Scholars such as Suvira Jaiswal, R. S. Sharma, and Vivekanand Jha characterize untouchability as a relatively later development after the establishment of the varna and caste system. Jha notes that the earliest Vedic text "Rigveda" makes no mention of untouchability, and even the later Vedic texts, which revile certain groups such as the Chandalas, do not suggest that untouchability existed in the contemporary society. According to Jha, in the later period, several groups began to be characterized as untouchable, a development which reached its peak during 600–1200 AD. Sharma theorizes that institution of untouchability arose when the aboriginal tribes with "low material culture" and "uncertain means of livelihood" came to be regarded as impure by the privileged classes who despised manual labour, and regarded associated impurity with "certain material objects". According to Jaiswal, when the members of aboriginal groups were assimilated into the Brahmanical society, the privileged among them may have tried to assert their higher status by disassociating themselves from their lower-status counterparts, who were gradually branded as untouchables.