Jason Bur'I theorized that a civilization of Progenitors must have come before the Ancients. In all of the Ancient literature he was able to translate, they always appeared to be counting forward from an established date, and they already appeared to be a relatively developed civilization by that date.
Further, he was able to determine that the calendar of Antiquity was based on the construction of the first Road, for which a date was given in another system as 7557, indicating an established chronology of over 7500 years BEFORE the founding of the empire. His book Forbidden History records this discovery.
Other evidence sugests that, for whatever reason, the Ancients who left this civilization dared not return.