Question

Write a broad description of how the cranial characteristics of the nasal bone morphology changed over time. Going from Sahelanthropus tchadensis to Homo sapiens.

Answer 1

The fundamental configuration of the nasal fossa is a highly conserved region. As one tracks the evolution from crocodilian to the mammalian skull, very little change can be observed in this region with its persistent and constant morphology seen in a great majority of mammalian groups.

In mammalian groups, the palate separates the nasal and oral cavities: Primary, secondary, and soft palate. The primary internal naris remains as a vestigial opening, i.e., the anterior palatine canal, which is topographically positioned between primary and secondary palate.

The primary nose fully opens behind a virtual, coronal plane through the anterior palatine canal, into both the respiratory and olfactory noses. Respiratory and olfactory noses are separated from each other by the transverse lamina, a thin, bony axial structure. Thus, the respiratory nose appears as two paramedian, long axial channels walled in on the inferior, lateral and medial sides by the reconfiguration of the primary palatal bones (vomer, palatine, pterygoid, and inferior turbinate bones) between the two maxillary bones and their palatine processes, and partitioned from the olfactory nose through the transverse lamina. The olfactory nose is completely embedded in the anterior cranial base, that is, the ethmoid bone.

The living primates (which include humans) are taxonomically classified in two suborders: Strepsirrhini and Haplorhini, the latter group includes our human ancestors[11]. Through the course of primate evolution, profound changes in the nasal fossa allow one to differentiate the haplorhines from strepsirrhines and all other mammals.

The word haplorhine means “dry nose” whereas strepsirrhine means “wet nose”. As a result, strepsirrhine primates exhibit wet noses similarly to dogs and cats. Haplorhine primates have a fused frontal bone suture as well as a fused mandibular symphysis. While both haplorhine and strepsirrhine primates have a complete orbital ring of bone, only the haplorhine exhibit a complete bony enclosure posteriorly separating the periorbital contents from the temporalis muscle as it traverses through the infratemporal fossa on its way to attaching the coronoid process of the mandible. The superior portion of the haplorhine nasal fossa is constricted by the orbital cones, which come about from the combined effect of orbital convergence and orbital frontation. Orbital convergence refers to the extent to which the orbital opening faces anteriorly improving stereoscopic vision that include the element of depth perception. Orbital frontation refers to what extent the superior and inferior margins are to the plane of the orbital opening so that more “frontated” organisms tend to view from the orbital socket more horizontally rather than superiorly. In most haplorhines, there is a considerable reduction of their snout length when compared to strepsirrhines. There is one more important anatomical distinction between these two subOrders of primates that resides within the nasal fossa and that is an absence of a transverse lamina in haplorhines, which translates in them not having a bony partition separating the respiratory and olfactory region within the nasal cavity proper.

Strepsirrhines, on the other hand, exhibit a partitioned respiratory and olfactory region within the nasal cavity by possessing a transverse lamina coupled by their complex ethmoturbinate system. But while the order of Primates is classified within the microsmatic group of mammals (this group shifted from an olfactory mode of existence to a visual reliance of subsistence), carnivores (classified as macrosmatic meaning their whole existence is based on smell) have the most complex and elaborate turbinate system in all of mammalia. Possession of four or more ethmoturbinates is found in strepsirrhines in contrast to the reported range of one to three pairs found in haplorhines. In addition, the haplorhine ethmoturbinates appear more reduced in size and are less intricately scrolled. Moreover, it appears that the tendency is toward a decrease and reconstitution of ethmoturbinate structural reorganization across the different haplorhine primate taxa.

While traditionally primates have been classified as microsmatic as mentioned above, other authors describe primates undergoing a reduction in their olfactory prowess. In 1970, Cartmill proposed the visual predation hypothesis of primate origins, which may help to explain this reduction. The visual predation hypothesis explains the adaptive significance of a variety of skeletal features that characterize modern primates as they transitioned to an arboreal mode of life. The change in orbital orientation enhanced stereoscopic vision, which was essential in the manually effective capture of food in a three dimensional setting of arboreal life but it may have initiated a cascade of morphological events to occur elsewhere in the craniofacial region particularly in the nasal area. As bony orbital modification and re-orientation occurred in these primates there was a concurrent reduction and re-arrangement of ethmoturbinate complexity to a more simple inferior-to-superior re-organization and, finally, a partial but not complete loss of olfactory mucosal area.

In humans, the evolutionary pattern of the nasal region as seen in the haplorhine non-human primates is continued in our species. The human nose appears as one organ with no morphological evidence distinguishing between the respiratory and olfactory noses. Studies of inspiratory airflow patterns in the nasal cavity, however, show the path of air flowing along the nasal floor and lower medial portion of the cavity (comparable to the inferior and middle meatus region) mimicking the respiratory pathway of an organism that possesses a transverse lamina[15].

