Tamil and Telugu belong to different branches of Dravidian. Tamil is South Dravidian I, and Telugu is South Dravidian II, also called South-Central. They split well over two thousand years ago. Their oldest number words still show the shared ancestor clearly. Setting the archaic forms side by side shows which language kept the old sounds, which one changed them, and where each language replaced a word outright.

THE SHARED CORE

Proto-Dravidian → Archaic Tamil / Archaic Telugu

1: *onṯu → onṟu / ondu
2: *ir(aṇṭu) → iraṇṭu / eraṇḍu
3: *muHn-ṯu → mūnṟu / mūnṟu
4: *nāl → nāṉku / nāluṅgu
5: *cay- → aintu / ēnu
6: *cāṯu → āṟu / āṟu
7: *ēḻ → ēḻu / ēḻu
8: *eṇ- → eṭṭu / eṇimidi
9: *toḷ-/*toṇ- → toṇṭu / tonbidi
10: *paHtu → pattu / padi

The two languages differ in the sounds, but the roots are the same for all ten.

TAMIL KEEPS THE OLD CONSONANTS

Archaic Tamil keeps the Proto-Dravidian alveolar ṟ in onṟu, mūnṟu, and āṟu. It also keeps the retroflex ḻ in ēḻu. Old Telugu still had these sounds in its earliest writing. That is why archaic Telugu has mūnṟu, āṟu, and ēḻu, written with letters that have since dropped out of use. Later, Telugu ṟ merged into r or ḍ, and ḻ merged into ḍ. This produced modern mūḍu, āru, and ēḍu. Tamil never made these mergers, so its archaic and modern forms stay close.

TELUGU SHOWS ITS OWN SOUND RULES

Telugu voices consonants that Tamil keeps voiceless or doubles. The Proto-Dravidian cluster *nṯ becomes nḍ in Telugu ondu, and Tamil has onṟu. Proto-Dravidian *paHtu becomes padi in Telugu, while Tamil doubles the consonant to make pattu.

Telugu also lowers vowels. Proto-Dravidian *i becomes e before a following a, so Tamil has iraṇṭu and Telugu has eraṇḍu. Modern Telugu reṇḍu goes further: it moves the r to the front of the word. This change is called apical displacement, and it is a well-known trait of South-Central Dravidian.

BOTH LOST THE INITIAL C-

Five and six both start with *c- in Proto-Dravidian. Neither language kept it. Proto-Dravidian *cay- became aintu in Tamil and ēnu in Telugu, and *cāṯu became āṟu in both. This loss is shared, but each language built its own form from what remained. Tamil aintu adds a -ntu ending. Telugu ēnu contracts the root. Telugu later gained a second form, aidu, which pushed ēnu aside as the word for five. Ēnu still survives in​​​​​​​​​​​​​​​​

REFERENCES

Krishnamurti, Bhadriraju. 2003. The Dravidian Languages. Cambridge: Cambridge University Press. (Chapter on numerals; also covers apical displacement and loss of initial *c-.)

Burrow, T. and M. B. Emeneau. 1984. A Dravidian Etymological Dictionary (DEDR), 2nd ed. Oxford: Clarendon Press. Online at the Digital Dictionaries of South Asia, University of Chicago (dsal.uchicago.edu).

Zvelebil, Kamil V. 1970. Comparative Dravidian Phonology. The Hague: Mouton.

Steever, Sanford B. (ed.). 1998. The Dravidian Languages. London: Routledge. (Separate chapters on Tamil and Telugu.)

Inspired by Insta post about Dravidian numerals by reyi-tsukka

According to Wikipedia, these symbols are dated to between 1000BCE and 300CE so I was wondering if there were any references in contempary literature such as the sangam texts.