The Superciliary arcs Are a bony crest of the frontal bone of the skull located above the receptacles of the eyes of all the primates. The human eyebrows are located on its lower margin.
In some cases they are crossed by a foramen (foramen): the foramen superciliar. Through the foramen, an arteriole or supraciliar artery is usually passed. This is considered an"inconstant anatomical accident"or a mutation that not all people have. In addition, this arteriole does not fulfill any special function.
Normally, in humans, Superciliary arcs protect each eye . In other primates, they are not arches, but the bone is continuous and not arched. The arches are separated by a shallow groove.
They are usually more prominent in men than in women, and vary in different ethnic groups. Differences between ethnic groups are explained as atavism or sexual diformism. In biology, atavism or regression is caused by a gene that was inactive at some point in the phylogenetic history but which manifests itself in its descendants.
Explanatory theories of the superciliary arcs
There are different theories that explain the development of this component of the frontal bone. The models that have been developed Superciliary arcs Have made it possible to better explain the disparate development in representatives of different ethnic groups at present of this bone. There are 2 points of view:
Spatial model
It has been proposed that the growth of this bone is related to the facial size, being the orbital development, that is of the eyes and the ocular position, secondary factors.
The size of this bone could be related to the development of the face and the neurocranium. The neurocranium is the covering of the encephalon, the cranial meninges and their adjacent membranous covers. This model is called spatial.
Biomechanical Theory
The presence of arcs is a reflection of the relationship between the orbit and the brain. In other words, during the development of the neurocranium, this superimposes the orbit, which does not allow the arcs to develop.
As the neurocranium grows, the orbits begin to move contrary to the brain. Arcs are a result of the separation of the orbit and the brain.
This last bio-mechanical theory proposes that the development of arcs is the direct product of the differential tension of mastication. Chewing is a digestive function that meets the molars and tongue. Their findings indicate that the arches disperse the tension caused by the force generated during mastication.
Functions in early hominids
The arches reinforce the weakest bones of the face in the same way that the chin reinforces the comparatively thin jaws. This was necessary for the earliest hominids because of the tension exerted on their skulls by the powerful chewing apparatus they had. To compare, one only has to look at the dentures of a Neanderthal and compare it with that of Homo Sapiens.
The arches are one of the last traces that were lost during the process of evolution towards the human being and in any case continue to appear thanks to the atavism. The size of the superciliary arches varies in the different primates, alive or fossils.
The closest living relatives of the human being are apes, which retain relatively pronounced superciliary arches. These are also called frontal bulls.
Research
Research on fossil remains of the homo has shown that the arches were reduced as the cranial vault grew. Thanks to evolution, the front of the brain changed shape, becoming flatter, while the eyes remained in front of the brain and the forehead turned vertical.
Caroline Wilkenson is a British forensic anthropologist, who works at Liverpool University John Moores. He specializes in facial reconstruction and has developed several investigations that touch on the subject of the superciliary arches. In her research the anthropologist determined the following:
Australoids have the largest forehead arches, similar in size to those of the Caucasoid, that is, the Caucasus man who has medial to large superciliary arches.
The Caucasoids occupy second place in superciliary arcs. Your forehead is usually tilted when the forehead arches are prominent. It has been determined that the Ainu ethnic group of Japan have deep eyes and large and prominent arches of the forehead.
The superciliary arches are divided into central and distal. In today's human beings, often only the central sections are preserved (if preserved at all). This contrasts with the pre-modern humans, who had steep and uninterrupted bows.
When studying fossils, anthropologists have proposed that the superciliary arches can be used to diagnose the sex of the fossil, since in men this bone was always more prominent. Other studies indicate that as the superciliary arches were reduced, wounds, bruises and spills were closer to the eyes and further away from the brain .
Among the changes in the skull that homo suffered to become homo sapiens are: increased brain volume, brain convolutions, complexity and neocortex (brain cells), sagittal crest disappearance (ie chewing muscles are Were progressively weakened by the change in diet from meat to vegetables and grains), disappearance of the superciliary arcs or torus supraorbitae and progressive recession of the face.
It is evident that all these processes indicate that one of two theories, the bio-mechanical theory and the spatial theory, is correct. In addition the dentition changed from 36 teeth to 32, the palate acquires a parabolic form, the canines are reducing their size and disappear diastemas or the spaces between the teeth.
Despite advances in the investigation of the evolution of the superciliary arches, it has not been possible to determine the period in which these bones became obsolete. These are present in all ancestors of homo sapiens to a greater or lesser extent.
References
- Russell, MD (1985). "The supraorbital torus:"A most remarkable peculiarity."". Current Anthropology. 26: 337.
- Wilkinson, Caroline. Forensic Facial Reconstruction. Cambridge University Press. 2004.