The book follows the evolution of mankind as it shapes surviving Purgatorius into tree dwellers, remoulds a group that drifts from Africa to a (then much closer) New World on a raft formed out of debris, and confronting others with a terrible dead end as ice clamps down on Antarctica.
The stream of DNA runs on elsewhere, where ape-like creatures in North Africa are forced out of their diminishing forests to come across grasslands where their distant descendants will later run joyously.