https://www.widewalls.ch/the-history-of-street-art/

Urban art is a diverse and constantly evolving expression. It seeks to surprise the viewer through messages that invite reflection, either through playfulness and irony, a political or social critique or a subversive invitation. The images that were carved in prehistoric caves today adorn the walls of the metropolis. Street art has emerged simultaneously in different parts of the world. The graffiti started showing up on the sides of train cars and walls.