The Aryans appear in Mitanni from 1500 BC as the ruling dynasty, which means that they must have entered the country as conquerors. - Thomas Burrow

Burrow, whose The Sanskrit Language (1973) is still the authority in this field, says: "Vedic is a language which in most respects is more archaic and less altered from original Indo-European than any other member of the family" (34); he also states that root nouns, "very much in decline in the earliest recorded Indo-European languages", are preserved better in Sanskrit, and later adds, "Chiefly owing to its antiquity the Sanskrit language is more readily analysable, and its roots more easily separable from accretionary elements than… any other IE language" (123, 289).

In addition, in his The Sanskrit Language T. Burrow finds a few traces of the Sanskrit language among the documents of the Kassite dynasty of Babylon: “In a list of names of gods with Babylonian equivalents we find a sun-god Suriyas (rendered Samas) which must clearly be identified with Skt Surya. In addition, Maruttas the war-god (rendered En-Urta) has been compared with Skt Marut … Among the kings of this dynasty one has a name which can be interpreted as Aryan: Abhirattas: abhi-ratha – ‘facing chariots in battle’.”

… in the case of Indo-European it is certain that there was no such unitary language which can be reached by means of comparison. It woul be easy to produce, more or less ad infinitum [,] a list of forms like Skt nabhi-, Gk omphalos 'navel', which although inherited directly from the primitive IE period, and radically related [,] are irreducible to a single original. In fact detailed comparison makes it clear that the Indo- European that we can reach by this means was already deeply split up into a series of varying dialects.