Was it a trick of the light that drew our Stone Age ancestors into caves to paint in charcoal and red hematite, to watch the heads of lions, likenesses of bison, horses, and aurochs in the reliefs of the walls, as they flickered by firelight? Or was it something deeper-a creative impulse, a spiritual dawn, a shamanistic conception of the world efflorescing in the dark, dank spaces beneath the surface of the earth where the spirits were literally at...