I'm coming a bit late to this so apologies if I repeat something or miss something. Apologies also that I've ended up going on at some length. This question has interested me since attended Richard Gombrich's Numata lectures in 2006 (that became the book What the Buddha Though). Something Prof Gombrich mentioned in passing was that Prof David Pingree had discovered clear evidence of Perisan influence in the Dīgha Nikāya.
In the Brahmajāla Sutta is a list of divination techniques which count as wrong livelihood for a bhikkhu. In fact these are in the section of text that is repeated in all of the first 13 suttas but is routinely left out of all by the first as a repetition. These divination techniques are so similar in form and content to a Mesopotamian Divination manual that survives in a cuneiform clay tablet that the Buddhists must have drawn on it as a source. Pingree argued that the obvious vector for spreading this text from Mesopotamia to India was the Achaemenid Empire (Ca. 550 BC- 330 BC) which exacted tribute from a province east of the Hindu Kush and West of the Indus. Whether this amounted to "control" is doubtful but the certain there was a Persian influence in the region, and almost certainly trade. Pingree sees other cultural influences in Indian literarture as well. Thus there was almost certainly no direct contact between Mesopotamian and Indian, but the Persian spread some Mesopotamian culture far and wide. For example, Persian administrators adopted first the cuneiform and then Aramaic writing systems.
See particularly: Pingree David. 1991. Mesopotamian omens in Sanskrit paper presented at La Circulation des biens, des personnes et des idées dans le proche-oriet ancien. Actes de la XXXVIIIe Recontre Assyriologique Internationale. Paris, 8-10 juillet. (Paper is in English).
With respect to any theory about the language of the Indus Valley civilisation I don't anyone has yet cracked it. The best efforts are those by Michael Witzel and Steve Farmer especially:
The Collapse of the Indus-Script Thesis: The Myth of a Literate Harappan Civilization. Steve Farmer, Richard Sproat, and Michael Witzel. http://www.safarmer.com/fsw2.pdf
Early Sources for South Asian Substrate Languages. Michael Witzel. http://www.people.fas.harvard.edu/~witz ... trates.pdf
Prof Witzel is kind enough to put much of his work online for free. Witzel argues that Munda is the best candidate for a language in the region of the Indus Valley based on loan words in Vedic. "An important result therefore is, that the language of the Indus people, at least those in the Panjab, must have been Para-Munda or a western form of Austro-Asiatic." (Early Sources: 15) But the fact is that we'll probably never know what language the Indus people spoke because the surviving evidence is so sparse.
One of the obvious conclusions about asuras and devas is that they are the common inheritance of the Indo-Iranian language speakers. A point is worth making here: language, culture and geography can be independent. So we know the Mitanni, for example, spoke an Indo-Iranian language of the Indic branch, and that they had some Indo-Iranian gods in their religion. But geographically they are far from other Indo-Iranian speakers. There is quite a good explanation for this in Frits Staal's book Discovering the Vedas
. It's more important when we look at the central Ganges plain. Someone has already pointed out that many languages and cultures were found in the region.
Asuras and devas are not distinguished in the earlier parts of the Ṛgveda. Varuṇa for example is known both as deva and asura. Sorry I don't have references for this to hand, but get hold of Doniger's selected translation of the RV and check the intro and the index - I'm sure I've seen an example or two in there. So deva and asura were just two names for gods in Indo-Iranian. Zarathustra decided that devas (Av daiva) and the people who worshipped them (probably Indic speakers) were demonic (ca 1000 BC?). In Vedic, a conflict between two groups of poorly defined rival gods emerged, with devas being more highly thought of. This contention between the groups is part of the narrative of Churning the Ocean of Milk (samudra manthanam). Aspects of this narrative are preserved in the Pali Canon - a testament to the Brahmanisation of Buddhism. By this time the asuras are demons who have their own loka. But note that sometimes Buddhists count 6 rebirth realms and sometimes 5 - where devas and asuras inhabit the same loka.
I suppose most of us are now familiar with the two civilations theory independently proposed by Johannes Bronkhorst and Geoffrey Samuel. On the whole I prefer Samuel's presentation of the data because he is less concerned with proving a point, and his version of events requires less revision of history. The Brahmins saw the area east of the Yamuna/Ganges confluence as uncouth, if not demonic. By the middle of the first millennium BC they were beginning to make inroads into that region, however. The Central Ganges Valley (CGV), is what Bronkhorst calls Greater Magadha, but at this time was more accurately called Greater Kosala. Magadha was still rising up to become a military power, whereas well before that Kosala-Videha was the centre of civilisation in the region.
