Comparative Evolutionary Models and the “Australopith Radiations”

Abstract

This paper makes a case for the more formal use of evolutionary models in trying to understand human evolution. As the fossil record for hominin evolution has accumulated, and the level of diversity recognized has increased, we have moved to viewing the evolutionary history of the lineage as a series of adaptive radiations, rather than as a process of continuous, within lineage, change. The australopithecines would be seen to represent one such radiation, diversifying phylogenetically and expanding geographically. It is assumed that this is a response to a combination of the evolution of bipedalism and the expansion of more open habitats. Such interpretations have been largely inductive, and little attention has been paid to the way in which processes such as adaptive radiations and dispersals have been analyzed more widely in evolutionary biology. In this paper the australopithecine radiation is examined in the context of a number of models that have been developed to identify adaptive radiations. The results suggest that while there is some evidence for adaptational directionality to the group, in other ways australopithecine evolution falls short of the criteria for an adaptive radiation. As an alternative, australopithecine diversity is looked at in the context of dispersal models and the distribution in Africa. Finally, as it is clear that such model-based approaches are very sensitive to scale, the pattern of early hominin evolution is compared to two events at different scales—the evolution of modern humans, and the diversity of the chimpanzee clade.