Energetics and the evolution of the genus Homo

The genus Homo as represented by Homo ergaster ( = early African Homo erectus) is characterized by a pattern of features that is more similar to modem humans than to the earlier and contemporaneous australopithecines and paranthropines. These features include larger relative brain sizes, larger bodies, slower rates of growth and maturation, dedicated bipedal locomotion, and smaller teeth and jaws. These features are phenotypic expressions of a very different lifestyle for the earliest members of the genus Homo. This paper considers the energetic correlates of the emergence of the genus Homo and suggests that there were three major changes in maintenance energy requirements. First, there was an absolute increase in energy requirements due to greater body size. Second, there was a shift in the relative requirements of the different organs, with increased energy diverted to brain metabolism at the expense of gut tissue, possibly mediated by changes in the proportion of weight comprised of fat. And third, there was a slower rate of childhood growth, offset by higher growth costs during infancy and adolescence. These changes, as well as energetic requirements of reproduction and bipedal locomotion, are considered in a discussion of one of the major transitions in adaptation in human evolution, the appearance of our own genus.