When presented with persuasive rhetoric, new theories of human origins can seem to be the coming consensus, as with those Stanley proposes. A major problem he and fellow paleontologists grapple with is the connection between Australopithecus (the "Lucy" fossil) and Homo erectus (the "Turkana Boy" fossil). The key, Stanley argues, is the movement of land masses millions of years ago between the Atlantic and Pacific Oceans that triggered an ice age that fragmented the African forests inhabited by Australopithecus, from an isolated population of which, through the accelerated processes of "punctuated equilibrium," emerged the Homo genus. In addition to that scientifically updated Great Chain of Being, Stanley dwells on pressures likely to have favored a change in Lucy's kin after anatomical stagnation for a million years. These he groups under inferences about carnivores and child rearing, which he headlines as the "terrestrial imperative" --what made it safe for hominids to descend from the trees. Read Less