The evolution of humanbipedalism, which began in primates about four million years ago, or as early as seven million years ago with Sahelanthropus, or about 12 million years ago with Danuvius guggenmosi, 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.
Energy efficiency
Human walking is about 75% less costly than both quadrupedal and bipedal walking in chimpanzees. Some hypotheses have 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 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.
Foot
Human feet evolved enlarged heels to bear the weight that evolution also 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.
Knee
Human knee joints are enlarged for the same reason as the hip – to better support an increased amount of body weight. The degree of knee extension has decreased. The changing pattern of the knee joint angle of humans shows a small extension peak, called the “double knee action,” in the midstance phase. Double knee action decreases energy lost by vertical movement of the center of gravity. Humans walk with their knees kept straight and the thighs bent inward so that the knees are almost directly under the body, rather than out to the side, as is the case in ancestral hominids. This type of gait also aids balance.
Limbs
An increase in leg length since the evolution of bipedalism changed how leg muscles functioned in upright gait. In humans the "push" for walking comes from the leg muscles acting at the ankle. A longer leg allows the use of the natural swing of the limb so that, when walking, humans do not need to use muscle to swing the other leg forward for the next step. As a consequence, since the human forelimbs are not needed for locomotion, they are instead optimized for carrying, holding, and manipulating objects with great precision. This results in decreased strength in the forelimbs relative to body size for humans compared to apes. Having long hindlimbs and short forelimbs allows humans to walk upright, while orangutans and gibbons had the adaptation of longer arms to swing on branches. Apes can stand on their hindlimbs, but they cannot do so for long periods of time without getting tired. This is because their femurs are not adapted for bipedalism. Apes have vertical femurs, while humans have femurs that are slightly angled medially from the hip to the knee, thus making human knees closer together and under the body’s center of gravity. This adaptation lets humans lock their knees and stand up straight for long periods of time without much effort from muscles. The gluteus maximum became a major role in walking and is one of the largest muscles in humans. This muscle is much smaller in chimps, which shows that it has an important role in bipedalism. When humans run, our upright posture tends to flex forward as each foot strikes the ground creating momentum forward. The gluteus muscle helps to prevent the upper trunk of the body from "pitching forward" or falling over.
Hip and pelvis
Modern human hip joints are larger than in quadrupedal ancestral species to better support the greater amount of body weight passing through them, as well as having a shorter, broader shape. This alteration in shape brought the vertebral column closer to the hip joint, providing a stable base for support of the trunk while walking upright. Also, because bipedal walking requires humans to balance on a relatively unstable ball and socket joint, the placement of the vertebral column closer to the hip joint allows humans to invest less muscular effort in balancing. Change in the shape of the hip may have led to the decrease in the degree of hip extension, an energy efficient adaptation. The ilium changed from a long and narrow shape to a short and broad one and the walls of the pelvis modernized to face laterally. These combined changes provide increased area for the gluteus muscles to attach; this helps to stabilize the torso while standing on one leg. The sacrum has also become more broad, increasing the diameter of the birth canal and making birthing easier. To increase surface for ligament attachment to help support the abdominal viscera during erect posture, the Ischia spines became more prominent and shifted towards the middle of the body.
Vertebral column
The vertebral column of humans takes a forward bend in the lumbar region and a backward bend in the thoracic 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. Also, the degree of body erection is significantly smaller to conserve energy.
Skull
The human skull is balanced on the vertebral column: The foramen magnum is located inferiorly under the skull, which puts much of the weight of the head behind the spine. Furthermore, the flat human face helps to maintain balance on the occipital condyles. Because of this, the erect position of the head is possible without the prominent supraorbital ridges and the strong muscular attachments found in, for example, apes. As a result, in humans the muscles of the forehead are only used for facial expressions. Increasing brain size has also been significant in human evolution. It began to increase around 2.4 million years ago but modern levels of brain size were not attained until after 500,000 years ago. Zoological analyses have shown that the size of human brains is significantly larger than what you would expect for their size. The human brain is in fact three to four times larger than its closest relative - the chimpanzee.
Significance
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 a problem 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.