The olfactory mucosa has been mapped to a small surface immediately inferior to the cribriform plate and to the upper portions of the nasal septum. The reduction of the olfactory mucosa seen in humans is strongly associated with the adaptive shift of a quadropedal locomotion gait to bipedality. Homo erectus is considered the first committed biped in our evolutionary history, which required the repositioning of the foramen magnum (a more anterior inferior placement) in order to balance the skull over the vertebral column and accommodate erect posture. These morphological changes had the effect of changing the orientation of the cribifrom plate from a vertical to a more horizontal manner. This resulted in a conversion of the mammalian olfactory nose into the human ethmoid complex, partitioned on each side in two clinically relevant compartments: The olfactory cleft medially and the ethmoid labyrinth laterally (in which the olfactory mucosa has disappeared).

Despite the evolutionary trend towards regression in the sense of smell, the embryologic development of the human nose is best understood when considering its olfactory origin, the subsequent respiratory reorganization, and the constriction of the ethmoid bone imposed by the orbital cones.

THE EVOLUTION DEVELOPMENT BIOLOGY (EVO-DEVO) OF THE HUMAN NOSE

The evo-devo approach, in comparison to the classical concept, explains why the nose is formed by a complex intricacy of different anatomical structures, and offers a rational explanation to this question[1] (the development of the paranasal sinuses, which occurs after birth, is not mentioned in this paper).

Phylogenically, the nose is exclusively an olfactory organ in fish, and the respiratory nose develops in crocodilians. Ontogenically, the growth and development of the olfactory nose precedes the development of the respiratory nose.

Development of the olfactory nose

Development of the olfactory capsule: The first embryologic evidences of the nose appear during the fourth week under the mask of two olfactory placodes on the frontal process of the embryo. Simultaneously, the corresponding wall of the brain undergoes rapid mitotic activity with a small bulge becoming visible and demarcating the olfactory region. Histologically, the future olfactory bulb and structures called the amygdaloid body and hippocampal formation are found in the forebrain.

Approximately, at five weeks Carnegie stage (CS) 15, (Carnegie staging is a method for dating embryos), the appearance of an olfactory pit is observed. This occurs with the invagination of the central portion of the placode. The invagination is in the direction of the adjacent brain where an olfactory elevation appears. Crest cells begin to gather together forming cords or filaments that travel within the mesenchyme.

At CS 16, the future olfactory bulb and the olfactory tubercle appear as elevations along the olfactory area of the cerebral hemispheres, or telencephalon. Between these two telencephalic elevated regions and the olfactory pit there is a significant and concentrated area of mesenchyme through which crest cells and olfactory epithelium must penetrate as they migrate to their destinations.

At CS 17 (approximately six weeks), the olfactory pit gives rise to the olfactory sac along with the development of the olfactory fin. The olfactory fin is an important structure as it separates the primitive nasal and oral cavities.

At CS 18 (6 ½ wk), the formation of the superficial fiber layer of the olfactory bulb originates from the fiber contribution of the olfactory nerve. There is an increase in the separation between the floor of the nasal (olfactory) sac and the oral cavity along with the appearance of a primitive olfactory septum between the olfactory sacs. The primordia of the olfactory centers, which represent the highly complex group of neurons, will be located near the juncture of the temporal and parietal lobes where they will continue to develop in the brain.

At CS 19, as vacuoles cultivate within the nasal fin they fuse with the nasal sac resulting in the sac’s enlargement. The nasal sac’s enlargement thins the nasal fin to a slender membrane before rupturing and forming the primitive choanae.

At CS 20 and 21, the various olfactory centers maintain their development within the brain while the olfactory epithelial fibers penetrate the sea of mesenchyme to establish connections.

At CS 22, the lateral walls of the olfactory sacs begin to fold over to form furrows and ridges, that increase the surface of olfactory epithelium.

At CS 23 (around the eighth week), the mesenchymal olfactory septum between the olfactory sacs has become cartilaginous and is now part of the olfactory capsule, which in itself is cartilaginous based. The olfactory capsule presents with its typical “M” shape morphology enveloping and separating both olfactory conduits from the brain.

During human foetus development from the ninth to tenth weeks, six major furrows develop along with their corresponding ridges or folds called ethmoturbinals (i.e., turbinates arising from the ethmoid) arising from the lateral aspect of the cartilaginous olfactory capsule. This cartilaginous precursor will undergo mineralization forming the ethmoid bone.