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Estudio evolutivo de la región cervical en hominoidea : morfología, integracíon e inferencia paleobiológica

Abstract : The main objective of this doctoral dissertation is to advance in the study of the morphology and evolution of the cervical vertebrae in Hominoidea. To reach this objective we have performed four studies using bony material (vertebrae and crania) belonging to both extant hominoid species, and also including the scarce fossil remains that from sub-tribe Hominina. Up until now, the scarce fossil material has limited the capacity to reconstruct the evolutionary history of the cervical spine. Thus, on top of classical morphological studies, using both traditional and geometric morphometrics, we have also applied statistical methods based on the principles of the theory of the evolution such as morphological integration, modularity and responses to selection, as a complementary approach to the fossil record.The results obtained in these four studies indicate that the morphology of the cervical vertebrae is related to postural and locomotor factors. These factors could have influenced the differences observed in the morphological and allometric patterns showed by H. sapiens regarding the rest of the hominoids. Also, the results from the analyses of integration and modularity indicate that there are differences in the pattern of integration showed by modern humans compared to that of the non-humans hominins (Pan and Gorilla). We consider that, despite some subtle differences, chimpanzees and gorillas could represent the ancestral patten for all the hominins, from which modern humans would have evolved. This break down from the ancestral pattern could be related to the selective pressures to bipedalism. From an evolutionary point view, these morphological changes in the human lineage did not occur in all the cervical vertebrae at the same time. Indeed, our results indicate there has been a relative stasis in the most caudal cervical vertebrae (i.e., C6-C7), whereas those located in a more cranial position evolved earlier and show a more derived morphology.Similarly, the results from the analyses regarding the relationship between the cranium and the cervical region, also reveal a distinct pattern of cranium-cervical integration for modern humans. The scarce Neandertal evidence seems to be roughly consistent with the distinct pattern showed by modern humans, which suggests a shared pattern for the (late) genus Homo. Finally, the last work, based on the study of the whole pre-sacral spine, shows that the thoracic vertebrae are internally the most integrated from the entire pre-sacral vertebral column. The high level of integration in the thoracic region decreases towards the most peripherally located vertebrae (i.e., C1-L5), where integration reaches its lowest values. The high integration in this region could have limited the ability of these vertebrae to respond to selection demands, probably caused by the functional constraints produced by their articulation with the thorax. In contrast, lumbar vertebrae are the most evolvable, and this could be due to functional factors related to the bipedal locomotion mode shown by modern humans, but also to developmental and genetic factors. We suggest that this evolutive pattern in the lumbar region in modern humans could also be present in all mammals. This hypothesis is based on the large variability shown by mammals in their locomotion modes, also in the high variation in the number of lumbar vertebrae, and in the ancestral body plane they shared due to the expression of the Hox genes.
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Mikel Arlegui. Estudio evolutivo de la región cervical en hominoidea : morfología, integracíon e inferencia paleobiológica. Biological anthropology. Université de Bordeaux; Universidad del País Vasco. Facultad de ciencias, 2019. Español. ⟨NNT : 2019BORD0280⟩. ⟨tel-02491673⟩

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