The Midi-Pyrénées, southern France. A spray-painted silhouette of a prehistoric hand, positioned low on the wall at Pech Merle, seems at odds with the other images in the cave—the fluidly stylized drawings of horses, mammoths, reindeer, and other herd animals of the prehistoric hunt. In fact, the hand stencils are the only naturalistic references to human beings at all, and the only subjects of any kind shown actual size. You could place your own hand over the stenciled hand of a cave painter, and even after 25,000 years of human evolution, your hand would fit.

The stencils are highly mysterious. Why would artists with enough skill to conjure magnificent animals in full motion, and who entered the caves to make paintings only after practicing ...

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