Olaf Stapledon’s Last and First Men http://www.amazon.com/exec/obidos/ASIN/0486219623/donhosek presents one possible future history on this kind of time scale. It is very much a product of the 1930s though, with rather disturbing racism, sexism and the kind of casual anti-Semitism that the rise of Hitler made impossible. Once he gets away from contemporary times, though, that tends to fade away and it’s very much a memorable book.