Zhen Li wrote:Probably a mixture of some kind of Munda-indigenous heritage, with Indo-Aryan. Since the names used in the Sutras for people and places in the Sakya kingdom are often of non-Indo-European origin. There's also the prominent evidence of trace Naga cults and tree cults, (not to mention Yakshas) which are definitely indigenous, and Mucilinda's name can be derived from Munda. There's also multiple stories in the Pali Canon about cousin-intermarriage among Sakya ancestors, which was forbidden in Brahmanical society. Also, indigenous burial customs were mound-based, in round mounds, while the Indo-Aryans abandoned the Kurgan-like mound burials centuries earlier in preference for square burial chambers. You will also note that many of the names of the Buddha's disciples are odd from the Brahminical perspective, such as Shariputra, who is named after his mother. There's also some evidence that the notion of a Mahapurusha was indigenous.
One thing I discovered recently was that, apparently, a number of ancient Indian names and Indic terms, including those in Buddhist literature, have cognates in Akkadian and other Mesopotamian languages. Malati J. Shendge suggests in her work The Language of the Harappans: From Akkadian to Sanskrit that the earlier Indus Valley civilization, which she identifies as the Asura culture described in Vedic literature, was Akkadian speaking. One noteworthy Indic name of interest also found in Akkadian and Sumerian sources is Kaśyapa (in Pali Kassapa). One of the Buddha's chief disciples was Mahā-Kaśyapa. The past buddha immediately prior to Śākyamuni was called Kaśyapa.
Kaśyapo Māricaḥ, PN composer of RV I.99, VIII.29; Rebhaḥ Kaśyapaḥ, family name of the composer of RV VIII.97. With this, cp. Sum. Kaššeba, king (priest-king?), Akk. Kaššāpu, sorcerer, (denotes sun-god Šamaš), also kašāpu, to use charms, bewitch, OB on, Kaššeba (=Šamaš)
In Indian sources, the name was borne by the husband of Aditi and father of the Ādityas (Varuṇa, Mitra, etc. seven). He was obviously a very ancient mythical personage who was connected with creation.
Malati J. Shendge, The Language of the Harappans: From Akkadian to Sanskrit (New Delhi, India: Abhinav Publications, 1997), 208.
This is not so surprising given that Akkadian loanwords exist in Vedic Sanskrit according to Thomas McEvilley:
In India in the late second millennium – the Middle Vedic period in terms of Sanskrit literary history – the reexpanding trade with the Near East brought with it elements of cultural diffusion. Contact with the Mesopotamian cultural stream may have left significant traces in the pantheistic hymns, of a type found widely in the Near East, in the the tenth book of the Rg Veda and in the appearance of Akkadian words in the Atharva Veda, both of which seem to have been taking shape at about the time the Near Eastern trade was revived.
Thomas McEvilley, The Shape of Ancient Thought Comparative Studies in Greek and Indian Philosophies (New Delhi, India: Motilal Banarsidass Publishers, 2008), 2.
McEvilley dates the wave of Akkadian influence to around 1000 BCE (this is incidentally during the Bronze Age dark age), further identifying two Akkadian words apsu and tiamat which originate from the Creation Epic.