I'm not really a genetics person so I will leave comments on that aspect of the paper to others. I think this model of human migration into the Americas is interesting. Although other theories propose several waves of migrations, this is the first that I am aware of that focuses on Beringia. In the past, Beringia has been of interest mainly for its utility in getting people across Siberia and into America (i.e. it was mainly considered a highway). What actually happened in Beringia was of little concern and the idea that people would occupy it in much the same way they occupy any other new territory seemed to be a foreign concept. In pointing out that it took several thousand years for people to move through the area the authors of the paper put the issue in a much more realistic light. This raises the question of how people, specifically hunter/gatherers (or foragers if you prefer) actually utilize new territory. Perhaps a survey of the ethnographic literature might help them flesh out some of the details. The one part of the paper that caused me some concern was the discussion of the impact of the archaeological footprint of the Amerind ancestors on Siberia, given that this was of about 7,000 years. On the other hand I am not very familiar with the archaeology of the area so a little more details would have been helpful. Overall, I think this was an excellant contribution to the literature.