After 6500 BC, evidence for the domestication of food crops and animals, construction of permanent structures, and storage of agricultural surplus appeared in Mehrgarh and other sites in what is now Balochistan. These gradually developed into the Indus Valley Civilization, the first urban culture in South Asia. Prehistoric sites of palaeolithic, mesolithic and neolithic origins have been discovered in the Siwalik hills of Dang district. The earliest inhabitants of modern Nepal and adjoining areas are believed to be people from the Indus Valley Civilization. It is possible that the Dravidian people whose history predates the onset of the Bronze Age in the Indian subcontinent (around 6300 BC) inhabited the area before the arrival of other ethnic groups like the Tibeto-Burmans and Indo-Aryans from across the border. By 4000 BC, the Tibeto-Burmese people had reached Nepal either directly across the Himalayas from Tibet or via Myanmar and north-east India or both. Another possibility for the first people to have inhabited Nepal are the Kusunda people. According to Hogdson (1847), the earliest inhabitants of Nepal were perhaps the Kusunda people, probably of proto-Australoid origin. Stella Kramrisch (1964) mentions a substratum of a race of pre-Dravidians and Dravidians, who were in Nepal even before the Newars, who formed the majority of the ancient inhabitants of the valley of Kathmandu.