We can know about early vedic period from
A. Archaeological excavations
B. The Rig Veda
C. Jatak KathaD. Contemporary culture

The Rig Veda is an ancient collection of Vedic Sanskrit Hymns and it is one among the four sacred texts viz. Yazur Veda, Sama Veda, Atharva Veda. It is organized in ten "books", or maṇḍalas. The hymns within each of the family books are arranged in collections each dealing with a particular deity: Agni comes first, Indra comes second, and so on. They are generally arranged by decreasing the number of hymns within each section.
Geographical distribution of the Vedic era texts. Each of the major regions had their own recension of Rig Veda (Sakhas), and the versions varied. The Kuru versions were more orthodox, but evidence suggests Vedic era people of other parts of Northern India had challenged the Kuru orthodoxy. The Vedic Sanskrit texts of the Rig Veda was transmitted remarkably unchanged, preserving, apart from certain prosodic changes the linguistic stage of the Late Bronze Age. Because of the faithful preservation of the text, the language was no longer immediately understandable to scholars of Classical Sanskrit by about 500 BC, necessitating commentaries interpreting the meaning of the text of the hymns. The Rig Veda is the earliest of specimens of Indo -European language we know about the Aryans. It also gives information about the early Vedic period.