This seems to be an unresolved question. My reading suggests a very deep
date for Zarathustra.
Some reference to the bible might be in
order. We have Daniel, who is putatively writing a book during the Babylonian
exile, but all responsible exegetes say these stories are Hasmonean in date.
Similarly, we have First, Second and Third Isaiah; the first would seem to
contain authentic pre-Babylonian material, while the second would seem to be
Babylonian and immediately post-Babylonian interpolations; the third is the
poet.
History writing, in the modern sense, did not exist until
Herodotus. Before Herodotus you don't so much have history as you have events
projected back or forward in time, as suits the mythic need, with a great deal
of editing involved. The authors of such books are not writing history in the
modern sense of the term.
Zarathushtra might not be particularly
historical. C.f. the historical Jesus vs. the Jesus as interpreted/created by
the Church.
The bible and Christianity/Judaism have been subjected
to more forms of searching analysis than just about any other academic topic
there is. The Avestan religion has not been subjected to this searching analysis
and consequently many interesting questions remain not merely unresolved, but
ultimately not even asked.
Zarathrustra may be a creature of the
Avestan religion, and not of history. Whatever the case, however, it is clear
that the Iranians completely displaced the Indics in the South Caucusus and
Persia -- in fact, everywhere except India. The Iranians encountered the Indic
religion -- which seems to have been remarkably similar to their own. As a
matter of ethnic pride and social superiority, it seems they differentiated
themselves from the Indics they were displacing via religion -- and were quite
successful. Maybe it was the Sassanids who did this, perhaps an earlier
group.
The question is not religion but caste. The elite Iranians
needed to differentiate themselves from the 'pagan' Indics. If they did, they
succeeded wonderfully. The later conversion to Islam, and the aftermath, would
likely have extirpated whatever remains of ancient Indic paganism which might
have persisted into the 700-800s CE.
Think of the ancient Christian apologetics, where the position of the
'heretics' is recoverable only though the writings of those who wrote against
them. Would we know of the Cathari without the polemics? Would the surviving
post AD 1, post AD 700s Indic-religionists of Iran have left records recoverable
to us consequent to the Sassanids and then later to the soldiers of Islam?
The idea of 'sacred scriptures' seems to have first occurred sometime after
700, progressing as time progressed. We get the Avesta, the first redaction of
the Hebrew scriptures -- and even Confucius in China. In Greece, we witness the
canonization of Homer and Hesiod as the basic books of the Olympian
Religion.
Whatever the Avesta is, it is not unreasonable to assume Zarathushtra is
the back-projected author, the religous author, the religious founder, the Moses
of Zoroastrianism, the Homer of Olympianism. Zarathustra is probably historical,
but he is also mythic. We have to treat him as do Jesus and Moses and
Homer.