The Kurds are the largest ethnic group ("group of people") in the world that does not have its own state. The Kurds are traditionally nomadic ("with no fixed living place") and live in a region in Asia called Kurdistan. Most of Kurdistan is mountainous. There are parts of Kurdistan in Turkey, Iraq, Iran, and in Syria. There is discussion in academia about the origins ("start") and migrations ("movements of people") of the Kurds in history. Many other groups have also lived in Kurdistan in the past. The ancestors of modern Kurds probably arrived in Iran in the middle of the 2nd millennium BC. Kurdistan was part of the empire of the Medes, then the Achaemenid Empire, the Seleucid Empire, the Roman and Byzantine empire, and then the Sassanid Empire.