Assyrian continuity is the theory of continuity between the modern Assyrian people, an indigenous ethnic minority in the Middle East, and the people of ancient Assyria. Assyrian continuity is a key part of the identity of the modern Assyrian people. No evidence exists of the original Assyrian population being replaced in the aftermath of the fall of the Assyrian Empire, contemporary scholarship almost unilaterally supports Assyrian continuity, recognizing the modern Assyrians as descendants of the Aramaic-speaking populations of the Neo-Assyrian Empire, which were composed of both the old native Assyrian population and of Aramean settlers in the Assyrian heartland. Due to a shortage of sources beyond the Bible and works by classical authors, western historians prior to the 19th century believed Assyrians to have been completely annihilated. Modern Assyriology has increasingly challenged this perception; today, Assyriologists recognize that Assyrian culture and people clearly survived the violent fall of the Neo-Assyrian Empire. The last period of ancient Assyrian history is now regarded to be the long post-imperial period, during which the Akkadian language went extinct but other aspects of Assyrian culture, such as religion, traditions, and naming patterns, survived in a reduced but highly recognizable form for nearly a millennium. The extinction of Akkadian and its replacement with Aramaic does not reflect the disappearance of the original Assyrian population; Aramaic was used not only by Aramean settlers but was also adopted by native Assyrians, in time even becoming used by the royal administration itself. Due to assimilation efforts encouraged by Assyrian kings, Arameans and other groups in the Assyrian heartland are also likely to quickly have self-identified, and been regarded, as Assyrians. The population of Upper Mesopotamia was Christianized between the 1st and 4th centuries AD. Aramaic-language sources from the Christian period predominantly use the self-designations ʾĀrāmāyā ("Aramean") and Suryāyā. "Assyrian" (Āthorāyā) also continuously survived as the designation for a Christian from Mosul (ancient Nineveh) and its vicinity. The reluctance of the overall Christian population to adopt ʾĀthorāyā as a self-designation probably derives from Assyria's portrayal in the Bible. The different terms did not refer to different groups, given that some individuals are described as both ʾĀrāmāyā and Suryāyā. The term Suryāyā, sometimes alternatively translated as "Syrian" or "Syriac", is believed to derive from the ancient Akkadian Assūrāyu. The academic consensus is that the modern name "Syria" originated as a shortened form of "Assyria". Assyrian nationalism developed near the end of