A small group of artists, carrying engraving tools, torches, and other supplies, assembled at the base of the towering limestone outcrop, near the mouth of a dark cave. Together, they entered the lightless space. For 40 minutes they wormed through passages and scrambled over speleothems. Their juniper-branch torches cast a warm, flickering light and shed bits of charcoal like bread crumbs along the trail. Nearly a quarter-mile from the cave entrance, they reached their canvas: a rugged limestone wall, 40 feet long and eight feet above the cave floor. They set the torches aside, lit stone lamps greased with marrow, and climbed onto a ledge that ran along the wall’s base. There, they began their work. At this spot in what’s now northern Spain’s Atxurra Cave, some 12,500 years ago, the artists carved scores of horses, bison, deer, and mountain goats. The animals’ outlines fluttered and winked in the light thrown by the lamps.