When it comes to the arrival of Indo-Aryans to South Asia a major question Indians always post is “if they are invaders why don’t they mention that in their mythology?” My standard rejoinder is straightforward: we have plenty of paleogenetic evidence that many populations are intruders, but their mythology doesn’t indicate that. If the Indian objection is to hold then why not others? Are all human populations autochthonous in their native lands?
And yet the most recent work suggests that steppe ancestry didn’t arrive in the BMAC region until 2000 BC. That means that the Cemetery H culture in Punjab dating to 1900 BC is the earliest likely candidate for Indo-Aryans in South Asia. The Rigveda was composed as early as 200 years after this date, or as late as 700 years. Could they have “forgotten” where they came from?
The Irish are one people who have preserved their mythology due to the gradual and indigenous nature of Christianization. But 2,500 years after their arrival en masse from the continent they had forgotten the details. But, the motif of invasion was preserved, though we don’t know if that is a memory of their past, or just a channeling of the mythos of the period when their folklore was written down. Another example might be the Japanese, who arrived about 1,100 years before the Heian period, the first flowering of literate civilization on the island. To my knowledge, their mass migration from southern Korea was mostly forgotten by then.
With all this in mind, I decided to reread Genetic origins of the Minoans and Mycenaeans. The conclusion of the paper is that it’s clear an appreciable, though minority, component of Mycenaean ancestry seems to have some affinity with Indo-European groups. The two candidates for the donors are people from the Eurasian steppe, or Copper age Armenians. For linguistic reasons that I can barely evaluate, I lean toward the former. This implies that the proto-Greeks arrived in the late 3rd millennium or early 2nd millennium. The mythology of ancient Greek as recorded by Hesiod and others in the Archaic period probably dates in part to the Bronze Age (some of the Greek gods are recorded in the Linear B tablets). To my knowledge the Greeks do not record when they arrived from outside of Greece.
This suggests that 1,000 years is sufficient for a forgetting, at least for a semi-literate society.
The last is key. Societies with written histories can maintain continuity. But what about oral societies?