Form and Function: Primate Locomotion

Khalil Betz-Heinemann – DI308

Note – images have not been included

Question 1.

How does the morphology differ in the four specimens and how do those differences reflect their different locomotor repertoires?

The four specimens being looked at are Mandrillus Sphinx (Mandrill), Hylobates Lar (Gibbon), Pan Troglodytes (Chimpanzee) and Homo Sapien (Human). The morphology being looked at is located in the anterior shoulder, with focus on the gleno-humeral joint. It includes the three bones; Clavicle, Scapula and Humerus in each specimen.

Mandrillus Sphinx

In the Mandrill the scapula is positioned in an anterior to posterior sagital plane at an angle of about 90 degrees thus placing the thorax of the animal at 90 degrees to the front limb, leaving the Mandrill with limbs positioned for quadrupedal locomotion. The clavicle is the smallest in length, in comparison to the size of the other specimens giving it the closest set shoulders. In addition to this the scapula socket sits directly on top of the head of the humerus (Figure 1). This results in the posterior thorax of the animal being placed on top of the front limbs instead of between them, therefore when moving in quadrupedal locomotion most of the animal’s body weight is supported by the two humerus’, allowing the muscles to focus primarily with movement (rather than carrying body weight). When looking at the scapula closely it can also be seen that it displays numerous distinguishable ridges suggesting that muscles are closely attached, thickset and more restricted than in the other three species. This to be expected as the other three specimens locomotion and behaviour requires more joint mobility. Overall the skeletal morphology of the Mandrill suggests a strong compact shoulder that supports a quadrupedally positioned body and therefore most suited to its terrestrial/arboreal quadrupedal lifestyle (Figure 2) (Platt 2009: pg 43).

Hylobates Lar

In the Gibbon all three shoulder bones are more gracile than the other three specimens (Figure 3), while the humerus and clavicle are the longest in comparison to body size. This gives it long slender arms set wide apart. This skeletal morphology reflects its arboreal brachiating, as the more gracile bones are lighter and so its environment can support its weight in addition to it saving energy as it is lighter. Having longer clavicle bones sets its shoulders wide apart so that when swinging its arms above itself they do not infer with its head. Additionally its longer arms maximise its swing distance making it an efficient brachiate. The scapula socket is non-restrictive, with the humerus head having a full 360 degrees to rotate allowing for maximum shoulder mobility. The high mobility, light frame and positioning of the shoulder bones as described all contribute to the Gibbon being highly adapted to an almost completely arboreal brachiating locomotive lifestyle (Fleagle 1992: Chp 2.8).

Pan Troglodytes

The Chimpanzee, a terrestrial quadruped, partially arboreal and bipedal, has a very robust humerus, scapula and clavicle. In comparison to the other specimens however, the clavicle is the most predominately curved (Figure 4). It is curved toward to the ground when standing quadrupedally creating a cradle for the anterior of its thorax to sit in. This is because its fore limbs are not directly below its thorax and therefore the clavicle is more important in supporting its thorax. Its scapula is positioned in a plane between that of the Mandrill and the Human, reflecting its body position in comparison to the angle of its arms, when moving quadrupedally. Because they knuckle walk this increases the distance from the ground of the anterior of their body in comparison to the Mandrill, so requiring the differential angle of the scapula as mentioned. The scapula also has a pronounced protrusion over the front of the humerus’s head so that it does not dislocate with the increased forward force placed on it during knuckle-walking, in comparison to the more downward force in the palm walking of Mandrills. The morphology of the shoulder bones provide both a partial ability for arboreal locomotion and allow mobility of the arms while in an upright position, along with being robust, giving the chimpanzee the increased choices of locomotion that it displays (Fleagle 1992: Chp 2.8).

Homo Sapien

Human’s shoulder morphology significantly reflects bipedal locomotion. When in an upright position the scapula faces toward the ground and across toward to the other shoulder, encasing part of the dorsal surface of the thorax (Figure 5). This balances the body weight, because in the Mandrill which is quadrupedal the scapula encases the side of the body. However if this were the case in humans it would place the weight of the shoulder disproportionately further out from a human’s centre of gravity, thus placing unnecessary stress on the clavicle. The clavicle in a human curves downward with gravity, as does the chimpanzees, however in a differing direction in comparison to the other shoulder bones, due to the different locomotion of both species. Though the curvature essentially fulfils the same purpose; that of supporting and holding up the anterior thorax. The scapula’s socket is non restrictive allowing the humerus’s ball to have an almost 360 degree rotational capacity. However it is slightly restricted by an upper protrusion on the scapula. This does not allow for the same arboreal mobility that the Gibbon enjoys, thus confirming human’s limited arboreal locomotive ability. Overall the human shoulder is designed to allow for a high mobility of the arms to interact with the environment, but the humerus is not as closely placed to the scapula as its support in carrying the bodies’ weight isn’t needed (Allen 2006: Chp 9).

