Pre-1890s

The Purusha Sukta:
The Rig Veda, the earliest Hindu text, contains a famous hymn that describes the different origins of the four castes (Book 10, Hymn 90, verses 11-12): in the Ralph Griffith translation (1896).

Kautilya's Arthashastra:
This well-known and influential early text (c.300's BCE and onward) is notoriously hard to date, but makes fascinating reading. Take a look  for example at Book 3; many of the penalties for various crimes are carefully graded according to caste. The Shamasastry translation (1923) is the standard one: [site]; another source for it: [site].

The Laws of Manu:
This is the text (c.1st c. CE?) that Dr. Ambedkar loved to hate; he and other Dalit protesters were later to burn it. The Laws of Manu explains that in the beginning of the universe the great abstract principle of Brahman created all things, including the four Varnas or ranked caste-groups: 'for the sake of the prosperity of the worlds, he caused the Brahmin, the Kshatriya, the Vaishya, and the Shudra to proceed from his mouth, his arms, his thighs, and his feet' (chapter I, verse 31).... 'But in order to protect this universe He, the most resplendent one, assigned separate (duties and) occupations to those who sprang from his mouth, arms, thighs, and feet' (I,87). At the top of this fourfold system is the Brahmin: 'As the Brahmin sprang from (Brahman's) mouth, as he was the first-born, and as he possesses the Veda, he is by right the lord of this whole creation' (I,93). At the bottom is the Shudra: 'One occupation only the lord prescribed to the Shudra: to serve meekly even these (other) three castes [varnas]' (I,91). Above all, the contrast between the two extremes of the hierarchy is made clear: 'But a Shudra, whether bought or unbought, he [=a Brahmin] may compel to do servile work; for he was created by the Self-existent (Svayambhu) to be the slave of a Brahmin' (VIII,413).

Outside the system entirely were the 'slaves' [dasyus]: 'All those tribes in this world, which are excluded from (the community of) those born from the mouth, the arms, the thighs, and the feet (of Brahman), are called Dasyus, whether they speak the language of the Mlechchhas (barbarians) or that of the Aryans' (X,45). Among those outside the system are groups produced by illicit unions among the different varnas, who 'shall subsist by occupations reprehended by the twice-born [=the three upper varnas]' (X,46). Among these occupations are 'catching and killing (animals) living in holes', 'working in leather' (X,49), 'carry[ing] out the corpses (of persons) who have no relatives' (X,55), and 'execut[ing] criminals' (X,56). These impure groups are to live in remote, wild areas or 'near well-known trees and burial grounds' (X,50). 'Their dress (shall be) the garments of the dead, (they shall eat) their food from broken dishes, black iron (shall be) their ornaments, and they must always wander from place to place' (X,52). 'A man who fulfils a religious duty, shall not seek intercourse with them; their transactions (shall be) among themselves, and their marriages with their equals' (X,53). 'Their food shall be given to them by others (than an Aryan giver) in a broken dish; at night they shall not walk about in villages and in towns' (X,54). The translation here is Buehler's (1886), the one that Dr. Ambedkar himself used: [site]; another [site].

by c.300's
By this time, specific caste groups (jati) of "untouchables" could be seen to exist. Discussion: Eleanor Zelliot, Encyclopedia of Asian History, New York: Scribner, 1988.

1852
The Maharashtrian caste reformer "Mahatma" Jotirao Phule (1827-1890) [site] [site] started, in Poona, the first school for Untouchable children. (K. N. Kadam, Dr. Bahasaheb Ambedkar and the Significance of his Movement: A Chronology, Bombay: Popular Prakashan, 1991, p. 63.)

1867
Subehdar Ramji Maloji Sanpal married Bhimabai Murbadkar. Both families belonged to the untouchable Mahar caste, and both were connected with the British Army. (K. N. Kadam, Dr. Bahasaheb Ambedkar and the Significance of his Movement: A Chronology, Bombay: Popular Prakashan, 1991, p. 64.)

1873
Jotirao Phule founded the Satya-Shodhak Samaj, or "Society in Search of Truth," an organization dedicated to liberating the low castes from Brahminical oppression. (K. N. Kadam, Dr. Bahasaheb Ambedkar and the Significance of his Movement: A Chronology, Bombay: Popular Prakashan, 1991, p. 65.)

1884
Col. Henry S. Olcott started four schools in Madras for Untouchable children. In 1880, in Ceylon, he and Madame Blavatsky [site] had become Buddhists. (K. N. Kadam, Dr. Bahasaheb Ambedkar and the Significance of his Movement: A Chronology, Bombay: Popular Prakashan, 1991, p. 66.)

1886
Gopal Baba Walangkar, another Mahar with a military background, retired from the army and settled at Dapoli, in Ratnagiri district. There he established the anti-caste organization "Anarya Dosh-Parihar Samaj." (K. N. Kadam, Dr. Bahasaheb Ambedkar and the Significance of his Movement: A Chronology, Bombay: Popular Prakashan, 1991, pp. 66-67.)

1888
Gopal Baba Walangkar, established the first Untouchable newspaper, Vital Vidhvansak. (Eleanor Zelliot, Dr. Babasaheb Ambedkar and the Untouchable Movement, New Delhi: Blumoon Books, 2004, pp 42-44.)