This work was supported by the College of Arts and Sciences, Stony Brook University, the Wenner-Gren Foundation, the Ford Foundation and the University of the Witwatersrand. I am grateful to the late P.V. Tobias, the late A.R. Hughes, the late J.W. Kitching, R.J. Clarke, B. Zipfel, C.K. Brain, E.S. Vrba and S. Potze for the opportunity to work on the fossils from Sterkfontein and Makapansgat, and for many hours of fruitful discussion. I thank Mike Lague for providing me with a draft manuscript from his work on the distal humerus. I am grateful to Ashley Hammond, William L. Jungers and Roberto Macchiarelli for their very constructive comments on the manuscript.