region have been dated back to at least the fourth millennium BC, including the Bronze Age Bactria–Margiana Archaeological Complex, the Andronovo cultures and the pro-urban site of Sarazm, a UNESCO World Heritage Site. The earliest recorded history of the region dates back to about 500 BC when much, if not all, of modern Tajikistan, was part of the Achaemenid Empire. Some authors have also suggested that in the seventh and sixth centuries BC, parts of modern Tajikistan, including territories in the Zeravshan valley, formed part of the ancient Hindu Kambojas tribe before it became part of the Achaemenid Empire. After the region's conquest by Alexander the Great it became part of the Greco-Bactrian Kingdom, a successor state of Alexander's empire. Northern Tajikistan (the cities of Khujand and Panjakent) was part of Sogdia, a collection of city-states which was overrun by Scythians and Yuezhi nomadic tribes around 150 BC. The Silk Road passed through the region and following the expedition of Chinese explorer Zhang Qian during the reign of Wudi (141BC–87 BC) commercial relations between Han Empire and Sogdiana flourished. Sogdians played a major role in facilitating trade and also worked in other capacities, as farmers, carpetweavers, glassmakers, and woodcarvers. The Kushan Empire, a collection of Yuezhi tribes, took control of the region in the first century AD and ruled until the fourth century AD during which time Buddhism, Nestorian Christianity, Zoroastrianism, and Manichaeism were all practised in the region. Later the Hephthalite Empire, a collection of nomadic tribes, moved into the region and Arabs brought Islam in the early eighth century. Central Asia continued in its role as a commercial crossroads, linking China, the steppes to the north, and the Islamic heartland. It was briefly under the control of the Tibetan Empire and the Tang dynasty from 650 to 680 CE and then under the control of the Umayyads in 710 CE. Samanid Empire The Samanid Empire, 819 to 999, restored Persian control of the region and enlarged the cities of Samarkand and Bukhara (both cities are today part of Uzbekistan) which became the cultural centers of Iran and the region was known as Khorasan. The empire was centered in Khorasan and Transoxiana; at its greatest extent encompassing modern-day Afghanistan, large parts of Iran, Tajikistan, Turkmenistan, Uzbekistan, Kyrgyzstan, parts of Kazakhstan, and Pakistan. Four brothers Nuh, Ahmad, Yahya, and Ilyas founded the Samanid state. Each of them ruled territory under