.The fourth episode takes place in the Great Rift Valley in northeastern Africa of the late Pliocene. The climate has changed, and now great grasslands have replaced trees. The episode focuses around a tribe of small hominids known as Australopithecus, one of the first apes able to walk upright and a close ancestor to humans. The Australopithecus has evolved to walk upright so as to better maneuver the plains as well as the climb the trees. However, it notes that although the Australopithecus looks human, it still only has the mind the size of a chimpanzee's. Some of the topics explored in the episode are the close social bonds among the tribe, how they use grooming as a means of communication, and how they work together to forage for food and to defend one another from attacks from such animals as an angry male Deinotherium, an ancestor of the modern elephant which they have to run from to avoid being crushed, and the feline predator Dinofelis. It touches upon how competing tribes of Australopithecus war among one another, although most of fighting is for show. It also explains the hierarchy in the tribe among the males (who are much larger than the females) and tells a story of how the dominating male is eventually overcome by another male, who wins the right to feed first at a carrion and to mate with the females. Another story tells of a young Australopithecus (nicknamed Blue) who tries to fit into the tribe after he is orphaned.