The Po is first certainly identified in the Graeco-Roman historians and geographers of the late Roman Republic and the early Roman Empire, long after the valley had been occupied successively by prehistoric and historic peoples: Ligures, Etruscans, Celts, Veneti, Umbri, and Romans. At that late date the ancient authors were attempting to explain the provenance of the name. Perhaps the earliest of these, Polybius[20] (2nd century BC), uses Pados (in Greek) and says that it was to be identified with the Eridanos of the poets.