When did speakers of Arabic start pronouncing ḏāl ذ as د dāl?: Any visitor to cities like Damascus, Beirut, or Cairo immediately notices the absence of this sound in the local vernacular, e.g. Classical Arabic hāḏā is pronounced hāda. Where does this come from and how old?
…What does this mean? Well, a change typical of the modern Arabic vernaculars is attested more than 500 years before the appearance of Islam and the Conquests disproving the common assumption that non-Classical Arabic forms in the modern vernaculars are the result of recent "corruption" to the language. Now what about the hypothesis of Aramaic influence? Macdonald suggests that our author was bilingual and that this change may have been influenced by Nabataean Aramaic. This is entirely possible, but so is the possibility that it is simply an internal development…
I don't believe Classical Arabic, as we study it in the grammar books, was every a spoken language and therefore cannot be ancestral to any modern vernacular. But we should be careful to which varieties we apply this label. Classical Arabic is a blanket term for the written register of Islamic civ. and it is regulated by the material collected and ranked by Arabic grammarians in their works, starting in the 8th c. CE. Qur'anic Arabic, for example, pre-exists 'Classical Arabic', although it contributes to it. The modern vernaculars descend from dialects of Old Arabic, one or more, pre-existing the formation of Classical Arabic. But they are not direct descendants of any one form of Old Arabic. Overtime they interact, mix, etc., so that many dialects today are a mosaic of Arabics.