Human skeletal changes due to bipedalism

The evolution of human bipedalism, which began in primates about four million years ago, or as early as seven million years ago with Sahelanthropus, has led to morphological alterations to the human skeleton including changes to the arrangement and size of the bones of the foot, hip size and shape, knee size, leg length, and the shape and orientation of the vertebral column. The evolutionary factors that produced these changes have been the subject of several theories.

Human walking is about 75% less costly than both quadrupedal and bipedal walking in chimpanzees. Some hypotheses have been supported that bipedalism increased the energetic efficiency of travel and that this was an important factor in the origin of bipedal locomotion. Humans save more energy than quadrupeds when walking but not when running. Human running is 75% less efficient than human walking. A study helped to prove that walking of living hominin bipeds is noticeably more efficient than walking of living hominin quadrupeds, but the costs of quadruped and bipedal travel are the same.

Human feet have evolved enlarged heels in order to bear the amount of weight that has also been increased. 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. Moreover, 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.

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