Skeletal changes due to erect posture and its implications

Man can stand erect, while anthropoid cannot’, Discuss the anatomical changes occurred in man due to erect posture.

Man alone has habitual bipedalism. Other animals assume occasional bipedalism but it is not habitual. The changes that have undergone in the skeleton of humans owing to the assumption of erect posture and bipedal gait are numerous. The specific changes effected in different parts of human skeleton are as follows:

1) Skull:

  • (i) The skull of man is well balanced on the first vertebral atlas.
  • (ii) Foramen Magnum: It is placed centrally at the base of the skull with the well balancing of the head and the face becoming vertical.

2) Vertebral Column:

When man stands erect the head remains in natural equilibrium, but at the same time, the weight of viscera in the abdominal and thoracic cavity tends to throw trunk in the forward direction. In order to avoid this, two anatomical arrangements have been made in the human skeleton.
(a) The elastic ligaments that interpose between vertebral laminae help the body to keep it in erect position.
(b) The four alternative curves present in the vertebral column tend to preserve the line of gravity and the trunk in the axis of sustaination that passes to the pelvis.

In this way the head and the trunk falls mainly on the pelvis. This condition has made the erect posture easy. The process of the vertebrae have also undergone a few changes in their structure. In tune with the erect posture, the externsar muscles, the spinal muscles and of the cervical ligament are in the direction and size of the sprainus processes.

3) The Thorax:

It is developed more transversely than that of the others mammals. The transverse diameter of the thorax is much greater than the dorso Ventral diameter. The flat end chest in throwing the centre of gravity of the body backwards towards the spine.

4) The Pelvis:

The pelvis of humans has undergone many changes.
(a) The human ilium has become extremely short.
(b) The sacral part of human ilium has acquired excessive breadth.
(c) The head of human femur is much larger than that of any other primate.
(d) The acetabulum region is very thicker and much larger than that of any other primate.
(e) The acetabulum is found situated in the centre of the pelvis when seen from the side.

All these changes in pelvis indicate the changes, occurred due to the assumption of, erect posture and bi-pedal gait on the ground.

5) The Limbs: Human adaptation of erect posture is related very much to the remarkable nature of limbs. In humans, the arms are shorter than the legs, which are relatively very long. Further, the physique has been modified for walking.

6) The Femur: The muscle attachments and ridges in femur are much more shortly defined and differentiated than in anthropoid apes. The backside of the femur possesses a rough ridge known as linea aspera, a Characteristically human feature. It has resulted to the great development of the extensor muscles, which play the most important roil in the erect posture and bi-pedal gait. These muscles are much less developed in man. Owing to the great development of this ridge or linea aspera, the cross section of the femur exhibits a prismatic form while in apes it is round and oval.

7) The Foot:

The foot of a man has witnessed remarkable changes due to the assumption of erect posture. When man’s ancestors began to walk erect, the function of the foot was completely changes, from grasping to supporting the whole body, which resulted in the following anatomical changes:

  • (a) Less of opposabifity of the great toe. Development of the shock-absorbing arch. Tendency towards mono-ductilism.
  • (b)The axis of the foot running through in between the first and second toe.
  • (c ) Formation of the medial and lateral arches.
  • (d) In the fifth digit reduction in the number of phalanges is in progress.
  • (e) The heel bone becomes larger and stronger.
  • (f) Wedge shaped development of the other tarsal bones.

The forearm of man are comparatively shorter than those of the anthropoid apes. The latter group have developed longer fore arm due to their practice of brachiating but in case of man we find different use of the arm. He applied it for lifting weights. In the case of apes there is elongation of the hand specially the fingers and the metacarpal bones. This arrangement helps brachiation. The thumb becomes less important and it is much shorter in comparison to the other fingers. On the other hand,in man the hand is no longer an organ of locomotion. Therefore, it has become shorter in length. Consequently, the meta tarsals have also became short.

To sum up, the influence of bi-pedal posture and gait in man can be seen in terms of modifications affected in the skull, vertebral column the pelvis and the limbs. In feet, the use of the legs alone for walking, and of the arms alone for other purposes, has brought many significant changes in the skeletal structure of man.

Implications

Several hypotheses have been proposed over the last century or so to explain the evolution of hominins. As bipedalism is the first trait on the road to modern humans, these hypotheses focus on the emergence of habitual bipedalism. Many have been refuted as new data is discovered.

