In order to explain the common features shared by Sanskrit and other Indo-European languages, many scholars have proposed the Indo-Aryan migration theory, asserting that the original speakers of what became Sanskrit arrived in what is now India and Pakistan from the north-west some time during the early second 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.