"This is what manifest destiny archeologists and anthropologists have been trying to prove for more than a century and are arguing today in the Kennewick Man case - that this hemisphere was totally empty until people started migrating across the Siberian land bridge and by sea from Europe and Polynesia."

I got a bit confused there - the alternative would be that all of us originated in America rather than Africa, and emigrated elsewhere across the Siberian Land Bridge and by sea, and I've never heard that suggested by anyone.