The Aramaic-speaking Assyrians are the indigenous people of Iraq and descendants of those who ruled ancient Akkad, Assyria and Babylonia. More generally speaking, the Assyrians (like the Mandeans) are descendants of the ancient Mesopotamians (Sumer, Akkad, Assyria, Babylon, Adiabene, Osroene and Hatra). They speak dialects of the Aramaic of the Assyrian and Babylonian Empires and have their own written script. They began to convert to Christianity in the 1st and 2nd centuries AD formerly having followed the ancient Sumerian-Akkadian religion (also known as Ashurism). There are believed to be no more than 500,000 Assyrians remaining in Iraq, with a large concentration in the diaspora. They are Iraq's fourth largest ethnic group after the Arabs, the Kurds and the Iraqi Turkmen.