Question 2.

Although both the chimpanzee and the Mandrill are walking on all fours, describe how their anatomy differs. What does this tell you about their evolutionary history?

The Mandrill and Chimpanzee are both arboreal and terrestrial. However the Mandrill is a quadruped terrestrially and arboreally, while the Chimpanzee is a knuckle-walking quadruped terrestrially and arm swinger arboreally. The shoulder morphology, as previously discussed, reflects this.

Digit morphology is also very sensitive to the natural selection of a primate’s locomotive demands. This is due to the fact that the feet and hands are always in contact with the environment a primate is moving through and so reflect their locomotion very specifically. Knuckle-walking means that the Chimpanzee can retain its long curved fingers for swinging arboreally, whereas the Mandrill has short thick fingers for its digitgrade locomotion (Platt 2009: pg 44). The longer curved fingers of the chimpanzee are more prehensile allowing them to be used for gripping branches more effectively or partaking in tool production and use.

The common ancestor for both these Anthropoids/Haplorhines is probably to be found among the Omomyoids (Allen 2006: Chp 9), however any of the possible ancestors that Science has suggested were not adapted to arboreal swinging or brachiating. They were probably evolved for arboreal quadrupedalism and leaping. Having curved fingers and being adapted for swinging is therefore a later adaptation, meaning the Mandrill’s palm walking quadrupedalism came first as inherited from the Anthropoid’s common ancestor. Fossil evidence shows that primate locomotion from the start, represents a shift from other vertebrates due to differential muscle recruitment associated with arboreal movement, even quadrupedally (Platt 2009: pg 55). This led to a whole new way of interacting with the environment and thus influenced both the behaviour and brains of the first primates.

However what distinguishes the Chimpanzee’s ancestral split from the Mandrill’s, is that it went further in adapting towards an arboreal lifestyle. This involved swinging or brachiating and so placed more emphasise for adaptation on the forelimbs, causing them to differentiate more. This can be seen in the difference between the two specimens’ anatomy and behaviour, where Chimpanzees’ fore limbs are very different from their rear limbs, in comparison to the Mandrills’ anatomy.

Whether it was due to resource availability, environmental change or possibly behavioural change as a result of a new locomotive strategy, the ancestor of the Great Apes (possibly excluding Orang-utans) returned to spend more time terrestrially. With their long curved fingers they cannot palm walk so they supports their weight by bunching their fists and knuckle walking. This allowed for capitalisation of both arboreal and terrestrial niches. Returning to the ground with such prehensile hands – previously used for brachiating or swinging – opened up the chimpanzees ancestors to increased manipulability of the environment and possibly as a consequence tool use.

In the case of the Mandrill, its anatomy suggests that its ancestor predates any non-quadrupedal arboreal locomotive behaviour, as it does not have curved fingers. In addition it has also returned to spend a good portion of its time terrestrially. As it has not gained long curved fingers it has remained palm walking, but due to more time now being spent on the ground quadrupedally, its digits have shortened and thickened with the impact of walking terrestrially. Its increased size in comparison to its ancestral Anthropoid, may be the cause of this return to the ground or an available outcome.

An additional anatomical change that supports the idea that the Mandrill’s ancestors were arboreal quadrupeds and the Chimpanzee’s became arboreal brachiates or swingers, is the presence of a tail. An arboreal quadruped requires a tail for balance and has been retained in the Mandrill, though seriously reduced now that it is more terrestrial. Whereas the Chimpanzee has no tail as it was lost when the ancestor of apes used brachiating or swinging, instead of leaping or jumping arboreally (Allen 2006: Chp 9).

Both species ancestral evolution has led them into the trees and back down to the ground with an additional increase in body size. However the key point in Chimpanzee ancestry was to acquire long curved fingers and then to return to an increasingly terrestrial lifestyle. This left the chimpanzee with a pair of highly prehensile hands and so as natural selection dictates, either use them or lose them. With a larger brain the opportunities that these hands provided for manipulating the environment has meant they have not been lost.

Question 3.

Describe, with aid of illustrations, three features of human skeletal anatomy that specifically relate to bipedalism.

Becoming a full time biped required the human skeleton to undergo many significant changes. However they are not as dramatic as they might first appear. Most primates can move bipedally, though not for long or efficiently. Therefore it is not a direct transition from quadrupedalism but a combination of differential muscle recruitment and changing behaviour over time (Platt 2009: pg 55).

