Although the Persian presence in Thrace was short-lived, it probably stimulated trade and first state formations among the Thracians. Mintings of Thracian coins started around 500 and may be an indicator for a variety of early tribal kingdoms. It has been suggested that the Odrysian kingdom might have had its origins in this period, even though the name of the Odrysians is notably absent from the numismatic evidence. The Odrysians eventually stepped into the light of history in the aftermath of the Persian failure in Greece, when they were mentioned by Herodotus, but without any further details. The Odrysians had their core territory in the valleys of the Hebros river and its tributaries Tonzos and Arda. Like other Thracian polities, the Odrysian tribal kingdom attempted to fill the vacuum left by the Persian retreat.