What group was at very bottom of the caste system and did not belong to any varna?

Scheduled castes and tribes (untouchables in the history) are not part of any varna. Only 4 varnas were existed. There are brahmin, kshatriya, vysya and shudra. Many groups of Shudras are now belong to forward castes and backward castes. However, the scheduled castes and tribes still facing the descrimination in every part of their lives. According to history the so called schedule castes (sons of the soil - Indigenous Warriors) were driven out by aryas and they became outcasted as these were the first warriors had fought aryan invaders.