Living beings are like rivers that arise in the mountains, states the Upanishad, some rivers flow to the east and some to the west, yet they end in an ocean, become the ocean itself, and realize they are not different but are same, and thus realize their Oneness. Uddalaka states in volume 6.10 of the Upanishad, that there comes a time when all human beings and all creatures know not, "I am this one, I am that one", but realize that they are One Truth, One Reality, and the whole world is one Atman.
Chandogya Upanishad 6.9
Given the violence and oppression of the Rig Vedic period, Sramana movements started emerging on the eastern periphery of Aryavarta, ie. Magadhi. This is likely an example of the thoughts pushing against the oppressive, harmful ways of Vedic Aryans yet NON DUALITY, INTROSPECTION, etc was re-appropriated as Vedic thought.