1748 edition of Clarissa separated speakers with “dashes or new lines”, but sometimes placed an opening quotation mark “at the exact point at which a quotation began, with a new ‘mark of silence’, or closing quotation mark (“), accompanying it where the quotation ended”. The 1765 edition of Moll Flanders showed changes in speaker with paragraph breaks, “though marginal inverted commas were retained for the occasional sententious quotation”. But by the end of the 18th century, Houston continues, “the growing pains of the double comma were largely past”.
– Don’t Be Scared: Dialogue Without Quotation Marks, Richard Lee @ the Guardian.