Human skeletal changes due to bipedalism

The evolution of bipedalism approximately four million years agocite book |author=Kondō, Shirō |title=Primate morphophysiology, locomotor analyses, and human bipedalism |publisher=University of Tokyo Press |location=Tokyo |year=1985 |pages= |isbn=4-13-066093-4 |oclc= |doi=] has led to significant changes in the anatomy of "Homo sapiens". The morphological alterations to the human skeleton that have occurred since the first bipedal hominid include changes in foot bone arrangement and size, hip size and shape, knee size, leg length, and vertebral column shape and orientation.

Foot

The human foot has been redesigned to act 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.cite book |author=Aiello,Leslie and Christopher Dean |title=An Introduction to Human Evolutionary Anatomy|publisher=Elsevier Academic Press |location=Oxford |year=1990] 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. Conversely, 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.

Hip

Modern human hip joints are larger than quadrupedal ancestral species to better support the greater amount of body weight passing through them, as well as 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.cite journal | author = Wang, W. | coauthors = Crompton, R.H.; Carey, T.S.; Günther, M.M.; Li, Y.; Savage, R.; Sellers, W.I. | year = 2004 | title = Comparison of inverse-dynamics musculo-skeletal models of AL 288-1 Australopithecus afarensis and KNM-WT 15000 Homo ergaster to modern humans, with implications for the evolution of bipedalism | journal = Journal of Human Evolution | volume = 47 | issue = 6 | pages = 453–478 | url = http://linkinghub.elsevier.com/retrieve/pii/S0047248404001265 | accessdate = 2008-04-02 | doi = 10.1016/j.jhevol.2004.08.007] 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 to balance. The shape change of the hip may have led to the decrease in the degree of hip extension, an energy efficient adaptation.

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 (the angle between the thigh and shank in a walking cycle) 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 increases balance.

Limbs

An increase in leg length since the evolution of bipedalism changed how leg muscles functioned in upright gait. In humans, the push in 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, as the human forelimbs are not needed for locomotion, they are instead optimized for carrying, holding, and manipulating object with great precision.cite book
title = Anatomy & Physiology: The Unity of Form and Function
first = Kenneth S. | last = Saladin
publisher = McGraw-Hill | ed = 3rd | year = 2003
pages = 286-287]

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 help maintaining the balance on the occipital condyles. Because of this arrangement, 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 (the occipitofrontalis) are only used for facial expressions.

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. Also, the degree of body erection (the angle of body incline to a vertical line in a walking cycle) is significantly smaller to conserve energy.

Significance

Even with much anatomical 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, with lower back pain among the leading causes of lost working days. These problems resulted because human joints are forced to support weight for which they were not originally designed to, now that the body’s full weight is borne on two legs rather than four. In fact, arthritis has been a problem since hominids became bipedal. Scientists have discovered instances of vertebral arthritis in prehistoric hunter-gatherers; arthritis is not merely pathology of modern lifestyles.cite book |author=Jacob C. Koella; Stearns, Stephen K. |title=Evolution in Health and Disease |publisher=Oxford University Press, USA |location= |year=2008 |pages= |isbn=0-19-920746-1 |oclc= |doi=] Physical constraints have made it difficult to modify the joints for further stability while maintaining efficiency of locomotion.

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