The following is from: Cosmos, Chaos and the World to Come (pp. 222-224), Norman Cohn. Yale University Press. Kindle Edition.
To return to the Book of Daniel: even linguistically there is something odd about the work, for chapters 2 to 7 are written, not like the rest of the Old Testament in Hebrew, but in the language of the Iranian empire, Imperial Aramaic; and they contain no less than twenty Persian loan-words.
More importantly, in chapter 2 there is an image which has a close parallel in Zoroastrian lore: the statue in Nebuchadnezzar’s dream, with its head of gold, breast and arms of silver, belly and thighs of bronze, legs of iron, and feet part iron and part terracotta. In the Iranian apocalypse Zand i Vahman Yasht (meaning ‘Commentary on the hymn in praise of the god Vohu Manah’), Zoroaster dreams of a tree with branches of gold, silver, steel — and iron mixed with clay.
Nebuchadnezzar’s statue and Zoroaster’s tree both symbolise the same thing: a succession of four historical periods. The concept of four ages, symbolised respectively by gold, silver, bronze and iron is to be found already in an eighth-century Greek work, Hesiod’s Work and Days; but the addition of iron mixed with clay is an innovation, and such a curious one that it cannot be coincidental. Nor is there much doubt as to which is the original, Nebuchadnezzar’s statue or Zoroaster’s tree. Although the extant version of the Zand-i Vahman Yasht is — like all Zoroastrian texts — late, its origin is very ancient. Some scholars hold that it goes back to the time of Alexander the Great, others that it goes back further still. What is certain is that it is far older than the Book of Daniel.
That being so, one would expect the Iranian interpretation of the dream to be more natural, more persuasive than the Jewish. And so it is — especially the interpretation of ‘iron mixed with clay’. In the Zand-i Vahman Yasht the image symbolises the age when ‘non-Iranians will be mixed with Iranians’ — that is, when the good strong iron of Zoroastrian Iranians will be weakened by an influx of infidel foreigners. In Daniel ‘iron mixed with clay’ is interpreted as the time when Seleucid rule will be weakened by unsuccessful dynastic marriages — a forced comparison if ever there was one!
There is other, even more convincing evidence of Zoroastrian influence. When Nebuchanezzar asks Daniel to interpret this same dream, Daniel invokes ‘a God in heaven who reveals mysteries … mysteries of what is to be’ — and the word for ‘mysteries’ is rz. It is the very same word as is used in the Scrolls to denote the secret knowledge which the Qumran community treasured above all things: knowledge of God’s plan for the world, and especially for the end of time. And it is a Persian word, much used by Zoroastrians in precisely the same sense.