Theory re Nethinim Origins

Michał Milewski, a co-administrator of the Family Tree DNA R1a project, in a
November 17, 2012 post stated:

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I have once suggested an alternative hypothesis. By contrast to the very popular Khazar theory, it seems to explain the characteristic association of this specific cluster with Levites. This theory assumes that the all R1a-AL members derive from a single lineage of Nethinims of Iranian (Persian, Median, etc.) origin, who could have arrived to Jerusalem with Ezra (the last wave of Jews returning from the Babylonian exile). During the subsequent period, the Nethinims were gradually incorporated into the Levites community (similarly to what happened to porters and singers). When the Jewish community suddenly shrank at the beginning of the first millennium AD (the population of Jews living in the Roman empire dropped from about 8 million to 1-1.5 million), this hypothetical R1a patrilineage was likely to become limited to one or two families living in one specific location (most likely in the Mediterranean region, i.e. in Italy or Spain). The next steps would include moving to Western Germany during the late Roman or Carolingian times and contributing to the demographic expansion of the Ashkenazi Jews.

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Milewski had previously suggested this theory in e-mails exchanged in August 2011.
Levite&HighPriest.Mourentore1743
Levites and High Priest: copper engraving by de Marne, 1743
(first issued by Jan Mourentore, Antwerp, 1599)