Emergence of the Nation. The iranian nation is one of the oldest continuous civilizations in the world. Upper Paleolithic and Mesolithic populations occupied caves in the Zagros and Elburz mountains. The earliest civilizations in the region descended from the Zagros foothills, where they developed agriculture and animal husbandry, and established the first urban cultures in the Tigris-Euphrates basin in present day Iraq. The earliest urban peoples in what is today iranian territory were the Elamites in the extreme southwest region of Khuzestan. The arrival of the Aryan peoples—Medes and Persians— on the iranian plateau in the first millennium B.C. E. marked the beginning of the iranian civilization, rising to the heights of the great Achaemenid empire consolidated by Cyrus the Great in 550 B.C. E. Under the rulers Darius the Great and Xerxes, the Achaemenid rulers extended their empire from northern India to Egypt.
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