In the early 1970s, South African summer sun searing his back, 14-year-old Stephen Townley Bassett dutifully followed his uncle into the shade offered by one of myriad caves dotted among the Cederberg Mountains of the Western Cape. Then he stopped and gawked. Splashed across the rock face in red and yellow hues was a striking collage of half-animal, half-human shapes. Dumbfounded, the teenager’s head swam with thoughts: Whose work was it? When, and why, did they create it?