In various sub-castes, people used to practice untouchability i.e., some of the people who were considered lower castes, for example, sweepers, cleaners, etc. were ignored, and there were rules such as others were not allowed to touch them, if they did, they were ignored by the community.
So why they used to do that?
For your question 'So why they used to do that?' :
Quoting from Dr.Koenraad Elst's 1994 Essay "Caste: The view from Belgium":
Untouchability originates in the belief that evil spirits surround dead and dying substances. People who work with corpses, body excretions or animal skins had an aura of danger and impurity, so they were kept away from mainstream society and from sacred learning and ritual. This often took grotesque forms: thus, an untouchable had to announce his polluting proximity with a rattle, like a leper.
Untouchability seems to originated from a notion of impurity associated with certain professions. This is not unique to Hindu society as it is popularly believed. Burakumin people of Japan, Baekjeong people of Korea were considered untouchable. It seems to have arisen from basic human nature to seek safety in an age where some diseases had no cure. Hence people who had the potential to carry diseases were kept away.
Untouchability is not mentioned in Vedic Samhitas. It does not have sanction in scriptures and it seems to have existed among non-Vedic Hindus too:
...the Tamils believed that any taking of life was dangerous, as it released the spirits of the things that were killed. Likewise, all who dealt with the dead or with dead substances from the body were considered to be charged with the power of death and were thought to be dangerous. Thus, long before the coming of the Aryans with their notion of varna, the Tamils had groups that were considered low and dangerous and with whom contact was closely regulated[1].
This reinforces that it was based on basic human instinct to seek safety rather than sanctioned by scriptures. So even within an untouchable caste, some sub-castes might have been considered untouchable by the members of the same caste on the notion of safety.
[1] Reference
This question has more to do with human nature than Hinduism. When people were divided into castes based upon their work, some castes declared themselves superior and called others inferior. Worshiping Gods was considered superior and noble work, so that caste became superior. Cleaning, washing etc. were considered inferior work, so that caste became inferior.
I don't know which resulted in which and so on, but these feelings resulted in customs like not allowing lower caste people into temples and not allowing them to read the Vedas. Those customs further reinforced the feelings. Untouchability spread like a superstition where everybody believed in it.
From this documentary India Untouched
It exposes the continuation of caste practices and Untouchability in Sikhism, Christianity and Islam, and even amongst the communists in Kerala.
Untouchability is prevalent in other religions too.
within Dalits, sub-castes practice Untouchability on the "lower" sub-castes, and a Harijan boy refuses to drink water from a Valmiki boy.
So untouchability is not just two-layered, it's multi-layered.
Caste hierarchy and discrimination is not sanctioned by the Vedas. I mentioning an excerpt from an article by Swami Venkatraman:
First, caste refers to jati, not varna. Jatis are the thousands of indigenous social- occupational groups, while varna refers to the four individualized societal functions described in many texts. Understanding this distinction is a necessary first step. The next is exploring whether the Purusha Sukta really sanctions a hierarchical and discriminatory caste system?
This is not mere academic curiosity. The solution to any problem relies on a correct diagnosis and even as they acknowledge the social history of caste-based discrimination in India, it is important for Hindus, and non-Hindus, to understand the terminology and know whether the Sukta really does sanction a birth-based hierarchy.
Below is a translation of the relevant verse from ‘Purusha Sukta’, which is the 90th Sukta of the 10th mandala in the Rig Veda, and it talks about the entire universe as the body of God (Purusha), and of all creation as emerging from Him.
From his mouth came forth the Brahmins
And of his arms were Rajanya made
From his thighs came the Vaishyas
And his feet gave birth to Sudras.
At a literal reading, this indeed appears to define a hierarchical system of classes with the Brahmins occupying the most prestigious position and the Sudras being the most inferior as they emerge from the feet. And this has pretty much become the dominant understanding of the verse among academics.
The best way to demonstrate the silliness of this interpretation of the 90th Sukta, is to actually assume it to be correct and then see where that leads us in terms of understanding the rest of the hymn. Thus, if the above verse indicates a hierarchical system, then presumably the body parts of the God (Purusha) from which everything in creation emerges, or the order in which the names are mentioned, or both, ought to be indicative of its superiority or otherwise.
Let us test this understanding against translations of the next two verses from the
90th ‘Purusha Sukta’:
Of his mind, the Moon is born
Of his eyes, the shining Sun
from his mouth, Indra and Agni,
And of his life-breath, Vayu
Space unfolds from his navel
The sky well formed from his head
From His feet, the earth and His ears the Quarters
Thus they thought up all the worlds.
If our assumption above were true, then the moon ought to be superior to the sun because the mind is superior to the eyes, and also because the moon is mentioned first. Moreover, based on where they emerge from, Indra (the king of the Devas) ought to be inferior to both Chandra (moon) and Surya (sun) and on par with Agni (fire), which also is illogical.
A similarly absurd comparison of the space, sky, earth with the ‘four directions’ will arise from the second verse. If the earth comes from the God’s (Purusha) feet, is it then inferior to the moon which comes from the mind?
There is clearly no hierarchy intended, but only symbolic meanings. This can be driven home more clearly, if one considers what the ‘Purusha Sukta’ says in its entirety. It describes the God (Purusha), as the perennial source of all creation, as having countless heads, eyes and legs, manifested everywhere beyond comprehension. All creation is but fourth a part of him and the rest is thus, ambiguous.
The Sukta describes a great Yajna, or a ritual sacrifice, called `Sarvahut’, or the ‘offering of all’. It was God (Purusha) himself who is worshipped in the Yajna, which is performed by Brahma, the creative power of the Purusha. The Devas, who are the senses of the Purusha, are the priests.
Thus, the beast of sacrifice, tied to the altar is the Purusha himself; all of nature is the altar; the Purusha’s heart is the fire, and the Purusha himself is sacrificed in the Yajna, which is the process of creation itself. The ‘Purusha Sukta’ does not intend to speak about human society and its organization.
The translation of one of the final verses states the essence of Hinduism clearly:
I know That Purusha who is glorious
Bright as the sun, beyond all darkness.
He who knows him thus Conquers death in this birth.
I know of no other way than this.
Consider the following now:
In the entire Rig Veda, it is only in the ‘Purusha Sukta’ that the four varnas are mentioned. However, the ‘Purusha Sukta’ itself does not use the word ‘varna’ and wherever the word occurs elsewhere in the Rig Veda, it is to be noted that it is not used to refer to the four types of people in society.
Moreover, Hindu sacred texts clearly relate ‘varnas’ to the ‘guna’ i.e., behavior and character, rather than the birth. The idea that different individuals of the same family can have different ‘varnas’ and those individuals had a choice of ‘varnas’ are present in the Rig Veda itself.
“I am a reciter of hymns, my father is a healer, my mother a grinder of corn. We desire to obtain wealth through various actions”-- Rig Veda 9.112.3
“O Indra, fond of soma, would you make me the protector of people, or would you make me a ruler, or would you make me a sage who has consumed soma, or would you bestow infinite wealth on me?” --- Rig Veda 3.44.5
“The four varnas were created by me according to differences in guna and karma; although the creator of this, know me as the non-doer being immutable.” -- Bhagavad Gita 4.13
http://swarajyamag.com/culture/caste-hierarchy-and-discrimination-not-sanctioned-by-the-vedas
It has it's origin in practicing cleanliness when contagious diseases were in abundance, which had no cure. So the people having professions related to 'unclean' things like wastes, leather etc were prohibited from touching, accessing public places and common water supplies like lakes and wells. In course of time this became a custom.
