Pssst, there's no such thing as 'true Europeans' either probably. Every nation is a nation of immigrants. It just depends on where you choose to draw the 'here before' line.
This 'where shall we begin' problem has to be resolved by common agreement. In the area of English law, the solution was to go back to the Great Flood, Noah's Flood and call that a clean slate.
tabula rasa. In America, this could be going back to the date of adoption of the Constitution of the United States. What other regions of the New World might do is up to them. Whatever science might discover is a side issue.
Anyway, that seems like an example of being practical about such things.