The invention of word spacing and punctuation:

”... the earliest hieroglyphic and alphabetic inscriptions had no punctuation symbols at all. No commas to indicate pauses and no periods between sentences. In fact, there weren’t even spaces between words. Nor did the early Greek and Roman writers use any form of punctuation. Knowing exactly how to read the words, where to put the intonations, pauses, etc., was an art, and one that required practice…. The use of spaces ( ) for interword separation didn’t appear until much later, roughly 600-800 AD. By the seventh century, the convention was quite common. In some early medieval manuscripts, two vertically aligned dots represented a full stop at the end of a sentence. Eventually one of the dots was dropped, and the remaining dot served as a period, colon or comma, depending on whether it was aligned with the top, middle, or base of the lowercase letters.” (from “History of Punctuation,” Complete Translation Services, Inc.)

Starter Links or References: Alberto Manguel, A History of Reading (New York: Viking, 1996): 47-50 | Complete Translation Services, Inc. article on “History of Punctuation”(article on site)