User:Aleong24/Human skeletal changes due to bipedalism

Foot[edit]
Main article: Foot

Human feet evolved enlarged heels. The human foot evolved as a platform to support the entire weight of the body, rather than acting as a grasping structure, as it did in early hominids. Humans therefore have smaller toes than their bipedal ancestors. '''This includes a non-opposable hallux, which is relocated in line with the other toes. The muscles that work along with the hallux has evolved to provide efficient push off. The push off would also require all the toes to be slightly bent up.'''

Humans have a foot arch rather than flat feet. When non-human hominids walk upright, weight is transmitted from the heel, along the outside of the foot, and then through the middle toes while a human foot transmits weight from the heel, along the outside of the foot, across the ball of the foot and finally through the big toe.This transference of weight contributes to energy conservation during locomotion. '''The long arch has also evolved to provide efficient push-off. The stiffening of the arch would be required of an upward gait, all considered that modern bipedalism does not include grasping of tree branches, which also explains the hallux evolving to line up with the rest of the toes.'''

Vertebral column[edit]
Main article: Vertebral column

The vertebral column of humans takes a forward bend in the lumbar (lower) region and a backward bend in the thoracic (upper) region. Without the lumbar curve, the vertebral column would always lean forward, a position that requires much more muscular effort for bipedal animals. With a forward bend, humans use less muscular effort to stand and walk upright. Together the lumbar and thoracic curves bring the body's center of gravity directly over the feet. Specifically, the S-shaped curve in the spine brings the center of gravity closer to the hips by bringing the torso back. Balance of the whole vertebral column over the hip joints is a major contribution for efficient bipedalism. The degree of body erection (the angle of body incline to a vertical line in a walking cycle) is significantly smaller to conserve energy.

'''The Angle of Sacral Incidence was a concept developed by G. Duval-Beaupère and his team at the University of René Descartes. It combines both the pelvic tilt and sacral slope to determine approximately how much lordosis is required for the upright gait to eliminate strain and fatigue on the torso. Lordosis, which the inward curvature of the spine, is normal for an upright gait as long as it is not too excessive or minimal. If the inward curvature of the spine is not enough, the center of balance would be offset causing the body to essentially tip forward, which is why some apes that have the ability to be bipedal require large amounts of energy to stand up. In addition to sacral angles, the sacrum has also evolved to be more flexible in comparison to the stiff sacrum that apes possess.'''

Significance[edit]
Even with much modification, some features of the human skeleton remain poorly adapted to bipedalism, leading to negative implications prevalent in humans today. The lower back and knee joints are plagued by osteological malfunction, lower back pain being a leading cause of lost working days, because the joints support more weight. Arthritis has been an obstacle since hominids became bipedal: scientists have discovered its traces in the vertebrae of prehistoric hunter-gatherers. Physical constraints have made it difficult to modify the joints for further stability while maintaining efficiency of locomotion.

'''There have been multiple theories as to why bipedalism was favored, thus leading to skeletal changes that aided the upward gait. The Savannah Hypothesis describes how the transition from arboreal habits to a savannah lifestyle favored being bipedal. This would also change the diet of hominins, possibly increasing the size of the brain and changing the skeletal structure of the skull. Transitions from the forests to the savannah meant that sunlight and heat would require major changes in lifestyle. Being bipedal on an open field is also an advantage because of heat dispersal. Walking upright reduces the amount of direct sun exposure and radiation in comparison to being a quadruped which have more body surface on top for the sun to hit.'''