IN her book on the Upaniṣads (Text and Authority in The Older Upaniṣads
. Leiden: Brill, 2008) Signe Cohen remarks that Brahmins in the CGV seem to be in conflict with Brahmins from the Āryavrata. The debate in the Bṛhadāraṇyaka Upaniṣad (BU) is between Yājñavalkya, representing the break away group aligned to the Yajurveda, and some Brahmins from the Āryavrata who trace their lineage to the Ṛgveda. A remnant of this distinction may well be found in the Pāli reference to "Brahmins from the Western region (brāhmaṇā pacchā bhūmakā
SN 42.6:", who appear to have been a distinctive group. One of the characteristics of the late Vedic was the introduction of the idea of rebirth destinations based on karma and an end to rebirth. Karma is either puṇya or pāpa and those who do puṇya actions (probably synonymous with Vedic rituals) go to the sun or moon (this is the famous Pañca vidyā that Gombrich finds traces of in the Tevijjā Sutta). Those who perform pāpa actions are reborn as worms etc.
In BU the introduction of these new concepts of ethical karma and rebirth is done by a king. This anomaly has drawn much attention but no consensus. All we can agree on is that kings teaching Brahmins (and not the other way around) is strange. Where did these new ideas come from. Bronkhorst argues that they came from the śramaṇa communities, but in order to do so he has to revise the chronology of Indian history in a way that is not entirely convincing. Everything points to the early Upaniṣads being a couple of centuries older than the earliest Buddhist texts.
In an article in the JOCBS (http://www.ocbs.org/ojs/index.php/jocbs/article/view/26
) I argued for another view based on some informal comments by Michael Witzel. The theory is that the new ideas came into the region from outside and that they were influenced by Zoroastrianism. I developed his notion that the Śākya tribe were Iranian in origin. "...tribes such as the Śākyas are largely absent from the Vedic literature and, where they are noticed, their customs are “strange”, but they are at the forefront in Buddhists texts, suggesting a late migration into the Bihar region; the name Śākya appears to be cognate with Śaka; burial practices in Magadha similar to Central Asia; incest marriages; and post-mortem judgement of actions of the body, speech and mind triad."
Sometime after the Ṛgveda was finished (ca 1200-1000 BC) but before the Buddhist texts were composed, a number of Iranian tribes settled in Panjab: including the Śākya, Vṛjji, Malla and many others mentioned in Pāli, but not in the Ṛgveda. They brought their own language and culture but assimilated quickly. Compare the Normans who arrived as Norwegian vikings and within a generation or two were fully assimilated French speakers. In 850 BC +/- 10 years the climate of all Eurasia shifted so that India was warmer and drier, which set tribes on the margins of the deserts in motion. They could not move into the Āryavrata because the Brahmins would not have welcomed them. So they moved east until they ended up on the margins of Kosala, and were subsequently absorbed into Kosala (and this may well be why some claims to relations with the Kosalan ancestor King Okkāku aka Ikṣvaku are preserved in the Canon).
The Śākyas interest us in particular of course since they are the Buddha's tribe. They claim to be descended from an sibling marriage (common in Iranian royalty, in imitation of Egypt) but shameful in the major Indian cultures. They speak of the actions of body, speech and mind determining one's afterlife destination. Their burial mounds (which become stupas in India) are unlike the Vedic mounds. The idea of cousin marriage, which someone has mentioned, is an artefact of the Pāli canon being written down in Sri Lanka where Dravidian marriage customs are the norm. There is no credible evidence for cousin marriage.
So these Iranian derived tribes arrive in the Central Ganges Valley. They meet indigenous people speaking Munda, Dravidian, Tibeto-Burman languages. They meet the first wave of Indic speakers (who are distinct from their Vedic speaking cousins - see Madhav Despandhe's essay on Indic dialects in Aryan and Non Aryan in India
). There is considerable interaction and the first śrāmaṇa religions emerge from this melting pot.
