Oh well. My view is that humans came to the Americas over and over, from various places, in various-sized waves, probably ripples timed with climate warmings, and left traces, and that the isolation of the continents is phony -- just as it is phony everywhere else in the world. Isolationism is political in origin, and is nothing more or less than yet another master race theory.
Personally I am going with the "Out of Africa" theory that the genetic research is demonstrating and it's getting better all the time.
According to the genes, my maternal ancestors went East to America across Berengia, and my paternal ancestors went West to England and France, and then got in a boat, continued West, and eventually met up in the middle of the North American continent.
If people from the European side of Eurasia did walk west or boat west before the Vikings, chances are that they're more closely related to me than you, since there were definitely Inuit in Greenland, and my own mitochondrial DNA is A, which is very common among the Inuit.
If you take a look at the faces of Native Americans from the earliest days, they look Asian. Not European.
I'm not saying Europeans couldn't have come here, but they didn't make any significant contributions before 1492. Even the Vikings didn't leave much of a trace.