The roots of Zoroastrianism are thought to have emerged from a common prehistoric Indo-Iranian religious system dating back to the early 2nd millennium BCE. The prophet Zoroaster himself, though traditionally dated to the 6th century BCE, is thought by many modern historians to have been a reformer of the polytheistic Iranian religion who lived in the 10th century BCE. Zoroastrianism as a religion was not firmly established until several centuries later. Zoroastrianism enters recorded history in the mid-5th century BCE. Herodotus' "The Histories" (completed c. 440 BCE) includes a description of Greater Iranian society with what may be recognizably Zoroastrian features, including exposure of the dead.