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This is a very tough question. So far I would opt for the Paleolithic continuity, although it doesn't really make sense based on the age estimate of this haplogroup (too young).
The Indo-Europeans hypothesis has the problem that the Balkans have far more I2a than R1a and R1b, unlike other IE regions. Ditto for the Slavs. Then, if it was Slavic, where would the previous population (from the Palaeolithic to the Roman era) have gone ? The Aromuns, presumably descended from the Roman-era population of the Balkans, have over 20% of I2a, more than the Slavic-speaking Macedonian neighbours.