the distant major ancient languages of the world. The Indo-Aryan migrations theory explains the common features shared by Sanskrit and other Indo-European languages by proposing that the original speakers of what became Sanskrit arrived in South Asia from a region of common origin, somewhere north-west of the Indus region, during the early 2nd millennium BCE. Evidence for such a theory includes the close relationship between the Indo-Iranian tongues and the Baltic and Slavic languages, vocabulary exchange with the non-Indo-European Uralic languages, and the nature of the attested Indo-European words for flora and fauna. The pre-history of Indo-Aryan languages which preceded Vedic Sanskrit is unclear and various hypotheses place it over a fairly wide limit. According to Thomas Burrow, based on the relationship between various Indo-European languages, the origin of all these languages may possibly be in what is now Central or Eastern Europe, while the Indo-Iranian group possibly arose in Central Russia. The Iranian and Indo-Aryan branches separated quite early. It is the Indo-Aryan branch that moved into eastern Iran and then south into South Asia in the first half of the 2nd millennium BCE. Once in ancient India, the Indo-Aryan language underwent rapid linguistic change and morphed into the Vedic Sanskrit language. Vedic Sanskrit The pre-Classical form of Sanskrit is known as Vedic Sanskrit. The earliest attested Sanskrit text is the Rigveda, a Hindu scripture from the mid- to late-second millennium BCE. No written records from such an early period survive, if any ever existed, but scholars are generally confident that the oral transmission of the texts is reliable: they are ceremonial literature, where the exact phonetic expression and its preservation were a part of the historic tradition. However some scholars have suggested that the original Ṛg-veda differed in some fundamental ways in phonology compared to the sole surviving version available to us. In particular that retroflex consonants did not exist as a natural part of the earliest Vedic language, and that these developed in the centuries after the composition had been completed, and as a gradual unconscious process during the oral transmission by generations of reciters. The primary source for this argument is internal evidence of the text which betrays an instability of the phenomenon of retroflexion, with the same phrases having sandhi-induced retroflexion in some parts but not other. This is taken along with evidence of controversy, for example, in passages of the Aitareya-Āraṇyaka (700 BCE), which