2016-04-01 - 2016-09-30 | Research area: EvoDevo
The emergence of the genus Homo is a key moment in human evolution. From 2 Myr ago the Homo lineage is documented by an increasingly rich fossil record, but the earliest phase remains poorly understood and the ancestral taxon elusive. Recently the newly discovered species Australopithecus sediba from southern Africa has been proposed as the direct ancestor of the Homo lineage, in particular based on cranial features of the holotype. This type specimen MH1 is a juvenile, but the diagnostic morphology was assumed to be unaffected by late growth into adulthood. As part of my PhD project I developed a new geometric morphometric protocol that can cope with the cranial morphological variation ranging from infant humans to adult crested gorillas. This new protocol is not only based on a high-density coverage of landmarks but also allows overlapping lines and surfaces to be analyzed concomitantly. Based on this protocol we present a 3D geometric morphometric assessment of the MH1 cranium to assess the estimated adult form of the specimen, and its implications for the proposed ancestry of A. sediba to the genus Homo. Using an ontogenetic comparative database composed by 243 crania of hominins and great apes we show that MH1 shares an ontogenetic trajectory with A. africanus supporting its initial classification in that genus. Phenetic analysis show a strong association of MH1 and Sts 52. When the adult morphology of both fossils is predicted using extant hominoids as a model they become significantly more similar to, but still more gracile than, mature A.africanus (Sts 5 and Sts 71). When the same developmental simulation is applied to Taung the specimen fails to cluster with Sts 5 and Sts 71, becoming similar to the adult predictions of MH 1 and Sts 52. An important conclusion is that, by itself, cranial gracilization within an austrolopithecus ontogenetic trajectory does not lead to a Homo like morphology. In this frame the cranial morphology of MH1 suits better in the broader debate of specific diversity within the South Africa Australopithecus genus than to the issue of the emergence of the genus Homo. Together with the chronology for the appearance of the genus Homo our findings make unlikely that A.sediba is a direct ancestral to it and highlights the importance of taking late changes of growth in consideration before taxonomic assessments are made.