TOF first ran across the notion of humans in America prior to the American Indians in a fiction! It was "Beringia," a 1990 story by Poul Anderson in his Time Patrol book The Shield of Time. It follows Wanda Tamberly, a time patrolman who has been studying this pre-Indian people (and has grown attached to them) when a band of Indians finds its way across the Bering Land Bridge. They bully the indigenous people (because they can. They are technologically more advanced.) But even when they don't, they over-awe the natives. But they also fear the native's powers of magic. After all, they are in their country and feeling very insecure, having fled their own land. Everything hits the fan, there are causal loops, and the time patrolman has to salvage her own reputation, too.