The earliest recorded history of the region dates back to about 500 BCE when much, if not all, of modern Tajikistan was part of the Achaemenid Empire. Some authors have also suggested that in the 7th and 6th century BCE parts of modern Tajikistan, including territories in the Zeravshan valley, formed part of Kambojas 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 BCE. The Silk Road passed through the region and following the expedition of Chinese explorer Zhang Qian during the reign of Wudi (141–87 BCE) commercial relations between Han China and Sogdiana flourished. Sogdians played a major role in facilitating trade and also worked in other capacities, as farmers, carpetweavers, glassmakers, and woodcarvers.