The causes and conditions that lead to the expansion of knowledge are multiple, but sometimes there arises a singular and powerful force. For more than a half a century, Phillip Tobias has exerted such an influence on our understanding of human evolution. His influence has been profound not only from his prolific scholarship, but in his generous gift of inspiring a generation of students and colleagues. It is auspicious indeed that Taung and Tobias entered our world in the same year.
What more have we learned in the 80 years that followed? First and foremost is that Dart was right: Australopithecus africanus was on the uniquely human branch of the tree of life (despite its ape-sized brain), was bipedal, and that Africa is truly the cradle of humankind as abundantly shown by discovery teams led by Tobias, Clarke, Broom, Robinson, the Leakeys, Howell, Johanson, Kimbel, White, Brunet, Hill, Senut, Pickford, Asfaw, Haile-Selassie and many others.
In the rich literature on why our ancestors became bipedal appear advocates of single hypotheses and those who take a more ecumenical approach. The beginning place has to be an appreciation of historical constraints. Our last common ancestor with the animal world was constrained by the fact that it had a body of a hominoid without typical mammalian quadrupedal specialisations. Terrestrially, chimps walk quadrupedally and bipedally with equal energetic expenditure, which is about 50% more expensive than the average mammal. This fact is based on studies of two young chimps, but ongoing studies at University of California, Davis of the bioenergetics of three adult female and two male chimps confirm this finding.
Does our current published sample of early hominins document the intermediate steps between ape-like and human-like gaits? There is a rich series of craniodental transformations through time from ape-like to human-like, but so far the intermediate steps between ape and human gaits are undocumented. All of the candidates for earliest hominin show signs of upright walking. The head balance of Sahelanthropus, the human-like thighs of Orrorin, the dorsally canted toes of Ardipithecus kadabba suggest the adoption of bipedality came before 6 to 5 million years ago. Between 4 and 2 million years there is abundant fossil evidence of bipedally adapted hominins, but analyses of this material often reveal surprisingly different complexes of locomotor traits that appear to indicate considerable diversity of adaptations.