unwritten Aramaic dialects emerged from Imperial Aramaic in Assyria. In around 700 BC, Aramaic slowly started to replace Akkadian in Assyria, Babylonia and the Levant. Widespread bilingualism among Assyrian nationals was already present prior to the fall of the empire. The language transition was achievable because the two languages featured similarities in grammar and vocabulary, and because the 22-lettered Aramaic alphabet was simpler to learn than the Akkadian cuneiform which had over 600 signs. The converging process that took place between Assyrian Akkadian and Aramaic across all aspects of both languages and societies is known as Aramaic-Assyrian symbiosis. Introduced as the official language of the Assyrian Empire by Tiglath-Pileser III (745–727BC), it became the language of commerce and trade, the vernacular language of Assyria in the late Iron Age and classical antiquity, and the lingua franca of the Neo-Assyrian Empire (911–605 BC), Neo-Babylonian Empire (605–539 BC), the Achaemenid Empire (539–323 BC), the Parthian Empire (247 BC–224 AD) and the Sasanian Empire (224–651 AD). Following the Achaemenid conquest of Assyria under Darius I, the Aramaic language was adopted as the "vehicle for written communication between the different regions of the vast empire with its different peoples and languages". After the conquest of Assyria by the Seleucid Empire in the late 4th century BC, Imperial Aramaic gradually lost its status as an imperial language, but continued to flourish alongside Ancient Greek. By the 1st century AD, Akkadian was extinct, though vocabulary and grammatical features still survive in modern NENA dialects. The Neo-Aramaic languages evolved from Middle Syriac-Aramaic by the 13th century. There is evidence that the drive for the adoption of Syriac was led by missionaries. Much literary effort was put into the production of an authoritative translation of the Bible into Syriac, the Peshitta (, ). At the same time, Ephrem the Syrian was producing the most treasured collection of poetry and theology in the Classical Syriac language. By the 3rd century AD, churches in Urhay in the kingdom of Osroene began to use Classical Syriac as the language of worship and it became the literary and liturgical language of many churches in the Fertile Crescent. Syriac was the common tongue of the region, where it was the native language of the Fertile Crescent, surrounding areas, as well as in parts of Eastern Arabia. It was the dominant language until 900 AD, till it was supplanted by Greek and later Arabic