Philology
The determination of the age in which Vedic literature started and flourished has its consequences for the Indo-Aryan question. The oldest text, the Rigveda, is full of precise references to places and natural phenomena in what are now Punjab and Haryana, and was unmistakably recorded in that part of India. The date at which it was composed is a firm terminus ante quem for the presence of the Vedic Aryans in India. In the academic mainstream view it was composed in mid to late 2nd millennium BC (Late Harappan) and OIT proponents propose a pre-Harappan date.
OIT proponents propose that bulk of Rigveda was composed prior to Indus Valley Civilization by linking archaeological evidence with data from Vedic text and archaeo-astronomical evidence.
Read more about this topic: Out Of India Theory