Where did the quotation mark originate?

Lynne Truss attributes an early form of the question mark to Alcuin of York. Truss describes the punctus interrogativus of the late 700s as "a lightning flash, striking from right to left. The punctuation system of Aelius Donatus, current through the Early Middle Ages, used only simple dots at various heights. This early question mark was a decoration of one of these dots, with the "lightning flash" perhaps meant to denote intonation.