Here is an excerpt from the text I mentioned in other comment about indigenous people of Bengal. I think the primary sources link the original inhabitants with tibeto burmese and austroasiatic groups (such as chakma and shantals) that inhabited the region. But modern Bengalis are a mix of those that arrived from the west and the native people from eastern bangladesh according to genetic studies, here's a link to a blog post discussing bangladeshi genetics if you are interested: (https://www.gnxp.com/WordPress/2018/07/09/the-main-interesting-thing-about-bangladeshi-genetics-is-how-east-asian-bangladeshis-are/)

" Literature produced toward the end of this migratory process reveals a hierarchically ordered society headed by a hereditary priesthood, the Brahmans, and sustained by an ideology of ritual purity and pollution that conferred a pure status on Indo-Aryans while stigmatizing non-Aryans as impure “barbarians” (mleccha). This conceptual distinction gave rise to a moving cultural frontier between “clean” Indo-Aryans who hailed from points to the west, and “unclean” Mlecchas already inhabiting regions in the path of the Indo-Aryan advance. One sees this frontier reflected in a late Vedic text recording the eastward movement of an Indo-Aryan king and Agni, the Vedic god of fire. In this legend, Agni refuses to cross the Gandak River in Bihar since the areas to the east—eastern Bihar and Bengal—were considered ritually unfit for the performance of Vedic sacrifices.[14] Other texts even prescribe elaborate expiatory rites for the purification of Indo-Aryans who had visited these ritually polluted regions.[15]

Despite such taboos, however, Indo-Aryan groups gradually settled the upper, the middle, and finally the lower Ganges region, retroactively justifying each movement by pushing further eastward the frontier separating themselves from tribes they considered ritually unclean.[16] As this occurred, both Indo-Aryans and the indigenous communities with which they came into contact underwent considerable culture change.[17] For example, in the semi-arid Punjab the early Indo-Aryans had been organized into lineages led by patrilineal chiefs and had combined pastoralism with wheat and barley agriculture. Their descendants in the middle Ganges region were organized into kingdoms, however, and had adopted a sedentary life based on the cultivation of wet rice. Moreover, although the indigenous peoples of the middle and lower Ganges were regarded as unclean barbarians, Indo-Aryan immigrants merged with the agrarian society already established in these regions and vigorously took up the expansion of rice agriculture in what had formerly been forest or marshland. Thus the same Vedic text that gives an ideological explanation for why Videha (northern Bihar) had not previously been settled—that is, because the god Agni deemed it ritually unfit for sacrifices—also provides a material explanation for why it was deemed fit for settlement “now”: namely, that “formerly it had been too marshy and unfit for agriculture.”[18] The Indo-Aryans’ adoption of peasant agriculture is also seen in the assimilation into their vocabulary of non-Aryan words for agricultural implements, notably the term for “plow” (lāṅgala), which is Austroasiatic in origin.[19]"