A professor I had in College (eons ago) postulated that humans could have walked over several times duing periods of ice advance and retreat when migration corridors would have been open, and could have been in North America as early as 32,000 years ago, no boats needed.
Professor Stephen Oppenheimer's Journey Of Mankind based on DNA studies places humans at Meadowcroft 25,000 years ago. They may have come across the Atlantic ice sheet and may be the source of the DNA haplogroup X present in northeast Indians.
I'm beginning to favor the 'Kelp Highway' idea for the west and South American incursions. There is a kelp forest offshore that stretches from to Japan to Chile and would have been an excellent source of food...(I used to dive for abalone in Northern California in the mid-60's.) Anyway, this would allow for an early entry and possibly explain the presence of old (Jomon/Ainu-like) skeletons that are unlike the modern Native Americans.
(Penon Woman)
"The most intriguing aspect of the skull is that it is long and narrow and typically Caucasian in appearance….Modern-day native Americans, however, have short, wide skulls…"