How did untouchability come into existence?

Untouchability refers to any belief or social practice which looks down upon people on account of their birth with certain caste labels. Such practices disallow them interaction with others or access to public places as equal citizens. So the Constitution made untouchability a punishable offence. 
P. Sainath travelled to various parts of the country and found that untouchability was still being practised against dalits or person belonging to scheduled castes. He found that in many places 
(a) Tea stalls kept two kinds of cups, one for Dalits one for others. 
(b) Barbers refused to serve dalit clients. 
(c) Dalit students were made to sit separately in the classroom or drink water from separate pitcher. 
(d) Dalit grooms were not allowed to ride a horse in the wedding procession. 
(e) Dalits were not allowed to use common hand pump or if they did, the hand pump was washed to purify it.