re: What would North America be like today had the American Indians been left alone?

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Go back and read your statement again, you claimed that they were more advanced 500 years ago than several societies today, and that is just plain false. 

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It is not false. Numerous indigenous societies today could not achieve their engineering feats alone if they tried.

I'm not arguing at all that they would have achieved anything close to what we see today, simply that their level of advancement would be more than certain other societies around the world. I don't think that's an illogical or farfetched assumption, at least compared to the thought of them being literally the least civilized society on the planet, as you had said.