Despite the immense geographic area and the vast time spans involved, the diverse civilizations that succeeded one another share unifying characteristics and form a whole. The first characteristic is the existence of a family of predominant written and spoken languages that date back five thousand years: Akkadian, Babylonian, Phoenician, Aramaic, Hebrew, and Arab all belong to the group of Semitic languages, some of which are still spoken today. The environment and lifestyles form the second unifying factor. The Tigris and the Euphrates define the "Fertile Crescent," which links the Mediterranean Sea to the Persian Gulf. During the Neolithic period (starting in the 8th millennium), humankind adopted a sedentary way of life based on agriculture and created the first settlements. Hierarchical societies developed, which encouraged the birth of cities and states in the 5th millennium. Economies grew increasingly complex and required accounting methods and tools. Writing developed around 3300 BC in Sumer, in the south of present-day Iraq. In the 3rd, 2nd, and 1st millennia, a series of empires emerged—those of Ur, the Akkadian dynasty, Babylonia, the Hittites, Assyria, and the Elamites in the southwest region of Iran—while trade flourished in the eastern Mediterranean, extending to Mari on the middle Euphrates and Ugarit on the Syrian coast. After the empire founded by Alexander the Great (332–323 BC) and continued by his successors, the Near East came under Roman rule (1st century BC).