Evolution of hominin cranial ontogeny

Craniofacial Evolution: From Australopithecus to Modern Man

ABSTRACT

Through the analysis and evaluation of the size and morphology of teeth and jaws, it is possible to differentiate animal species including man, allowing the theory of human evolution to be established. Hominin evolution is characterized by two main features, the transition to bipedality and the increase in brain size. This had an important impact on the structure and function of the hominine skull. The aim of this article is to retrace the evolutionary steps that led to Homo Sapiens, the direct ancestor of modern man. For the following paper, scientific articles from Medline (PubMed) and Google Scholar database were used from 1929 to 2018. Hominid evolution is characterized by 2 main features, the transition to bipedality and the increase in brain size. The fossils found show that both trends have had an important impact on the morphology and function of the skull of modern man. The basicranial flexion and growth have led to the facial modifications typical of modern man. The main changes concern the mandibular evolution with the appearance of the chin, the formation of the Spee curve due to mandibular arch growth, the formation of the temporomandibular joint as a hinge joint with a sliding socket as the condyle and a better function and maximum efficiency than its predecessors thanks to respect for the divine proportions of the face. Knowledge of ontogenesis allows us to know the factors underlying human form and function, analyze and evaluate them in the clinical context.

Evidence for independent brain and neurocranial reorganization during hominin evolution

Throughout hominin evolution, the brain of our ancestors underwent a 3-fold increase in size and substantial structural reorganization. However, inferring brain reorganization from fossil hominin neurocrania (=braincases) remains a challenge, above all because comparative data relating brain to neurocranial structures in living humans and great apes are still scarce. Here we use MRI and same-subject spatially aligned computed tomography (CT) and MRI data of humans and chimpanzees to quantify the spatial relationships between these structures, both within and across species. Results indicate that evolutionary changes in brain and neurocranial structures are largely independent of each other. The brains of humans compared to chimpanzees exhibit a characteristic posterior shift of the inferior pre- and postcentral gyri, indicative of reorganization of the frontal opercular region. Changes in human neurocranial structure do not reflect cortical reorganization. Rather, they reflect constraints related to increased encephalization and obligate bipedalism, resulting in relative enlargement of the parietal bones and anterior displacement of the cerebellar fossa. This implies that the relative position and size of neurocranial bones, as well as overall endocranial shape (e.g., globularity), should not be used to make inferences about evolutionary changes in the relative size or reorganization of adjacent cortical regions of fossil hominins.

Ontogeny of the maxilla in Neanderthals and their ancestors

Neanderthals had large and projecting (prognathic) faces similar to those of their putative ancestors from Sima de los Huesos (SH) and different from the retracted modern human face. When such differences arose during development and the morphogenetic modifications involved are unknown. We show that maxillary growth remodelling (bone formation and resorption) of the Devil's Tower (Gibraltar 2) and La Quina 18 Neanderthals and four SH hominins, all sub-adults, show extensive bone deposition, whereas in modern humans extensive osteoclastic bone resorption is found in the same regions. This morphogenetic difference is evident by ∼5 years of age. Modern human faces are distinct from those of the Neanderthal and SH fossils in part because their postnatal growth processes differ markedly. The growth remodelling identified in these fossil hominins is shared with Australopithecus and early Homo but not with modern humans suggesting that the modern human face is developmentally derived.

Unlike modern humans, Neanderthals had large and projecting faces. Here, the authors show that the maxilla of modern humans is distinct from those of the Neanderthal and Middle Pleistocene hominins from Sima de los Huesos because their growth processes differ markedly during the postnatal period.

The influence of life history and sexual dimorphism on entheseal changes in modern humans and African great apes

Entheseal changes have been widely studied with regard to their correlation to biomechanical stress and their usefulness for biocultural reconstructions. However, anthropological and medical studies have demonstrated the marked influence of both age and sex on the development of these features. Studies of entheseal changes are mostly aimed in testing functional hypotheses and are mostly focused on modern humans, with few data available for non-human primates. The lack of comparative studies on the effect of age and sex on entheseal changes represent a gap in our understanding of the evolutionary basis of both development and degeneration of the human musculoskeletal system. The aim of the present work is to compare age trajectories and patterns of sexual dimorphism in entheseal changes between modern humans and African great apes. To this end we analyzed 23 postcranial entheses in a human contemporary identified skeletal collection (N = 484) and compared the results with those obtained from the analysis of Pan (N = 50) and Gorilla (N = 47) skeletal specimens. Results highlight taxon-specific age trajectories possibly linked to differences in life history schedules and phyletic relationships. Robusticity trajectories separate Pan and modern humans from Gorilla, whereas enthesopathic patterns are unique in modern humans and possibly linked to their extended potential lifespan. Comparisons between sexes evidence a decreasing dimorphism in robusticity from Gorilla, to modern humans to Pan, which is likely linked to the role played by size, lifespan and physical activity on robusticity development. The present study confirms previous hypotheses on the possible relevance of EC in the study of life history, pointing moreover to their usefulness in evolutionary studies.

Reciprocal influence of masticatory apparatus, craniofacial structure and whole body homeostasis

There are evidences that the evolution into Homo erectus was partially induced by masticatory muscular dystrophy caused by a gene mutation, which in turn increased brain capacity and led to bipedalism. It is generally accepted that the morphology and function of mammalian skull are partially controlled by epigenetic mechanisms. Archeologic evidences support that the masticatory apparatus have influenced the mechanical stress distribution in hominin skull, and consequently changed craniofacial morphology and function. Even after evolution into H. erectus, alterations in food properties by civilization and cultural preferences have caused modification of human masticatory pattern and accordingly craniofacial structure. Since there are evidences that prehuman and human masticatory apparatus has been influenced the craniofacial and whole body morphology and function, this apparatus in turn might influence whole body homeostasis. Plausible reciprocal influencing mechanisms of the masticatory apparatus on the whole body homeostasis might be (1) direct mechanical influence on the craniofacial structure, (2) distortion of cerebrospinal fluid circulation, and/or (3) several neural/humoral routes. Based on these backgrounds, the hypothesis of the present study is that the morphology and function of masticatory apparatus influence the whole body homeostasis and these interactions are reciprocal. Therefore, human masticatory apparatus, at the present time, should be kept in its physiological status to maintain the whole body homeostasis. We recommend basic and clinical approaches to confirm this hypothesis.