The History of Anglo-European Punctuation (Heavily Abridged!)
Most of this information is based upon David B. Thomas, “Whence the Semicolon?,” Early English and Norse Studies (London: Methuen, 1963) 191-195 and Malcolm Parkes' Pause and Effect: An Introduction to the History of Punctuation in the West (Berkeley: U of California P, 1983). The rest is personal observation of punctuation conventions in late Middle English and Early Modern English manuscripts and facsimiles of manuscripts. In brief, it can be summarized thusly: punctuation conventions change based on the needs of speakers and hearers (at first) and then writers and readers (next) and finally printers, who need regular spelling and punctuation patterns to calculate how much type and paper a given text will use up. Blame the printers, and science, for spelling and punctuation regularization, but conventions always existed.
Punctuation in its Earliest Stages: Greek Rhetoricians
Many of the terms we now use for punctuation marks once were technical terms to describe parts of an oration.
1) “Paragraph”—(from paragraphos—to write beside): a line in the text’s margin to denote a change of speakers, hence a change in source, intention, topic, etc., which would need a shift in the performer’s spoken pitch. The paragraph symbol (¶) originated as a scribe's marginal indication that the text had introduced a new line of thinking, and became a printer's type ornament which was called, in English, a "pilcrow" (which the OED says is a deformation of the French pelagraphe for something which stands beside the writing [graphe]).
2) “Period”—(from periodos—a circuit, around or away): a dot in the line marking the end of a logical expression made up of several clauses.
3) “Colon”—(from kolon—poetic jargon for a unit of meter): part of a rhythmical period, indicated by two dots in the line to connect that part with the next part as part of a chain containing two to six groups of words.
4) “Comma”—(from komma—a short clause; from koptein, to cut): a pause or separation between a set of related ideas, similar to the caesura or middle-of-line break in epic poetry
(“Hwæt we gardena in geardagum”
“Oft we have heard in elder days”—Beowulf’s first line)
5) “Commation” or section—like a comma in a list of like elements split like the “sections” of an orange or of an Euclidean solid (e.g., “cats, like the Siamese, Burmese, and Himalayan…”--note that this example uses the "serial" or "Oxford" comma before the final "and," which some style sheets reject).
Punctuation’s Middle (Classical) Era: Latin “minuscule” script, that which was used for all but capital (“majuscule”) letters
This is a sample line with comma (subdistinctio) to divide with a dot on the scribe’s guide-line a phrase. or two. and a colon (media distinctio) half-way above the line to separate a clause (sorry—couldn’t make Microsoft put the dot half-way up) : following such a clause might be another independent clause followed by a superscript “period” or dot above the line like this.
Middle English to Early Modern and Modern English Printers’ and Authors’ Punctuation Conventions—
1400 Chaucer dies. Surviving manuscripts of Chaucer's works, all produced after his death, contain almost no punctuation other than the capital letter beginning sentences and the virgule (/) or full stop, also known as the period, usually seen at the ends of major rhetorical units in verse (roughly our "paragraph"). No quotation marks were used. Readers had to figure out dialogue on their own.
1468 Gutenberg dies. Punctuation of the Bible was regularized by Jerome and Alcuin, and the text was printed as it had been copied by manuscript scribes, character by character, with no allowance for change. Secular books, when they came to be printed in the incunabular period (1451--1501), tended to have highly individualized punctuation that depended on the "house style" of individual printers.
1475-91 William Caxton, England’s first and most successful printer in the days of “incunabula” (cradle books—the earliest), uses the double-virgule (//) for full stops and occasional periods for ends of paragraphs. He invents the “chapter” or “capitulum” based on manuscript divisions marked by scribes with large capital letters (i.e., “majescules”).
1536-42 Sir Thomas Wyatt’s diplomatic correspondence, preserved in the Pipe Rolls archives, contain at least one semi-colon as we now know them, a comma below a period separating a complex sentence’s clauses, but not necessarily two independent clauses. A period is used for a “full stop” common, “virgule” (/) still used for a full stop instead of a period if the writer uses no commas, but commas and periods used together. Usage, like spelling, is according to the ear and the author’s mood.
1561 Aldus Manutius, Venetian publisher of the famous Aldine Press editions of Humanist works, for his printing establishment proposes use of the semi-colon in printed books to regularize coordination of long clauses. Defined the period as the end of a complete grammatical unit (i.e., a sentence). The wide European success of Aldine Press books spread the printer's standardized usages of the semi-colon and period.
1564 Shakespeare born. All of the quarto editions of his plays, printed without his permission, were very lightly pointed.
1600 Frequent use of the semi-colon in English manuscript correspondence.
1616 Shakespeare dies. Seven years later, in 1623, Hemmings and Condell combined with Henry Jaggard, a successful printer, to organize production of the First Folio edition of Shakespeare's plays. Subsequent editions, like the Fourth Folio of 1685, continued to follow Jaggard's shop's pointing conventions.
1623 Ben Jonson’s English Grammar lays out rules for usage, including punctuation, but as a prescriptive grammar it is slow to gain popularity against the tide of common usage.
1642-49 English Civil War—transformation of government from monarchy to parliamentary dictatorship; end of press censorship but closing of the theaters, and emergence of the popular press as a news source for newly literate commoners.
1755 Samuel Johnson publishes the Dictionary of the English Language in which he argues that English spelling and punctuation should be regularized so that the English people can more reliably understand each other's writing.
Semi-Colon’s Emergence from a Printer’s Abbreviation
. = a period
, = a virgula (after the “/” ["forward slash"] is abandoned as a sub-period clause divider because it is too like the letter “l”)
q; = a scribal abbreviation mark for “que” and “banq;” for “banque” (bank)
Last Stage: Semi-colons indicate a pause or distinction between ideas greater than those indicated by a comma and less than those indicated by a colon, but within those demarcated by an initial capital and a terminal period.
For more detailed discussion of the evolution of punctuation, see Malcolm B. Parkes, Pause and Effect: An Introduction to the History of Punctuation in the West (Berkeley: U of California P, 1983) [Available at the Goucher Library, 411.P245p]