From elegant line drawings in the 30,000 year old Caves D’Arcy in central France, to the triumphal graffiti of Russian troops who captured the Reichstag in 1945, we have scratched, etched and painted from time immemorial. In A Brief History of Graffiti - part of BBC’s pop art season - Richard Clay guides us through cave art, revolutionary posters and contemporary street art, to unravel the enigma of graffiti.