Bipedalism is the ability to walk on two legs. It allows hominids to walk with their arms completely free. This gives them the advantage of being able to make and use tools efficiently. Arms can also be used for work, for social display, and communication. Walking on two limbs is also more energy efficient than walking on four limbs. This gave the early hominids more energy more energy to reproduce and thus had a better chance of producing offspring that had the bipedalism trait. The evolution of human bipedalism was as a result of various morphological alterations on the human skeleton. Various indicators of bipedalism on the skeleton include the vertebrae column, the knees, and the hips. Adaptations of these parts of the skeleton aided locomotion in different ways.
Vertebrae column
Maintaining proper balance is important when one walks on two legs. The walking cycle involves lifting one leg off the ground swinging it forward while the other leg is on the ground creating balance. Proper balancing is achieved in humans because the center of gravity is located close to the center of the pelvis. In apes walking on four limbs, the center of gravity is located near the body’s center on the torso and reduces the body’s balance. As legs move forward and backward during the walking cycle, the center of gravity shifts from one section of the pelvis to another. It makes a pattern that resembles the figure 8.
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The lumbar curvature of the spine aids to bring the center of gravity close to the midline of the body and above the feet. The lumbar region in humans is also flexible and allows for bending forward at the hips and also bending backward and sideways. Apes have no lumbar curve that can pull the upper body back over the pelvis. Their weight is thus what pulls their bodies forward. Humans have an additional lumbar curve which positions the body inward, over the pelvis. This positions the weight of the human body forward above the hips. The legs can thus be easily straightened while the center of gravity is lowered increasing stability and efficiency.
The lumbar vertebrae in human are different in number and size compared to that of apes. Humans mostly have 5 larger lumbar vertebrae while large apes can have 4 lumbar vertebrae that are smaller than that found in humans (Williams and Russo, 2015). The greater the size and number of the vertebrae form a flexible back. This allows the hips and trunk to move forward when walking. In apes, the lower back is less flexible and they must thus shift the hips a greater distance forward when it decides to walk in a bipedal motion.
Knees
The knee is a critical adaptation that keeps the body's center of gravity balanced over the stance leg during the walking cycle. The knee joint is made up of several bones that include the femur, the patella, and the tibia. Other primates have a more mobile knee compared to humans which helps them in climbing. The top of the tibia in apes is also more concave which increases the rotation of the knees. Humans need a stable knee when walking. The top of the tibia is thus flat in humans so as to reduce rotation of the knees.
The femur in apes is straight and does not angle inwards like that of humans. Human knees thus close together when standing because of the femurs or the upper leg bones angle inwards. The angle formed allows the weight of the upper body to be better centered towards the feet. This increases balancing since the weight of the body is supported at the midline. Walking becomes more efficient as humans are not forced to rotate the body from side to side with every single step. The human femur is also quite long and this increases the stride length enabling walking that is more efficient (Frelat et al., 2017).
Human knee joints are also enlarged and robust to provide a better support for an increased body weight. The degree of extension of the knee is decreased and the knee forms a double knee action. The double knee action decreases the energy lost by vertical movement when walking. Humans can thus walk with their knees kept straight with the thighs being bent inwards. The knees are placed almost directly under the body and not outside like in apes. Such gait helps in balance.
Hips
The hips of the modern human are large compared to animals that walk on four limbs. The larger hips support the greater amount of body weight which passes through them. The hip also has an alteration in shape where it is broader and shorter in shape. The alteration in shape provides a stable support of the trunk when walking upright. The human pelvis also has larger hip joints and large pelvic outlets. Such adaptations allow for efficient bipedalism (Gruss and Schmitt, 2015).
In bipedal animals, the hips balance and support the weight of the torso during movement. However, the increase in the size of the pelvic outlet caused the hip joints to be repositioned further from the centerline of the body. More force is thus exerted on the hip joint as the joint moves far away from the body’s center of gravity. This can affect stability since the weight of the torso usually presses down to the middle section of the body. Such an issue is resolved through adaptations within the femur and pelvis.
In conclusion, the human body has several anatomical modifications that enable bipedalism. However, there are other features of the human skeleton that remain poorly adapted. For instance, the joints support more weight and this can lead to issues like arthritis in later stages of life. The general adaptations are better adapted to walking on two limbs compared to other apes to foster efficiency when walking.
References
Frelat, M. A., Shaw, C. N., Sukhdeo, S., Hublin, J. J., Benazzi, S., & Ryan, T. M. (2017). Evolution of the hominin knee and ankle. Journal of human evolution , 108 , 147-160.
Gruss, L. T., & Schmitt, D. (2015). The evolution of the human pelvis: changing adaptations to bipedalism, obstetrics and thermoregulation. Phil. Trans. R. Soc. B , 370 (1663), 20140063.
Williams, S. A., & Russo, G. A. (2015). Evolution of the hominoid vertebral column: the long and the short of it. Evolutionary Anthropology: Issues, News, and Reviews , 24 (1), 15-32.
