Skeletal changes due to erect posture

Bipedalism, or the ability to maintain an erect posture, is one of the most distinguishing characteristics of a human being’s appearance. It is estimated that the human way of walking with an erect posture is about 75% more energy efficient than the method of walking of other quadrupeds or bipedal animals. 

The process of switching to bipedalism began about four million years ago in primates. This form of erect body posture has resulted in several changes and adaptations of the human skeleton. The different parts of the skeletal system are modified in different ways.

Skull

Because of the erect posture of humans, the skull is balanced at a pivotal point on top of the vertebral column. A major part of the brain is transferred behind the spine by the foramen magnum below the skull. A flat facial structure balances the weight of the head. This is how humans can maintain an erect posture without developing a prominent suborbital ridge and strong muscles found in apes. 

Vertebral column

The human vertebral column has a particular bend called the lumbar curve. The spine in human beings is bent forward in the lower (lumbar) region and backwards in the upper (thoracic) region. The lumbar curve makes it possible for humans to maintain an erect posture because, without the lumbar curve, the vertebral column would be perpetually bent forward. This would cause a great amount of muscular effort for any bipedal. But because of the lumbar curve, human beings can easily stand and sit straight. This special shape of the vertebral column ensures that the centre of gravity of the human body is directly above the feet, helping to maintain balance while standing erect. 

Thorax

The thorax changes its shape with the change in the axis of the body. The ribs in human beings are more curved, making the thorax barrel-shaped. This makes the thorax’s dorso-ventral diameter smaller than the thorax’s transverse diameter. This facilitates the erect posture of human beings.

The pelvis

There is a significant amount of body weight around the human pelvis. This is accommodated by a large pelvic joint. The shape of the pelvic joint is shorter and broader. The shape of the pelvis has altered to bring the vertebral column closer to it. So when walking with an erect posture, the vertebral column can rest on the pelvic joint, thus making it easier to walk erect. The ball and socket joint on which bipedal walking is dependent is relatively unstable. The proximity of the vertebral column to the hip joint makes it easier for the muscles to balance the body without expending too much energy. The degree of hip extension in humans is less due to the shape of the hip. The ilium is the pelvis of humans evolved to change to short and broad from long and narrow, and the iliac crest is curved. 

Limbs

The leg muscles of the ankles in human beings facilitate the push that makes walking possible. The leg bones of human beings have evolved to be lengthy. This increased length makes it possible for the leg to be swung forward without the use of muscles. The leg bones can make walking possible on their own. This leaves the arms and hands to be specialised for holding and carrying things. This specialised function of the forelimbs makes them shorter in comparison to other primates. Human beings can walk in an erect posture due to this modification of limb lengths. 

Knees

The bodyweight of human beings needs extra support while standing erect. For this reason, the knee joints are enlarged. The degree to which the knee can extend is also decreased, aiding in better maintenance of balance while walking. Since there are two extension/flexion movements in the knee during one walking cycle, it is known as ‘double knee action’. This double knee action dramatically reduces the expenditure of energy that would have happened otherwise when the centre of gravity moves along a vertical axis. 

Feet

The early hominids had feet that could curve and grasp things. Humans evolved feet that are like platforms to help support the body’s weight while walking. For this reason, the toes of humans are smaller than those of other primates. Human beings also lack opposable toes. When other primates walk, the body’s weight is transmitted from the heel along the sides of the feet and then onto the middle toes. This happens because the feet of the primates are flat. The feet of human beings, on the other hand, are arched. 

Conclusion

There have been several skeletal changes due to erect posture in the human skeleton. The different joints have adapted to accommodate the motions required to maintain an erect body posture. Several bones have been altered in shape and angled differently to help with balance and locomotion. The erect body posture helps conserve energy and enables human beings to develop specialised functions and manipulate objects with great accuracy and detail.