Kurgan hypothesis, the north-west of the region is also considered to be the source of the root of the Indo-European languages. The horse-drawn chariot appears in the 3rd millennium BC, by 2000 BC, in the form of war chariots with spoked wheels, thus being made more maneuverable, and dominated the battlefields. The growing use of the horse, combined with the failure, roughly around 2000 BC, of the always precarious irrigation systems that had allowed for extensive agriculture in the region, gave rise and dominance of pastoral nomadism by 1000 BC, a way of life that would dominate the region for the next several millennia, giving rise to the Scythian expansion of the Iron Age. Scattered nomadic groups maintained herds of sheep, goats, horses, and camels, and conducted annual migrations to find new pastures (a practice known as transhumance). The people lived in yurts (or gers) – tents made of hides and wood that could be disassembled and transported. Each group had several yurts, each accommodating about five people. While the semi-arid plains were dominated by the nomads, small city-states and sedentary agrarian societies arose in the more humid areas of Central Asia. The Bactria-Margiana Archaeological Complex of the early 2nd millennium BC was the first sedentary civilization of the region, practicing irrigation farming of wheat and barley and possibly a form of writing. Bactria-Margiana probably interacted with the contemporary Bronze Age nomads of the Andronovo culture, the originators of the spoke-wheeled chariot, who lived to their north in western Siberia, Russia, and parts of Kazakhstan, and survived as a culture until the 1st millennium BC. These cultures, particularly Bactria-Margiana, have been posited as possible representatives of the hypothetical Aryan culture ancestral to the speakers of the Indo-Iranian languages (see Indo-Iranians). Later the strongest of Sogdian city-states of the Fergana Valley rose to prominence. After the 1st century BC, these cities became home to the traders of the Silk Road and grew wealthy from this trade. The steppe nomads were dependent on these settled people for a wide array of goods that were impossible for transient populations to produce. The nomads traded for these when they could, but because they generally did not produce goods of interest to sedentary people, the popular alternative was to carry out raids. A wide variety of people came to populate the steppes. Nomadic groups in Central Asia included the Huns and other Turks, as well