Linguistics
The early linguistic hypothesis points to Indo-Aryan languages as intrusive into South Asia, supposedly some time in the 2nd millennium BC. This is heavily debated as recent genetic studies do not support an "invasion" of central Asians. Based on linguistics, geography, description of rivers, life as described in the verses, the Rig Veda has been dated to have been composed before 1500 BC.
Read more about this topic: Indo-Aryan Migration