The evolution and diversification of Pleistocene Homo

Doctoral Thesis


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University of Cape Town

The morphologically diverse and geographically expansive Pleistocene Homo fossil record continues to be a topic of debate. Recent fossil discoveries have highlighted the diversity, as well as the difficulty of identifying evolutionary relationships, within our lineage. Previous studies have focused on making distinctions between inter-and intra-specific variation, with relatively poor understanding of population structure or the evolutionary forces which have shaped the complex phenotypic diversity within our genus. The focus of this thesis is to expand our current understanding of the cranial and mandibular variation within Pleistocene Homo by assessing patterns of variation within our lineage, exploring the morphological link between newly discovered Homo (and Homo-like) fossil species and existing Homo taxa, investigating the evolutionary processes acting during the emergence and diversification of our genus, and considering the possible ancestor-descendant relationships at the transition from australopith to Homo. Analyses are performed on three-dimensional scan data (landmarks and interlandmark distances) collected from specimens of Pleistocene Homo. To provide context, robust and gracile australopiths are incorporated due to their temporal and/or spatial correspondence. The core of this thesis consists of four manuscripts. A suite of quantitative methods are utilized in these manuscripts to evaluate the morphological diversity within this hypodigm. These include statistical tests developed from quantitative evolutionary theory, Mahalanobis' distances, Generalised Procrustes Analysis, and Euclidean Distance Matrix Analysis. The results of these analyses emphasise the importance of neutral evolutionary forces in shaping morphological diversity during the evolution of Homo. However, they also indicate that adaptive evolution /selection contributed to the differences in masticatory morphology within our genus, played a significant role in the dispersal of Homo out of Africa, and may have been an important driver of diversification during the transition from Australopithecus to Homo, as well as between Homo rudolfensis and other Homo groups. Importantly, they show that specimen affiliations, the effect of selection, as well as patterns of variability vary depending on the skeletal region analysed and extant model choice. Finally, they highlight the large amount of morphological variability during the emergence and evolution of our genus, supporting the coexistence of a diversity of forms, and the presence of multiple lineages. Taken together, these results reveal a complex evolutionary scenario shaping the diversity within Pleistocene Homo and their possible ancestors, challenging previous notions of a linear evolutionary trajectory. This conclusion emphasises the need for future research on hominin diversity to incorporate evolutionary process into models of evolutionary change.

Includes bibliographical references