This Smithsonian Museum video explains evidence suggesting that European “Solutrean” people travelled to North America by boat while ice sheets covered the northern part of the Earth.  The anthropologist Dennis Stanford explains that “Clovis” stone projectile points that were found throughout the Americas are very similar to styles that were created by the Solutrean culture in Europe, however such artifacts have not been discovered in the Alaska and Siberian region.  He also explains the difficulties of finding archeological sites in the Eastern US from that time period because much of it is now underwater.

[NOTE: There are issues with the Solutrean Hypothesis though, as this video by Stephan Milo explains.]