Characterizing the Evolutionary Path(s) to Early <i>Homo</i>

<div><p>Numerous studies suggest that the transition from <i>Australopithecus</i> to <i>Homo</i> was characterized by evolutionary innovation, resulting in the emergence and coexistence of a diversity of forms. However, the evolutionary processes necessary to drive such a transition have not been examined. Here, we apply statistical tests developed from quantitative evolutionary theory to assess whether morphological differences among late australopith and early <i>Homo</i> species in Africa have been shaped by natural selection. Where selection is demonstrated, we identify aspects of morphology that were most likely under selective pressure, and determine the nature (type, rate) of that selection. Results demonstrate that selection must be invoked to explain an <i>Au. africanus</i>—<i>Au. sediba</i>—<i>Homo</i> transition, while transitions from late australopiths to various early <i>Homo</i> species that exclude <i>Au. sediba</i> can be achieved through drift alone. Rate tests indicate that selection is largely directional, acting to rapidly differentiate these taxa. Reconstructions of patterns of directional selection needed to drive the <i>Au. africanus</i>—<i>Au. sediba</i>—<i>Homo</i> transition suggest that selection would have affected all regions of the skull. These results may indicate that an evolutionary path to <i>Homo</i> without <i>Au. sediba</i> is the simpler path and/or provide evidence that this pathway involved more reliance on cultural adaptations to cope with environmental change.</p>