WHAT ARE BROW RIDGES? - ANATOMY AND PHYSIOLOGY - 2024

The brow ridges are a bony ridge of the frontal bone of the skull located above the eye sockets of all primates. The eyebrows of humans are located on their lower margin.

In some cases, they are traversed by a foramen (hole): the superciliary foramen. Through the foramen, a supraciliary arteriole or artery usually passes. This is considered a "fickle anatomical feature" or a mutation that not all people have. Furthermore, this arteriole does not fulfill any special function.

Normally, in humans, the brow ridges 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 sexual atavism or diformism.

In biology, atavism or regression is caused by a gene that became inactive at some point in phylogenetic history but manifests itself in its descendants.

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Explanatory theories of superciliary arches

There are different theories that explain the development of this component of the frontal bone. The models that have been developed of superciliary arches have made it possible to better explain the uneven development of this bone in representatives of different ethnic groups today. There are 2 points of view:

Spatial model

It has been proposed that the growth of this bone is related to facial size, with orbital development, that is, of the eyes and ocular position, being 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 brain, the cranial meninges and their adjacent membranous covers. This model is called spatial.

Bio-mechanical theory

The presence of the arches is a reflection of the relationship between the orbit and the brain. In other words, during the development of the neurocranium, it overlaps the orbit, which does not allow the arches to develop.

As the neurocranium grows, the orbits begin to move contrary to the brain. The arches are a result of the separation of the orbit and the brain.

This last bio-mechanical theory proposes that the development of the arches is the direct product of the differential stress of mastication. Chewing is a digestive function performed by the molars and the tongue. Their findings indicate that the arches disperse the tension caused by the force generated during chewing.

Functions in early hominids

The arches strengthen the weaker bones of the face in the same way that the chin strengthens the jaws that are comparatively thin.

This was necessary for the first hominids because of the stress that the powerful chewing devices they had on their skulls. To compare, you just have to look at the teeth of a Neanderthal and compare it with that of Homo Sapiens.

The arches are one of the last features that were lost during the evolution process towards the human being and in any case they continue to appear thanks to atavism. The size of the brow ridges varies in different primates, living or fossil.

The closest living relatives of humans are the apes, which retain relatively pronounced brow ridges. 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 part of the brain changed shape, becoming flatter, while the eyes were in front of the brain and the forehead became vertical.

Caroline Wilkenson is a British forensic anthropologist, working at Liverpool John Moores University. She specializes in facial reconstruction and has developed several investigations that address the subject of brow ridges. 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 Caucasian man who has medium to large brow arches.

The Caucasoids rank second in superciliary arches. Their forehead is generally sloping when the arches of the forehead are prominent. The Ainu people of Japan have been determined to have deep eyes and large, prominent arches of the forehead.

The superciliary arches are divided into central and distal. In modern humans, often only the central sections are preserved (if preserved at all). This is in contrast to pre-modern humans, who had steep, unbroken arches.

When studying fossils, anthropologists have proposed that brow ridges 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 brow ridges were reduced, the wounds, bruises and strokes were closer to the eyes and further away from the brain.

Among the changes in the skull that homo suffered until they became homo sapiens are: increase in brain volume, cerebral convolutions, complexity and neocortex (brain cells), disappearance of the sagittal crest (that is, chewing muscles are were progressively weakening thanks to the change of diet from meat to vegetables and grains), disappearance of the superciliary arches or torus supraorbitae and progressive recession of the face.

It is evident that all these processes indicate that one of the 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 shape, the canines gradually reduce their size and the diastemas or the spaces between the teeth disappear.

Despite advances in research on the evolution of the superciliary arches, the period in which these bones became obsolete has not been determined. These are present in all the ancestors of Homo sapiens to a greater or lesser extent.

References

1. Russell, MD (1985). "The supraorbital torus:" A most remarkable peculiarity. "". Current Anthropology. 26: 337.
2. Wilkenson, Caroline. Forensic Facial Reconstruction. Cambridge University Press. 2004.