Just one point I would like to make regarding humans in North America. Earlier a poster (I think it was Lion King) suggested that the existence of evidence of a Mastodin killed by a spear proved that humans were in North America a lot earlier than has been thought. Well I'm afraid that simply isn't true.
Mastodons became extinct about 11,000 years ago (Mammoths about 4,500 years ago), but humans are known to have lived in the North American continent only for the last 25,000 to 40,000 years.
The DNA of a 24,000 year-old skeleton discovered neal Lake Baikal in southern Siberia shows not only a close genetic relationship to today's Native Americans, but it also shows that he descended from people who had lived in Europe and Western Asia. It is almost certain that the ancestors of all Amerind people migrated to the Americas across the Bering Land Bridge which exists from about 25,000 to about 11,500 years ago until it was flooded in the big melt at the end of the last ice-age.
You have to go back to 130,000 years ago before there is enough ice locked up to lower the sea levels sufficiently for another land bridge to form in Berengia. There is no evidence of humans living in Siberia or North America at that time.