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9.3: Bipedal Anatomy

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    The majority of bipedal characteristics involve the hip (or pelvic girdle) and lower limb. However, as will be seen below, certain skull and trunk characteristics are also adaptations for bipedal locomotion. In addition, we have inherited many aspects of our upper bodies from our ape ancestors and those will all be discussed in the following sections.

    Skull

    The skull consists of the bones of the braincase and face and the mandible (lower jaw). The foramen magnum  is the hole in the occipital bone situated in the base of our skulls (see Figure). It is where our spinal cord exits the cranial vault.

    clipboard_e3b54f887164f0ab14af725890130912a.png
    Figure \(\PageIndex{1}\): Foramen magnum indicated from inside skull vault. “Crane4 Foramen magnum” by Didier Descouens is licensed under CC BY-SA 3.0.

    In hominins, the foramen magnum is positioned more anteriorly than in the other apes because our head sits on top of our vertebral column. Thus while the earliest hominins had very ape-like faces, the position of the foramen magnum shows that they were bipeds.

    Spine

    Ape spines are not as flexible as monkeys’ spines, giving us better upper body support since we are more upright than most other primates. Our vertebrae increase in size and robusticity from top to bottom so that our lumbar vertebrae are very large; they sit on the fused vertebrae of the sacrum, which is firmly attached to the hip bones. The sacrum is large and broad and curves inward (as does the coccyx) to help support the organs. Thus our spinal column is a strong supporting structure for the upper body. We hominins have two larger curves in our backs relative to the other apes, the cervical curve  and the lumbar curve. The fact that our heads are more upright than nonhuman apes means that the cervical vertebrae must form a more concave curve, i.e. the superior aspect of our neck is arched back relative to theirs. The more pronounced lumbar curve forms when we stand up and begin toddling about. The joints between the lumbar vertebrae are easily strained and it is thus important to maintain strong back and abdominal muscles throughout life, to aid in the stability of the region.

    Pelvis

    Our pelvis is unique and interesting. It has changed significantly from an ape pelvis (see Figure). The pelvis is made up of three bones: the two lateral bones, termed innominates or os coxae, and the sacrum. Collectively, they form a basin-like structure that holds our internal organs while providing support for our upper bodies. Each innominate  consists of three bones that fuse during development: the ilium, ischium, and  pubis. They meet at the hip joint. Hominin innominates became shorter and broader, so that the ilium wraps around laterally from an earlier, more posterior position.

    clipboard_e1aab6ce3d8c830be5da1af677c2b1d50.png
    Figure \(\PageIndex{2}\): Left innominates of chimp (left), australopith (center), and human (right). Illustration by Keenan Taylor.

    This changed the orientation and action of our hip muscles, allowing for our striding gait and the ability to balance our weight on one fully extended leg while the other leg is in the swing phase. A portion of the gluteus maximus  muscle inserts behind the hip joint in hominins (versus lateral in chimps), and thus instead of abducting the femur (moving it out laterally, as when doing jumping jacks), it changed to a powerful hip extensor (backward motion) for running.

    Legs

    The lower limb consists of the femur of the thigh, the tibia and fibula of the leg, seven tarsal bones of the ankle, five metatarsals of the body of the foot, and phalanges of the digits (three per toe and two per big toe or hallux). The head (proximal ball-like structure) of the hominin femur is large. The femur angles medially (inward) from hip to knee, so that our upper body weight is transferred down through our hip joints to our knees. This is termed the carrying or bicondylar angle.

    The knees of quadrupedal apes are directly below the hip joint, so there is more strain on the knee joints when they walk bipedally (see Figure). Unlike apes’ knees that are chronically flexed, our knees are capable of full extension; each locks into place when the other leg is in swing phase, giving us a stable supporting leg. Each gluteus mediusmuscle alternately supports the opposite side of the torso and pelvis, so that it does not slump on the unsupported side.

    clipboard_e97270cc58f36e59db3ca94bd5cf1032f.png
    Figure \(\PageIndex{3}\): Pelvic girdle and weight loading on knee joint. Illustration by Keenan Taylor.

    Foot

    Our feet have changed dramatically from a mobile, grasping structure to a rigid, supporting one. The tarsal bones of the human ankle are large and robust for support. The joint between the distal tibia and fibula is fairly immobile, so that the two bones are firmly lashed together. Together, they articulate with the talus (most superior tarsal bone) in a hinge joint. We have lost much of the mobility of an ape foot and thus have become less agile in climbing over time. The calcaneus or heel bone is very large and robust and, along with the ball of the foot (distal end of the first metatarsal) and the area below the baby toe (fifth metatarsophalangeal joint), forms a tripod structure. Our feet have three arches for support, shock absorption, and forward propulsion; they are the medial and lateral longitudinal arches and the transverse arch. Hominin toes became shorter and less curved over time.

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