The oldest written materials existing in India belong to the Vedic Aryans. It is within these works that Samasāra doctrine appears for the first time. Having lived with the indigenous people for some time, Aryan people adopted certain religious themes assimilating them into their own belief system, or substituting them for the parallel beliefs of their own culture; among those beliefs, doctrines of Fravahar, Totemism (kinship of humans with plants) and ancestor worship are plays the most part. These doctrines when  is penetrated into India are merged with the ideas of incarnation and embodiment of the gods (which are themselves rooted in Mesopotamian and Middle Eastern traditions) and the belief in totemism and are ultimately manifested as Samsāra doctrine. In this way, Gods like Viṣnu, that is closest to Samasāra, have somehow been substituted for the Arian Mithra which is most closely linked to Fravahar (manifestation) doctrine. Although Samsāra was openly expressed in Upaniṣads, there is some evidence in Vedas of the return of the soul indicating an earlier belief in Samsāra in the period before writing of Upaniṣads.