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THE CRANIAL'S BASE OF THE HOMININS _x001F_EVOLUTION

The base area of the cranium, for instance, the basioccipital, temporal segments and the sphenoid communicates significant awareness involving morphological variations within the hominin clade. The outlines of the basicranial morphology exist in a couple of hominin species assessments and diagnoses. The employment of the parsimony examination of documented cranial and dental data is vital in the prediction of the cranial base morphology that is expected in the confirmation of the hypothetical final collective ancestor of the PanHomo group. Multiple instances of recorded data also provide predictions regarding the primitive state of the cranial base for each hominin clade thus providing an brilliant picture of the evolution of the cranial base in various agencies of notable subclades found in the hominin clade. This research outlines the significance cranial base morphology in the evolution of the various hominids before after the advent of the genus Homo.

Introduction
The base segments of the cranium, for example, the basioccipital parts, the sphenoid region, and the temporal bones have been established to undertake major morphological alterations in the hominin lineage (Profico et al. 2017). Research has established distinctions in the basicranium notable in multiple hominin species examinations. The multiple discoveries of various craniums present subtle distinctions of the cranial base amongst various representations of hominin fossil records. Vital parts of the cranium such as the petrous parts situated in the temporal bone are usually the densest sections of the cranium and avail significant details regarding the hominin clade. Petrous part is more distinct amongst the ancient ancestor than the later relatives in the Homo clade. Unfortunately, the cranial bases amongst some of the Homo erectus cranial specimens discovered in the Indonesian regions such as Java are broken. The damage has been linked to anthropogenic activities, for instance, abstraction of the brain of the members who had died (Lieberman, Pearson & Mowbray, 2000). The cranial base is a vital segment where several purposes converge. Such functions include breathing, food consumption, and ingestion, posture position, bodily balance and many other roles. Due to the endless list of functions assigned to the cranial base, many anthropologists have harbored the belief that the cranial base is vastly incorporated structure whose modifications alters the various roles of the hominin clade (Profico et al. 2017).
Exterior morphology of the cranial base
Research on the cranial base has been established in two main segments; the midline morphology otherwise called the sagittal and the study of the cranial base as an entire unit. Several morphometric studies on the sagittal concentrate on the physical features such as the comparative lengths and angular associations of the constituents of the midline of the cranial base part (Lieberman, Pearson & Mowbray, 2000). On the contrary, an abundance of studies on the cranial base as a unit has been focused on the gross morphology which is not only viewed from the endocranial exterior, but also from underneath in the segments such as norma basilaris. Such studies mainly employed linear variables in comparing the antero-posterior parts of the parasagittal elements of the cranial base, the lengths between two-pronged bodies, for instance, the vascular in comparison of the relative breadths of the parts. They also entailed comparison of the components, angular factors to match with the alignment of the petrous bones and the tympanic portions of the temporal bones (Profico et al. 2017).
Historically, many palaeoanthropologists had considerably rejected it and chose to examine other cranial parts such as the face or even the cranial vault. The approach has since shifted especially after the introduction of better forms of investigating the cranial base. Such discoveries were the introduction of the non-destructive forms of gathering information from the parts of the cranial base, for instance, the bony labyrinth. Assessments by the researchers have portrayed the small differences amongst the closely related primates, for instance, difference in the semicircular canals cavities portends disparities in the posture levels and the locomotor model of the primates. Application of the subsequent technologies such as computed tomography (CT) and another recent innovation such as micro-CT in the extraction of vital information regarding the bony labyrinth situated in the intact petrous bones has ignited the desire to study the cranial base.
Research methodology
A study was conducted whereby fourteen fossil hominin taxa were entailed in a cladistics assessments (Profico et al. 2017). Some of the included fossil taxa include Ardipithecus ramidus, Kenyanthropus platyops, Australopithecus anamensis and many other to fill a list of fourteen taxa. The study left out three notable taxa such as Ardipithecus kadabba and two others due to limit availability of cranial base information. The study also incorporated living samples from the hominoid group like the Homo sapiens and two additional distantly related outgroups, for instance Colobus guereza to assist in determination of the features of the primates (Nevell & Wood, 2008).
Findings and discussion
The findings from the research exposed several information regarding cranial base. The research focused heavily in distinguishing parts of the cranial base such as temporal bone, petrous, mastoid and temporomandibular joint, tympanic, squamous temporal bone, external cranial base flexion and even occipital bone (Profico et al. 2017). The findings regarding cranial base morphology as exposed in the norma basilaris has been apportioned into studies concerned with identification of comparatively discrete, small-scale, taxonomically characteristic morphological structures and also assessments that were focused on seeking large scaled based variations in magnitude of the parts of the cranial base. The study showed more significance in undertaking larger scale studies aimed at both the relative and absolute impacts of the sphenoid and the temporal bones on the breadths of the cranial base and also highlighting the alignment of the tympanic and petrous parts especially on the temporal bone (Nevell & Wood, 2008).