The reasons for why human’s ancestors became bipedal in comparison to their ape cousins remaining quadrupedal, are numerous and not fully agreed upon. Though as evidence (Pontzer 2007) shows our bipedal locomotion is actually more energy efficient than bipedal and quadrupedal locomotion in other Great Apes (Figure 6). One human skeletal property that assists this efficient bipedalism in humans in comparison to other Great Apes is a double curved spine. The cervical curve and lumbar curve of the human spine both help maintain the correct centre of gravity needed for remaining upright while walking bipedally. The double curve places our head and torso in a vertical line above our feet (Figure 7). If the spine were not curved the weight of the head and thorax would be transferred directly down the spine to the posterior of the pelvis. This would move the centre of gravity to our dorsal side and make us more likely to fall backward. Additionally the double curved spine acts like a spring meaning that energy is regained by the body when walking as stepping down curves the spine and then its release provides a slight upward force, allowing the muscles to focus more on movement. Additionally if the spine weren’t curved and springy, then the direct transference of stress to the posterior pelvis through the vertebra when running bipedally, could fracture the pelvis and damage vertebrae. The surfaces of the vertebra in the human spine are also larger in comparison to other apes, spreading the weight of carrying the upper body and reducing stress on themselves by spreading it over a larger surface area.

The adaptations of knee joints in humans are also vital to being bipedal. The knee joint is the largest joint in the human body reflecting its vital role in human locomotion. It must maintain a balance between supporting the entire weight of the body while maintaining its flexibility.

The stability of the human knee joint is maintained during motion by the Medial Collateral joint and Lateral Collateral Ligament, which are positioned on either side of the knee. They hold the Femur and Tibia in alignment but also allow the knee to bend while held together by the Cruciate ligaments (Rajendran 1985). The surface area of the tibia and femur at their meeting point in the knee are also very large to spread the impact imposed when standing or moving bipedally. They therefore better support the increased amount of body weight placed on them in comparison to a quadruped, where it is spread over four joints.

One of the most important aspects of the bipedal knee in comparison to that of other Great Apes is that humans can extend their knees and lock their legs straight. Chimpanzees for example use muscle power to support their body weight when moving or standing bipedally, which is not as efficient. This locking mechanism works because the articular contours of the Femoral and Tibial condyles can fit together snugly to act as a hinge (Rajendran 1985). When the leg is fully straightened the quadriceps then cause this hinge joint then swivel inwards into a locked position (Figure 8). This means that we can stand upright without having to exert any force in our quadriceps and so waste no energy.

As a bipedal organism the human head in relation to the ground is different. When looking at ‘Figure 9’ we can see that the human head is placed directly on top of the spine at approximately 90 degrees to the ground, whereas the Chimpanzees’ head and spinal column are at around 45 degrees to the ground. This is reflected directly by the socket in the skull where the top of the spine fits; the Foramen magnum. It is positioned so that whatever an animals’ most common posture is, the centre of gravity remains in the centre of the body, maintaining balance and the animal can still see objects at its level. If the Foramen magnum in a human were placed further back like in a Chimpanzee (Figure 10) this would cause two problems. First the head would hang forward causing the centre of gravity to shift ventrally requiring a human to constantly strain their neck muscles to remain upright. This would not be energy efficient and cause constant muscle strain. Additionally the strain on the upper spine being constantly bent forward to fit into the skull would place stress on it. Otherwise the alternate option would be to continuously look upward to keep the spine straight, leaving human’s vulnerable to unseen attack and unable to look at the environment at their level.

As we can see (Figure 10) the Foramen magnum has shifted toward the base of the skull as primates have become more bipedal, to compensate for the problems it would otherwise cause and allow the skull to sit easily atop the spine without the need for large neck muscles to hold it up (Martin 1992: pg 78). All of these specific anatomical adaptations reflect human bipedalism specifically and without them it bipedalism would be less energy efficient, cause more stress on the anatomy and leave human’s less adapted to the bipedal environment.

Bibliography

Allen, J. S. Anton, A. C. & Stanford C. (2006). Biological Anthropology 2 edition. Pearson: New Jersey.

Fleagle, J. G. & Martin, R. (1992). The Cambridge Encyclopaedia of Human Evolution. Cambridge University Press: New York.

Platt, M. & Ghanzfar, A, (2009.) Primate Neuroethology. Oxford University Press U.S.A. : New York.

Pontzer, H. (2007). Chimpanzee locomotor energetics and the origins of human bipedalism. Proceedings of the National Academy of Sciences of the United States of America:vol. 104, no. 30 12265-12269.

Rajendran, K. (1985). Mechanism of locking at the knee joint. Journal of Anatomy: 143: 189-194.

http://www.thefreedictionary.com – Spelling of scientific terminology.

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