Martha Foley and Dr. Curt Stager continue a discussion of animal extinction that began with a look at the book, "The Ghosts of Evolution," by Connie Barlow.Before the last Ice Age, North America was more like Africa is today and contained many species of megafauna including three kinds of mammoths, giant ground sloths, native horses, camels, and rhinos.All of them disappeared very shortly after Neolithic hunters crossed the land bridge from Asia.