New findings have been established regarding the hominoids, for instance, there is extensive agreement amongst many molecular biologists confirming that the Pan�Homo deviations happened around 4 and 8 Million years ago. Multiple resources are currently entailed in investigating the fossiliferous deposits over the time to widen the initial hominin hypodigm. Due to the scantiness of the fossil-based evidence concerning a great, a majority of the pre-1.5 Million-year-old taxa documented in the recognized in speciose examinations of the first hominin created the phylogenetic associations which have been are still believed to be provisional. The current existent fossil evidence results have however assisted in predicting the anticipated cranial base morphology of the last common ancestor (LCA) for the e Pan-Homo clade. Such prediction have been successfully contrasted with certain characters amongst the modern humans and other similarly contrasted with qualities existent amongst the chimpanzees (Lieberman, Pearson & Mowbray, 2000).
Such studies have also contributed in highlighting around eight possible cranial base synapomorphies. Synapomorphies are elementary variations observed within the living humanoids as contrasted to the predictions from the experiments. The findings noted two synapomorphies as the posterior skull length and comparatively more forwardly located foramen magnum in togetherness with enhanced ambiguity in multiple other features (Nevell & Wood, 2008).
Evolutionary finding in the Paranthropus subclade
The study established that while there are substantial linkages between P. aethiopicus and (P. robustus, P. boisei) after analysis of total evidence, the cranial base features do not avail sufficient evidence to back the association amongst the taxa. Paranthropus cranial base framework showed a homoplasy (Lieberman, Pearson & Mowbray, 2000). A homoplasy is an attribute feature shared by a group of species but does not exist in their common forefather. The research also elaborated typical cranial base features that are synapomorphic in Paranthropus group and did not portray signs of homoplasy especially founded on the cranial and dental outline set. For instance, the lateral increased length of the mastoid progression in relation to the supramastoid crest which is equally shared among the members of the Paranthropus. Another feature shared by all the Paranthropus taxa is the horizontally placed peripheral auditory meatus and overlapping of the parietal bone and occipital bone especially at the asterion (Profico et al. 2017). There was the establishment of nearly 14 cranial base features that suggests the existence of parallel evolution between Paranthropus and Homo groups.
Evolution process in the Homo subclade
Several features bond the genus Homo especially based on the coronal alignment of the petrous bone, the characteristics of deep and thin posterior digastric notch and even abridged pneumatization of the temporal squama. Other features also increasingly flexed cranial base, anterior disposition of the foramen magnum, and even summarized enclosure of longus capitus. The H. habilis H. ergaster, H. sapiens share a reduced distance notable between the TMJ and the occlusal plane as one of the major feature linking them. The analysis of the cranial base has thereafter appeared in many of the subsequent analysis. For instance, the study of calvaria that was discovered in 2000 in Ileret, in Kenya. The study established that H. habilis showed its temporomandibular joint (TMJ) have maintained a primitive state of having a mediolaterally moderately broad (Lieberman, Pearson & Mowbray, 2000). The primate also had both the tympanic and the petrous are fairly sagittally positioned thus creating notable alignment. In comparison, the TMJ of the H. erectus is considerably slim mediolaterally, and the tympanic is comparatively coronally aligned while the petrous is vehemently sagittally placed. The study indicates that the cranial base is considerably well distinct and represented multiple collections hominins obtained from Dmanisi.
Conclusion
In brief, cranial base morphology has contributed massively to the hominin systematics as notable from the attempts of the initial investigation into the hominin records. Based on Raymond�s assignment of Taung to Au. africanus back in 1925, he founded his reasons on the placement and alignment of the foramen magnum. The skull framework has constantly contributed in the classification of the hominin into their appropriate genus and species. The prominence of cranial base morphology is majorly connected to its taphonomic in combination to its morphological features. Petrous bones are characteristically dense, properly safeguarded and carnivores rarely crush them. Multiple fossils collected within a period of 20 years expose a huge bulk of the cranial base which has been essential in augmenting the list of the temporal assortment of hominin fossils.

Bibliography
Profico, A., Piras, P., Buzi, C., Di Vincenzo, F., Lattarini, F., Melchionna, M., ... & Manzi, G. (2017). The evolution of cranial base and face in Cercopithecoidea and Hominoidea: Modularity and morphological integration. American Journal of Primatology.
Nevell, L., & Wood, B. (2008). Cranial base evolution within the hominin clade. Journal of anatomy, 212(4), 455-468.
Lieberman, D. E., Pearson, O. M., & Mowbray, K. M. (2000). Basicranial influence on overall cranial shape. Journal of Human Evolution, 38(2), 291-315.

July 24, 2021
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Science

Subcategory:

Biology

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