. The sources regarding Arsaces' life differ greatly. He is mostly known from Greek and Roman sources, who were hostile to him and his dynasty due to the later
Roman–Parthian Wars. In
Iranian national history his descent is traced back to several mythical figures, such as being a descendant of either
Kay Kawad, Kay Arash, Dara the son of Homay, or
Arash, the heroic archer-figure. The affiliation of Arsaces with Arash is due to the resemblance in their names and Arsaces' coins imitating him as a bowman. According to
Roman historian
Ammianus Marcellinus, Arsaces was a bandit of low birth, who invaded and conquered
Parthia, killing its satrap
Andragoras, who had recently declared independence from the
Hellenistic Seleucid Empire. The most accepted theory is the one by the Greek geographer
Strabo: according to him, Arsaces was a
Scythian chieftain, who became the leader of the
Parni, one of the three tribes of the
Dahae confederacy of
Central Asia. The Dahae relied their strength completely on horseback, and thus possessed an extremely mobile force, which was able to retreat to the south of the
Aral Sea when endangered. Because of this, other empires met complications in their efforts trying to control them. Some historians such as Hashem Kazemi and Mostafa Dehpahlavan present an alternative hypothesis that Arsaces was a native Parthian instead. The Dahae had originally lived between the
Jaxartes in the 4th-century BC, but gradually moved southward, possibly in the early 3rd-century BC. They first migrated southeast to Bactria, but were driven away, and as a result changed their course to the west. They gradually started settling in Parthia, a region in the south-eastern part of the
Caspian Sea, that almost corresponded to present-day
Khorasan Province of Iran and southern
Turkmenistan. The region was then under the rule of the Seleucids. By 282/1 BC, Parthia was under considerable Parni influence. The Parni were not the only to migrate to Parthia, as the region was constantly receiving new waves of Iranian migrants from the north. The Parni were an eastern Iranian tribe, who practised
Iranian polytheism. By the middle of the 3rd-century BC, however, they had been assimilated into the local Parthian culture; they adopted Parthian, a
north-western Iranian language, and became adherents of
Zoroastrianism, even giving themselves Zoroastrian names, such as Arsaces' father, Phriapites, whose name was derived from
Avestan *Friya pitā ("father-lover"). Arsaces himself was probably born and raised in Parthia, speaking the Parthian language. According to the French historian Jérôme Gaslain, Arsaces could have arguably spent much of his life in the Seleucid lands, and may even have belonged to the local elite of Parthia. The Dahae frequently served as horse archers in the armies of Greek rulers, from the
Macedonian Alexander the Great () to the Seleucid
Antiochus III the Great (). This implies that Arsaces, who is described as an "experienced soldier" in classical records, may have served as a mercenary under the Seleucid rulers or their governors. == Reign ==