Earlier, I posted a lecture about the questions of who were the Native Americans and when did they arrive.
apparantly a special about it is/was shown on PBS, and the video link is here.
my one caveat would be that maybe the later arrivals didn't wipe out the coastal immigrants but intermarried (something that is discussed in video two of the series).
number two mentions that this didn't just happen in Europe/the Levant but also in Africa.
rewrite the history books
and if you look hard enough, maybe you might find footprints in the sand:
WesternDigs article here.