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Ontogenetic Evolution of the Human Chin

Kenisha Barretto
B.Sc. in Biotechnology
Thakur College of Science and Commerce

The theory of natural selection is the best supported theory with evidence from a wide range of scientific fields. From the earliest known hominid, such as Sahelanthropus tchadensis to the current Homo sapiens, we have learned to adapt and live. The mental protuberance commonly known as the human chin is an automorphic trait specific to modern humans. However, various forms of this protrusion have been identified in our various extinct ancestors. There is no concrete evidence as to the reason for which we have this protrusion.

According to the American Association of Anatomy, the changes in the developmental timing, spatial locations, or sizes of intramembranous or cartilaginous craniofacial growth centers could create species-specific facial morphology quite early, suggesting that morphological differences between species result from early (infant or possibly prenatal) ontogenetic processes. Traditional interpretations of the evolution of the facial skeleton within the hominin lineage stress the influence of three major factors: a reduction in canine sexual dimorphism (change in appearance apart from changes in the sexual organs), adaptations to a tougher diet, and later a combination of a dramatic increase in brain size, and adaptations to a less tough diet.

It is argued that articulate human speech is impossible without a lowered larynx and an expanded region above it, as there is no skeletal evidence of the humanoid vocal tract. Hence, the transformation of the Hominid dialect could be another factor of influence. However, it is impossible to assess linguistic competency by observing the insides of reassembled fossil cranium that are incomplete, battered, and distorted.

The early hominid skull is more similar to apes rather than the modern humans. Humans have big brains and a small jaw, whereas apes have a small brain and a broad jaw. Also, the canine teeth of apes are large and pointed as opposed to small and non-protruding teeth of humans. Regardless, the combination of improved tools and techniques along with the use of fire for cooking surely contributed to the reduction of the jaw.

The progressive integration of the vocal tract in humans and chimpanzees implement a strong point, that the various protrusions of the mental region detected in our extinct ancestors could have grown from a common developmental pathway in response to the constraint of space at the back of the vocal tract.

Morphological changes of chimps and humans
Similar shape changes in chimpanzees (a-b) and humans (d-e) simultaneously include: 1) the forward projection of the mental region; 2) the relative horizontal reduction of the tongue and that of the oral cavity; 3) the cranial base movement; 4) the forward positioning of the cervical column and the hyoid bone.

A study conducted by Michael Coquerelle, et al., concluded that the withdrawal of the face underneath the brain has reduced the width of the vocal tract. This space restriction is caused by both the backward positioning of the upper mid-face naturally occurring with the movement of the cranial base, and the forward positioning of the cervical column and hyoid bone due to the development of upright posture. By distinction, the size of the tongue has remained the same, indicating rearrangement at the back of the vocal tract. This indicates that the mental prominence of modern humans is a by-product via the reshaping of the tongue, which is coping with the retraction of the face and the development of upright posture.

The results finally showed that Neanderthal and Aterpuerca who have similar or somewhat close protrusions (chins) like humans have achieved a similar space accomplishment of the vocal tract. This morphological feature appears during early fetal life, but the craniofacial context and factors linked with its development remain unclear. During early postnatal life, this feature becomes smoother and thicker as the mental region project forwards. This suggests that the inverted T- relief (chin) may be associated with the same factors as those associated with the prominence; however, this needs further exploration.

Reference (Mar-21-A5)


Author biography: Kenisha Barretto is a student currently pursuing a B.Sc. in Biotechnology. She aspires to have career in Biotechnology and its related fields. She aims to make innovations to help solve the ever-increasing problems of the world. Her goals are to someday work for the top industries and institutes of the world.

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