Dr Richard Clay goes in search of what it is that has made us scribble and scratch mementoes of our lives for more than 30,000 years. From the prehistoric cave paintings of Burgundy in France, through gladiatorial fan worship in Roman Lyons to the messages left on the walls of Germany's Reichstag in 1945 by triumphant Soviet troops, time and again we have wanted to leave a permanent record of our existence for our descendants. And it may be that this is where what today we call art comes from - the humble scratch, graffiti.