Niede Guidon still remembers her astonishment when she glimpsed the paintings. Preserved amid the bromeliad-encrusted plateaus that tower over the thorn forests of northeast Brazil, the ancient rock art depicts fierce battles among tribesmen, orgiastic scenes of prehistoric revelry and hunters pursuing their game, spears in hand. “These were stunning compositions, people and animals together, not just figures alone,” said Dr. Guidon, 81, remembering what first lured her and other archaeologists in the 1970s to this remote site where jaguars still prowl. Hidden in the rock shelters where prehistoric humans once lived, the paintings number in the thousands. Some are...