  • The first hypothesis was the hunting hypothesis proposed by Charles Darwin. He suggested that bipedalism gave the first hominins an advantage in that it freed up their hands to carry weapons used to hunt animals.
  • The patchy forest hypothesis suggests that the emerging mosaic environment that began emerging at the end of the Miocene made bipedalism advantageous. The phrase mosaic environment in this case refers to an environment that had patchy forest interspersed with grasslands that eventually became the African savannas that we know today. This caused food resources to become spread out over the landscape. For traveling long distances, bipedalism is more energy efficient than quadrupedalism. Traveling bipedally freed up hands for carrying provisions and the early hominins could have easily fed from both terrestrial and arboreal resources.
  • The provisioning hypothesis states that having hands free to carry food allowed males to provision females and offspring. Since much of the females energy went to child-rearing, the ability of a male to provision her and her offspring would have been an attractive quality. Those males who could walk more efficiently bipedally while carrying food would have been prime mate material, allowing both the male and female to reproduce successfully.
  • Another model accounts for better dispersion of body when the head is raised and less surface is exposed to the sun during the hottest time of the day. This might have played an important role in the thermoregulation of the brain in early hominids for the development of brain.

Summary :

The evolution of bipedalism and erect posture in humans has led to significant skeletal changes compared to our quadrupedal ancestors. These adaptations can be observed in various parts of the human skeleton, including the skull, spine, pelvis, legs, and feet. Here is a detailed examination of these changes:

Skull and Spine

  1. Foramen Magnum Position:
    • Quadrupeds: The foramen magnum (the hole through which the spinal cord passes) is positioned towards the back of the skull.
    • Bipeds: In humans, the foramen magnum is located more centrally under the skull, allowing the head to balance directly on top of the spine.
    • Example: The more forward position of the foramen magnum in the fossil hominid Australopithecus afarensis compared to that of a chimpanzee.
  2. Spinal Curvature:
    • Quadrupeds: Have a single, C-shaped curve in their spine.
    • Bipeds: Humans have an S-shaped spine, with distinct cervical (neck), thoracic (upper back), and lumbar (lower back) curves. This S-shape helps to balance the body’s weight over the pelvis.
    • Example: The lumbar curve in humans, which is more pronounced than in quadrupeds, helps to support the upper body and absorb shock during walking.

Pelvis and Hip

  1. Pelvis Shape:
    • Quadrupeds: The pelvis is long and narrow, optimized for a quadrupedal gait.
    • Bipeds: The human pelvis is shorter and broader, with iliac blades that are curved to support abdominal organs and provide a stable platform for bipedal locomotion.
    • Example: The broad and short pelvis of Australopithecus afarensis (e.g., the famous “Lucy” fossil) compared to the elongated pelvis of chimpanzees.
  2. Hip Joint:
    • Quadrupeds: The hip joints are oriented more towards the back of the body.
    • Bipeds: Human hip joints are oriented to support an upright posture, with larger femoral heads and a more robust acetabulum (hip socket) to bear the increased stress from bipedal locomotion.
    • Example: The human femur has a larger and more robust femoral head compared to that of quadrupedal primates.

Legs

  1. Femur:
    • Quadrupeds: The femur is straight and the knees are positioned more laterally.
    • Bipeds: The human femur is angled inward (bicondylar angle), bringing the knees closer together under the body’s center of gravity, which aids in balance during walking.
    • Example: The angled femurs in humans create a more efficient bipedal gait compared to the straight femurs in quadrupeds.
  2. Knee Joint:
    • Quadrupeds: The knee joint is less adapted for weight-bearing in an upright position.
    • Bipeds: The human knee joint is larger and more complex, with a locking mechanism that provides stability during standing and walking.
    • Example: The presence of a locking mechanism in the human knee joint allows for prolonged periods of standing without excessive muscle fatigue.

Feet

  1. Foot Structure:
    • Quadrupeds: Feet are adapted for grasping and climbing, with an opposable big toe.
    • Bipeds: Human feet are adapted for walking, with a non-opposable big toe aligned with the other toes, a prominent arch for shock absorption, and a stiff midfoot for efficient push-off.
    • Example: The loss of the opposable big toe and the development of arches in the feet of early hominids like Homo habilis and Homo erectus.
  2. Toes:
    • Quadrupeds: Longer toes for grasping branches.
    • Bipeds: Shorter toes that facilitate forward propulsion and stability during walking and running.
    • Example: The shorter and straighter toes in humans compared to the longer, curved toes of tree-dwelling primates like chimpanzees.

Additional Considerations

  1. Muscle Attachments:
    • Changes in the skeletal structure are accompanied by shifts in muscle attachments to support bipedal locomotion. The gluteal muscles, for instance, have a different orientation in humans to aid in balance and propulsion.
  2. Joint Adaptations:
    • The joints in the lower limbs, especially the hip, knee, and ankle, have evolved to bear the weight of an upright body and to provide the necessary range of motion for efficient bipedal walking and running.

Fossil Evidence

  1. Australopithecus afarensis:
    • The famous “Lucy” fossil exhibits many traits indicative of bipedalism, such as a more human-like pelvis and angled femurs.
  2. Homo erectus:
    • Fossils show a more modern human body plan, with adaptations for long-distance walking and running.
  3. Ardipithecus ramidus:
    • This early hominid shows a mix of bipedal and arboreal traits, indicating a transitional form in the evolution of bipedalism.
Join Telegram