Skeletal changes due to erect posture and its implications

Introduction:

In the history of primate evolution man evolved from non human primates. Gradual evolution from terrestrial quadrupedalism (pronograde) to semi-erect posture (clinograde) and finally to a fully erect posture (orthograde) with bipedal locomotion is the way how the evolution of locomotion from simians to man has been interpreted.

Man deserves its name when he could stand erect and walk bipedally. We stand with our back vertical, legs straight and our arms and hands hanging down freely on the side of the body.

On the other hand our primate relatives, even if they stand, they do it with a slopping back, bent legs and arms and hands hanging down in front of the body. The assumption of such an erect posture has consequences on our skeletal system. Anatomical modifications that took place on the skeletal system due to assumption of erect posture vary in magnitude in different parts of the skeleton.

Changes are observed to be more prominent on skull, vertebral column, pelvic girdle, femur and foot.

Changes in Pelvic Girdle due to assumption of Erect Posture:-

When an individual stands erect, his whole body weight is to be transmitted on the hind legs through pelvic girdle. Therefore the pelvis plays a great role in getting an erect posture and naturally had experienced a number of changes.

It is said that pelvis changed rather more than the vertebral column during the evolution of habitual erect posture and bipedal gait (Buettner Janunsch).

Some of the associated changes are:

1. Among the quadrupeds, the pelvis may be like a bony cylinder flattened from side to side, with its long axis parallel to spine, and the spine parallel to the ground.
   When an animal stands up with its spine more or less perpendicular to the ground, the girdle is now to transmit the weight of the entire body. As a result of this new stress the pelvis is now flattened downward and expanded laterally; it becomes lower and broader. Its anterior or superior end spreads out, making it funnel shaped with the small orifice directed downward.
2. It was observed that human pelvis had experienced a mosaic evolution. The three bones that make up the pelvis- ischium, ilium and pubis did not develop and change at the same rate.
   The ilium experienced change most and ischium the least. The ischium of man is almost the same as that of living pongidae (Washburn). The ilium shortened and broadened and bent back on ischium, and resembled fan shaped in appearance. Among the primates, the extreme shortness of human ilium puts man in position of lower pelvis relative to trunk height than any other primate (Hooten).
3. The excessive breadth of the sacaral part of the human ilium also helps in transmitting the whole body weight.
4. The ilio-sacral articular surface of the ilium is placed far above the acetabulum in other primates, as such there is no bony structure opposite to acetabulum. In case of man the lower part of the sacrum lies opposite to the acetabulum.
5. Proportionately, human pelvis exhibits much larger and thicker acetabular region than that of other primates. This also helps in transmitting and supporting the body weight femur whose head articulate with it.
6. The acetabulum lies in the middle of the ilium and ischium, unlike those of pongids among whom the acetabulum is located well below.
7. The sacrum is shorter and broader and the inlet of the pelvis forms a broad bottomless basin which supports pelvic viscera.

Changes in Femur due to assumption of Erect Posture:-

In an erect standing animal the entire weight of the trunk and upper extremity is to be transmitted from the pelvis to the femur which in turn supports the same. Therefore in order the bear the load of the weight, the femur has to experience a number of modifications from the femur of the quadrupedal or semi erect postured primates.
The following are the modifications are observed on the femur of man due to assumption of erect posture.

1. In contrast to other primates, human legs (lower extremity) are longer than arms, as such the femur is long and stout. To meet the pressure of the body weight. Infect femur is the longest, largest and strongest single bone of human body.
2. The condyles (lateral and medial) are comparatively larger than those of other nonhuman primates because of the fact that joints of the leg need to have large and stout structures for supporting the body weight.
3. The ridges for muscle attachments in the femur are much more sharply defined and differentiated than other non-human primates. To be more particular, the linea aspera � a prominent rough ridge which runs along the length of the diaphysis on the posterior surface is distinctly present. It is the ridge where the extensor muscles responsible for erect posture and bipedal gait are attached to.
4. The femur is angled outward from the knee rather than standing upright as observed among the great apes. This carrying angle ensures that the knee is brought well under the body and closer to the line of action of body weight.
5. The callo-diaphysial angle i.e. neck-shaft angle is larger in man compared to apes and other primates. This obtuse angle helps to support heavier weight as compared to lower angle.
6. The poplitial surface is also more concave as compared to apes. This may also have some significance in erect posture.

Changes in Foot due to assumption of Erect Posture:-

With the assumption of erect posture and bipedal walk, the hind limb is no longer a grasping organ but locomotory organ. As such changes necessary for modifying this grasping foot into an organ capable of supporting the weight of the body is fairly clear.

1. The great toe and its supporting metatarsal bones must be brought into the line of the long axis of the foot; for a divergent toe, sticking out like a thumb would be a hindrance to ground walking, since it would be stubbed almost constantly. Therefore, all the toes are in a line.
2. The big toe is no more opposable since the foot as a whole is not a grasping organ. Because of this, it also became shorter than the thumb.
3. The four lesser toes being no longer used for grasping now underwent a process of shortening and degeneration.
4. The great toe is not separated from other toes and all the meta tarsal are bound together by a common ligament.
5. The weight of the body is transmitted from heel to great toe during walking. Therefore the foot has been modified with the development of transverse and longitudinal arches which help to absorb shocks.
6. Elongation and strengthening of the calcaneum is another modification so that the weight is easily transmitted.

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