If it was possible to ask Spencer Wells a question it would be:

After reading your book "Journey of Man" there are two perplexing issues that come to me. The first would be to embrace the assumption that Man travelled to Australia from Africa along a coastal route rather easily, but gave no credibility to the same situation as a possibility for colonizing North America, rather than the "land bridge" of Beringia as the only route. Coastal hopping by early Man could rationalize the finding of "Kennewick" bones and similar finds in Spirit Cave Nevada, not to mention even older recent findings in South America.
The second issue would be that the author discounts the possibility of a "Kennewick" type Man because there are no traces of DNA found in modern Native American populations to support this theory.
But we know that Neanderthal existed in Europe before Modern Man, and we also know that Europeans did not descend from Neanderthal because there is no DNA in modern Europeans to support this theory.

Why would there be such difference in conclusions on the part of Mr.Wells in the same publication?