A ruling line descended from the Yüeh-chih, a people that ruled over most of the northern Indian subcontinent, Afghanistan, and parts of Central Asia during the first three centuries of the Christian era. It began as a nomadic tribe in the 2nd century BC. Under Kaniska I (fl 1st century AD) and his successors, the Kushan kingdom reached its height. It was considered one of the four great Eurasian powers of its time (the others being China, Rome, and Parthia). The Kushans were instrumental in spreading Buddhism in Central Asia and China and in developing Mahayana Buddhism and the Gandhara and Mathura schools of art. The Kushans became affluent through trade, particularly with Rome. After the rise of the Sasanian dynasty in Iran and of local powers in northern India, Kushan rule declined.