The fact that refutes the land bridge as being the only route is that the indigenous peoples of Central and South America were far more advanced than those of North America. If the land bridge was the only route, we would expect to see the most advanced tribes in the northern hemisphere where they had lived longer and we more established. Just the opposite is true. My guess is there were at least 2 routes - a land bridge in the north, and island-hopping in the south.