Australopiths were not likely to have gone on diets but they certainly had them. Diets provide the energy and nutrients that enable survival and reproduction. Natural selection favors those individuals who are best at transforming their diets into surviving offspring, who will, in turn pass on the genes that made them and their parents successful. For this reason, diets fundamentally shape species’ biology, from how they distribute themselves in relation to resources to the contours of their molar cusps.

Because what a species eats is a keystone adaptation, scenarios of human evolution often begin with changes in diet. In the 1950s, the South African paleoanthropologist John T. Robinson (1923–2001), presented an argument about early human evolution (48, 49), now referred to as the “Dietary Hypothesis.” According to Robinson, the cranial and dental differences between South African hominins Paranthropus robustus and Australopithecus africanus pointed to a major dietary difference between them. Robinson reasoned that the large muscles of mastication (as inferred from bony anatomy) and expanded molars of Paranthropus robustus were adaptations to a tough vegetarian diet, including roots and bulbs, that required a great deal of chewing. His analysis of craniodental morphology in Australopithecus africanus, however, led him to conclude that this species was the founder of a new line of more omnivorous, tool-using hominins that could respond more flexibly to changing environmental conditions. This is the line, he argued, that eventually gave rise to us.

Raymond Dart's ideas (50) about early human evolution were likely to have influenced Robinson. Dart's study of South African cave sites led him to believe (incorrectly, it turned out [51]) that hominin hunting was responsible for the animal bones accumulated there. In his view, hunting transformed our peaceful ape-like ancestors into intelligent, weapon-wielding, blood-thirsty predators. More recent ideas about the importance of meat in human evolution de-emphasize the act of hunting as a prime mover of human evolution, focusing instead on the nutritive benefits of meat in facilitating human evolution. Humans and chimpanzees do differ in their meat-eating habits.