Abstract
The type specimen of Australopithecus sediba (MH1) is a late juvenile, prompting some commentators to suggest that had it lived to adulthood its morphology would have changed sufficiently so as to render hypotheses regarding its phylogenetic relations suspect. Considering the potentially critical position of this species with regard to the origins of the genus Homo, a deeper understanding of this change is especially vital. As an empirical response to this critique, a developmental simulation of the MH1 cranium was carried out using geometric morphometric techniques to extrapolate adult morphology using extant male and female chimpanzees, gorillas and humans by modelling remaining development. Multivariate comparisons of the simulated adult A. sediba crania with other early hominin taxa indicate that subsequent cranial development primarily reflects development of secondary sexual characteristics and would not likely be substantial enough to alter suggested morphological affinities of A. sediba. This study also illustrates the importance of separating developmental vectors by sex when estimating ontogenetic change. Results of the ontogenetic projections concur with those from mandible morphology, and jointly affirm the taxonomic validity of A. sediba.
Highlights
Only a single, relatively complete cranium of Australopithecus sediba has been recovered from the Malapa fossil site, belonging to the type specimen MH1.1 Dating to 1.977±0.002 Ma,[2] the Malapa hominins exhibit a unique, mosaic morphology, possessing features that align them with both the genus Homo as well as other species of australopith[1]
As the third principal component accounts for a substantial amount of the total variation (15.2%) in the Principal component analysis (PCA) using only fossils, we examined the distribution of fossil taxa along this axis
In other words, had MH1 lived to adulthood, its craniofacial morphology would look similar enough to its present, juvenile cranial morphology that we can reliably differentiate the taxon using the cranium of this particular specimen
Summary
Only a single, relatively complete cranium of Australopithecus sediba has been recovered from the Malapa fossil site, belonging to the type specimen MH1.1 Dating to 1.977±0.002 Ma,[2] the Malapa hominins exhibit a unique, mosaic morphology, possessing features that align them with both the genus Homo as well as other species of australopith[1] Based on this intermediate morphology, Berger et al.[1] suggested a possible ancestor– descendant relationship between A. sediba and the genus Homo, with the possibility of A. sediba representing the direct ancestor to H. erectus, or otherwise a close sister group to that ancestor. Kimbel[8] criticised the use of the juvenile mandible in multivariate statistical comparisons with other species, based on the potential of continued growth and development for impacting linear measurements.[9]
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