Systems of diverse beliefs and practices of the culturally and linguistically related group of ancient peoples who inhabited the Iranian plateau and its borderlands, as well as areas of Central Asia from the Black Sea to Khotan (modern Hotan, China). In religion and culture, both the northern and southern Iranians had much in common with the ancient Indo-Aryan-speaking peoples of the Indian subcontinent, although there was much borrowing from Mesopotamia as well.