Agriculture in the Old World is a relatively recent invention, sometime around 8000 B.C.
There is very little evidence of human habitation of the Americas before 12,000 B.C.
The Ice Sheets retreated enough to allow crossing of the Bering Straits only shortly before that.
Megafauna still existed around 11,000 B.C.
While all of these assertions are constantly being revised I believe they are still considered valid. If you think otherwise, point me to the evidence.
I tend to agree with the newer models that indicate that the Western Hemisphere has been a very interesting place for a very long time. Try poking around Ernest's Gods, Graves, & Glyphs list. Some of it is a stretch, a lot of it is serious. It is one of the better easily accessible collections and it's free. If you want more, ask blam.
Look up some of these sites: Monte Verde, Topper, Cactus Hill and of course Calico. I will post a link that may help.
Browse through this link to see a number of old sites.