KARMA & REBIRTH
I have a forthcoming paper (wrangling with the editor at present) which describes the history of karma and rebirth like this. Generally speaking people in India in 800 BC believed in a rebirth eschatology. One died in this world (ayam lokaṃ) and was born in the other world (paraṃ lokaṃ) - the old binary distinction is often see in Pāli. One lived there for a long time until one died and was reborn here. Around and around. And everyone was happy with this because those who had gone before (Skt. pretaḥ < pra√i) were there. One was either living here with one's family, or there with one's ancestors. Now someone in this thread argues against rebirth being the Vedic view, but this was disproved some time ago by Joanna Jurewicz. see for Example: Rebirth eschatology in the Ṛgveda in search for roots of transmigration. Indologica Taurinensia
, 34 (2008). It now seems clear that Ṛgveda has evidence of this basic binary kind of rebirth eschatology that is described by Gananath Obeyesekere in his book Imagining Karma
And then the Śākyas arrived and, being at least influenced by Zoroastrianism, they believed in a single destination afterlife based on actions of body, speech and mind. What emerged was a hybrid system that gradually became more complex. The hybrid model has a round of rebirth that continues based on actions of body, speech and mind unless one makes an effort. This version of the afterlife portrays that round as unattractive (for the first time) and offers an alternative single destination. A fairly simply hybrid like this is what appears in Bṛhadāraṇyaka Upaniṣad. Later versions, as found in Buddhist and Jain texts is more sophisticated with multiple rebirth destinations, but terminology (like ayaṃ/paraṃ lokaṃ) often reflecting an earlier stage of development.
The Śākyas retained the idea that judgements about the afterlife destination where impersonal and impartial. And quite uniquely in India the mechanism for deciding one's afterlife destination did not involve a deity or anthropomorphic representation. Karma is still a supernatural force, still carries out all the same functions as moral gods, but is not personified, except to some extent in the figure of Yama who presides over hell. This is an odd transformation I have yet to fully investigate, because in the Vedas Yama is the first human being to find his way to the ancestors, a trail blazer, and thus ought to live above the sky with them. How he comes to live in the underworld is a mystery to me.
Thus my theory, backed by minimal evidence, is that the Śākyas injected something new into the two cultures picture from outside the region. It spread both east and west (most theories describe transfer of ideas east to west [Bronkhorst], or west to east [almost everyone else]). The Śākyas had assimilated and probably spoke an Indic language by this time, but this does not mean that they were culturally similar to Brahmins. Brahmins were well established in Kosala by the time of the Buddha and probably making further inroads, but they are treated as outsiders with outlandish notions in early Buddhist texts. But Brahmins soon become converts to Buddhism and had a major impact on how Buddhism developed. Brahmin families were literate, cultured, and sophisticated in their philosophy and outlook (i.e. they were the opposite of the cartoon bad guys we usually portray them as). Brahmin converts to Śrāmaṇa religions were scorned by the orthodox, but this did not prevent them from giving Buddhism quite a few ideas and practices. No where is this more clearly seen than in the biography of the Buddha which is tuned to Brahmanical sensibilities in innumerable ways, and more as times goes on. Not least in our founder carrying a prestigious and quintessentially Brahmin name: Siddhārtha Gautama. This name is very difficult to explain for a non-Brahmin even if we take him to be a kṣatriya, which I think is extremely unlikely. (I'm flogging an article about this at the moment, but no takers yet - a draft is here
My understanding is that the Buddha was probably culturally a mix of Iranian stock and indigenous inhabitants of the Himalayan foothills who spoke Munda, Dravidian and/or Tibeto-Burman languages. His tribe most likely spoke an Indic language and was thoroughly acclimatised to India except for a few remnants that point to Iran. During his lifetime and afterwards there was considerable and growing influence from Brahmins. Some minor influences from Persia are also detectable. Early Buddhism is a very mixed bag culturally. And one of the themes I'm exploring on my blog at present is the poor fit between some aspects of Buddhist ethics and Buddhist metaphysics. I take this to be evidence for the composite nature of Buddhism from the earliest period. I've quipped that it does not look like the work of one committee, let alone one person!
My inclination is away from seeing the Buddha as a founder figure, to seeing him more as a culture hero - a legendary embodiment of the values of the Śākyas (after all Buddhists were referred to as Śākyas for many centuries in India, particularly by their philosophical opponents). Thus he is more like Gilgamesh, Maui, or King Arthur than Jesus, Mohammed or Guru Nanak. He is an idealised, but also, by the time we meed him, Brahmanised, Śākyan.