Early Americans

During the waning of the last Ice Age an ice free corridor opened up between the Laurentide ice sheet and the Cordilleran ice sheet.  It was thought that people migrated over several generations from the Russian Far East, crossed the Beringia land bridge and into Alaska, & Canada.  Then South through an ice free corridor and into the interior of the continent.  There is Archeological evidence that people were in the America's before the ice free corridor opened up.  An alternative route was people boating south along North America's Pacific Coast.  Then again, there might have been several episodes of glacial resurgence and retreat creating ice free corridors that opened and closed multiple times.  It is  pretty much inconceivable that people migrated traveled over 1000 miles across the surface of the ice sheet from the North to the South.   The ice free corridor was created as temperatures warmed, and ice sheets retreated as well as by the rain shadow or (precipitation shadow) on the east side of the Rocky Mountains.  

Archeological sites with evidence of human's having killed and butchered large mammals (above). 

Clovis spear points (below).