"The structure of the text has been more extensively studied, already by Bergaigne (1878-83) and Oldenberg in the 19th century. From the latter's Pro… - Michael Witzel

This leads to the conclusion that the Indo-Aryan influence, ... was powerful enough from early on to replace local names, in spite of the well-known conservatism of river names. This is especially surprising in the area once occupied by the Indus Civilisation where one would have expected the survival of older names, as has been the case in Europe and the Near East. At the least, one would expect a palimpsest, as found in New England with the name of the state of Massachussetts next to the Charles river, formerly called the Massachussetts river, and such new adaptations as Stony Brook, Muddy Creek, Red River, etc., next to the adaptations of Indian names such as the Mississippi and the Missouri.

So far archaeology and palaeontology, based on multi-variate analysis of skeletal features, have not found a new wave of immigration into the subcontinent after 4500 BCE (a separation between the Neolithic and Chalcolithic populations of Mehrgarh), and up to 800 BCE.

Something of this fear of the horse and of the thundering chariot, the "tank" of the 2nd millennium B.C. is transparent in the famous horse 'Dadhikra' of the Puru king Trasadasya ("Tremble enemy" in RV 4.38.8) ……..The first appearance of thundering chariots must have stricken the local population with terror similar to that experienced by the Aztecs and the Incas upon the arrival of the iron-clad, horse riding Spaniards.