Caratterizzazione morfo-dinamica della regione occipitale del cranio nel genere Homo: Una prospettiva evo-devo

The PhD project is based on 3D geometric morphometrics analysis (GMM) of the rear portion of the human cranium (the area surrounding the posterior fossa) in fossil and recent samples. The aim is not simply to quantify the differences in occipital morphology between human species and lineages, but also to relate these differences to the phenomenon of encephalization, which should be regarded as the primary determinant for the variability in cranial morphology within the genus Homo, from both the ontogenetic and evolutionary perspectives. With the emergence of the genus Homo at the beginning of the Early Pleistocene, the non-allometric increase of cerebral volumes (encephalization) – and the subsequent development of the cranium along differential growth vectors (mainly antero-posterior directed) – has led to disturbance of the postural equilibrium between the skull and the spine that occurred in the Late Miocene with the transition to bipedalism and its maintenance among the australopithecines. This disturbance resulted in functional responses, both direct and indirect, in terms of increased instability in the patterns of cranial ossification (due to differential brain growth, highlighted by the presence of ontogenetic stress indicators such as sutural ossicles on the rear braincase) and in the increase and rearrangement of the nuchal muscles that supports and rotates the skull on the spine. The increase in muscle masses has lead in many fossil human species (e.g. Homo erectus , Neanderthals) to the occurrence of large insertions and associated bony structures such as tori, ridges and pits. Only with the emergence of our species, in consequence of the achievement of a more globular cranial shape and moving of the nuchal muscles forward and beneath the cerebral and cerebellar volumes, a new and more stable head balance is regained. Within this evolutionary and developmental melieux, the occurrence and distribution of cranial discrete traits known as hypostotic features (interpreted as an expression of "ontogenetic stress") versus hyperostotic feature, are evaluated in various hominin OTUs by multivatiate statistics. Different patterns are observed in the comparison between Middle Pleistocene samples from Africa, Asia and Europe. Using 2block partial least-square analysis, the non-metric variables (discrete traits) are compared to changes in morphology of the cranial vault recorded by 3D geometric morphometrics. These differences appear to be consistent with geographically-distinct trajectories of encephalization. In this framework, the peculiar (i.e., globular) cranial morphology of H. sapiens emerges from the fossil record of late H. heidelbergensis in Africa, which is characterized by a pattern of "ontogenetic stress" affecting the mid-lateral regions of the cranial vault (in relationship with the expansion, elevation and bending of the parietal lobes of the brain). This suggests a sharp structural reorganization in H. sapiens, which minimises the "ontogenetic stress" and allows to recover a morpho-functional balance, combining high degrees of encephalization with a radial distribution of expansion forces that act on the cranial vault. By contrast, in examining the transition in Europe from H. heidelbergensis to H. neanderthalensis, the analysis carried out in this thesis supported the notion that the increased encephalization – involving first the occipital region, and then the biparietal vault – is associated with the maintenance of a plesiomorphic architecture in the cranial (and encephalic) morphology, with increased instability in growth and osteogenetic processes, as shown by the extreme levels of hypostosis recorded among the Neanderthals. The influence of the muscular loading forces acting on the external surface of the squama of the occipital (on the nuchal scale in particular) considered as the most powerful external “constrains” in relationship with the occipital interspecific shape variability, are also considered by finite element analysis (FEA).