Most people look to the simple creature; the lemming.
Each year adult lemmings unwittingly commit suicide by charging head-long into the Bering Sea. Scientists say it's because for millenia, lemmings migrated across the "land bridge".
Supposedly, because this annual migration was imbedded in their DNA, lemmings continue to die in the ocean....looking for their long lost "land bridge".
I'd also go with a scenerio that included glaciers. If in a period back in history that glaciers prodiced a "land bridge" early man wouldn't need boats...although boats would work. Humans are a curious lot, and it they can see land across some water...it's only a matter of time before someone goes there and comes back, even if it's just to say "I was there".
While Viking settlements in Newfoundland and Nova Scotia have been noted, there are are stone structures with massive stone obelisks in New York State, Maine, and New Hampshire. The structures are believed to have been made betwen 1,600 and 2,000 BC. This timeframe is believed to be far older than North American Clovis Man or other "migratory" peoples that made use of the Bering Strait "land bridge". Some scientists say that Celtic peoples had temporary settlements in these locations...other scientists say they were built by ancient Minoan explorers. No one seems to know for sure.
So much for Columbus discovering North America.
There have also been discoveries of Chinese anchors in ports ranging from California to Peru...these anchors are dated as old as back to 1,000 BC.
BTW: When Russia still owned Alaska there was a small trickle of Russian citizens who volunteered to settle the region prior to the American Civil War. You will find their ancestors with russian names to this day.
I'd love to see more research done. The evidence is out there.
ROX