Thank you, Daverupa!
This can be complemented by articles:
Michael Witzel
The Development of the Vedic Canon and its Schools : The Social and Political Milieu
Both the Malla and Vrji apparently immigrated into the east only after the end of the Vedic period, but well before the time of the Buddha (c. 400 B.C.). This must have been one of the last great infiltrations in Vedic times of western peoples into the lower Gan gå area. More or less about this time the so-called second urbanization began as well.
Nevertheless, the settlement pattern in the east was not as homogenous as it was in the more western areas where the indigenous population had become Indo-Aryan in language and culture since the Mantra period. Instead, the Kosala-Videha area was one of great mixture of peoples. There were some earlier eastern Indo-Aryan settlers, the local Munda people and some Tibeto-Burmese elements. Then, various types and groups new immigrants entered from the areas further west. These were some brahmanically oriented tribes but also other non-orthoprax Indo-Aryan tribes such as the Malla and Vrji. They immigrated from northwestern India into Bihar which had been already settled by the old, para-Vedic Indo-Aryan tribes such as the Iksvåku, Kosala, Kåśi, and Videha.
Many of these tribes, including the Śakya to whom the Buddha belonged, are called asurya in ŚB. For it is the Sakya and their neighbors, the Malla, Vajji, etc. who are reported in the Påli texts as builders of high grave mounds, such as the one built for the Buddha. According to ŚB 12.8.1.5 the “easterners and others(!)” are reported to have round “demonic” graves, some of which may have been excavated at Lauriya in E. Nepal. These graves are similar to the kurgan type grave mounds of S. Russia and Central Asia. However, the origin of the Śakya is not as clear as that of the Malla and Vrji. They may very well have been (northern) Iranian, and would then constitute an earlier, apparently the first wave of the later Śaka invasions from Central Asia.
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The eastern region thus supplied the ideal ferment for the meeting of ideas and the development of new concepts. Just as the break-up of the old tribal society of the Rgveda saw strikingly new developments in ritual and the emergence of the brahmanical pre-scientific science of homologies (bandhu), the new stratified and partly aristocratic, partly oligarchic society of the east witnessed the emergence of many of the typically Upanisadic ideas.
By the time of the Buddha (c. 400 B.C.), wandering teachers of all sorts were normal appearances in the towns and villages of the east (Dīghanikåya 2). We get a glimpse of the earlier state of this phenomenon when Yåjñavalkya leaves home (BĀU 4.5.15). If we may trust the BĀU and ŚB accounts of Uddålaka's travels in the Panjab, he reached both the western and the eastern ends of Vedic India in his travels. In fact, the geographical horizon of the early Upanisads stretches from Gandhåra to Anga.
http://www.people.fas.harvard.edu/~witzel/canon.pdfPossible Iranian Origins for Sākyas and Aspects of Buddhism
Jayarava Attwood
http://independent.academia.edu/Jayarav ... f_Buddhism