The list of common deities and concepts will make it clear that the Zoroastrian system is essentially the same as the Vedic one. The presence of Indra in the list of the daevas seems to mirror the relegation of Indra that started in the Puranic times, where instead of connecting to Svar through the intermediate region of which Indra is lord, a direct worship of the Great Lord (Visnu or Siva) was stressed. This innovation is not counter to the Vedic system since the triple division is a recursive order. The devas are a part of the good forces in the Zoroastrian system under the label of yazata (yajata, the adored-ones).
The Zoroasatrian mythology remembers the Vedic sages and heroes such as Kavi Susravah (Kay Khosrau), Kavi Usanas (Kay Us). The names Ksatra Vrya (Shahriyar) and Suvarnah (Khwarrah, Farrah) help find the logic of late Persian names. The daeva in modern Persian are known as deev. The commonality of the fire ritual is well known. Less known is the ritual of the nine-nights (barashnom i no-shab), which is like the Indian ritual of the same name (navaratri). The No Roz occurs on the day of the spring equinox, just as the festival of Indra does.
Zarathushtra made a clear distinction between the good way (ashavant) and the false way (dregvant). The pre-Zoroastrian religion of Iran is clearly Vedic. Zarathushtra's innovation lay in his emphasis on the dichotomy of good and bad. But in details it retained the earlier structure of the Vedic divinities and their relationship as well as the central role of the fire ritual.
The Pahlavi texts distinguish between the states related to the spirit and the body as menog (Sanskrit manas) and getg (Sanskrit gathita). The idea of Consciousness being primary is expressed in the theology as the creation, first of menog and then getg. In the beginning both these are perfect but later, due to "mixture" there is trouble. In general, evolution proceeds from the menog to the getg state. This is similar to the evolution from sattva to tamas.
The Pahlavi word for "confession" is patt, which is similar to patita, fallen. Purification is done by yozdathra, suddhi. Herodotus states that the "Persians built no temples, no altars, made no images or statues" (Herodotus 1.131-2). Arrian in the Indica (7) says that Indians "did not build temples for the gods." To the outsider also, the two religions of the Persians and the Indians looked similar. Elsewhere, I have summarized the evidence  regarding the presence of the Indian religion in West Asia in the second millennium BC. This spread appears with the Kassites in 1750 BC in Mesopotamia who worshiped Surya, and later for centuries in the empire of the Vedic worshiping Mitanni. These ruling groups represented a minority in a population that spoke different languages. Other Vedic religion worshiping groups were undoubtedly in the intermediate region of Iran, which itself consisted of several ethnic groups including the Elamite and the Turkic.
Zarathushtra brought a new element into the picture from the northeast. Linguistically, he happened to be "h" speaking in opposition to the Indic "s" speaking, as in haptah versus saptah for week, or hvar versus svar for the Sun. He also brought the categorization of good versus evil onto the framework to create a new structure which was to be influential in the shaping of the Judeo-Christian tradition.
The old Vedic religion survived for a pretty long time in corners of Iran. The evidence of the survival of the devas comes from the daiva-inscription of Khshayarshan (Xerxes) (ruled 486-465 BC) in which the revolt by the daiva worshipers in West Iran is directly mentioned.
Scholars generally take the use of daiva in the inscription as a misprint for daeva. Whether that is true or not, the inscription does point to the presence of diverse beliefs within the region during the middle of the first millennium BC. Furthermore, the presence of the Mitanni does support the notion of the daiva worshipers to the West of the Iranians.
The extensive spread of the Vedic religion in Iran prior to Zarathushtra explains how the Zoroastrian "reform" left the basic system unchanged. The similarities in the ritual offering made by the Zoroastrians and the Hindus are well known. These offerings include the milk, water, the sap of plants, cakes of rice or wheat, fruit and vegetables, butter.
The spread of the Vedic system also explains how the Mitannis , as an Indic-name using ruling minority, remained connected to their Vedic traditions. They were neighbours to the pre-Zoroastrian Vedic Iran and thus they should not be seen as an isolated group. The chronological framework presented by the parallels between the Zoroastrian and the Vedic systems is in consonance with the idea that the Vedic people have been in India since at least 5000 BC, as confirmed by the astronomical references in the Vedic texts and the absence of archaeological evidence regarding influx of people into India after that time .
The Puranas speak of the Vedic people in Jambudvpa and beyond the Himalayas in the
North, in Uttara-Kuru. It appears that subsequent to the collapse of the Sarasvati-river based economy around 1900 BC, groups of Indians moved West and that might have been responsible for the Aryanization of Iran, if it wasn't Aryanized earlier. This movement seems to be correlated with the presence of the Indic Kassites and the Mitannis in West Asia. 
In such a scenario, the Uttara-Kuru tribes, who were a part of the larger Vedic world, may have pushed Westwards in a process that must have continued for millennia and taken myths from the Indic region to Europe. This was not a process of invasions, but rather a complex process with some migration and some cultural diffusion. One should note that about 10,000 years ago, most of northern Europe was under ice in the last Ice Age and the inhabitants of ice-free southern Europe were speakers of non-Indo-European languages such as the Basque, Etruscan and Finnish of later times. To the extent the Uttara-Kuru tribes moved West, they must have intermarried with local populations to emerge as different European tribes.
The divergence in the names of the stars, which were central to the Vedic ritual, suggests that there existed variation in the traditions, reflecting local custom and influence of other cultures. If the date of Zarathushtra being 258 years before Alexander is correct, then the syntheses of Zarathushtra and the Buddha, one extolling wisdom (medha) and the other intelligence (buddhi), occurred at almost the same period. The use of temples is late in the Zoroastrian and the Hindu traditions and it may have been a response to the popularity of the Buddha image and the sangha that administered it.
Unfortunately, Avestan and later Zoroastrian studies have not made use of fieldwork of social customs in Kashmir and Punjab. It is my hope that research done on parallels in India and also on the Vedic religion prior to the coming of Zarathushtra will be most useful. The understanding of the Zoroastrian religion would benefit from a systematic comparison with the Vedic texts and by considering the Sanskrit etymologies of the Avestan words.
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