III.

Structure and Physiology

The details of skeletal structure distinguishing Homo sapiens from the nearest primate relatives—the gorilla, chimpanzee, and orangutan—stem largely from a very early adaptation to a completely erect posture and a two-footed striding walk (bipedalism). The uniquely S-shaped spinal column places the center of gravity of the human body directly over the area of support provided by the feet, thus giving stability and balance in the upright position. Other mechanical modifications for bipedalism include a broad pelvis, a locking knee joint, an elongated heel bone, and a lengthened and aligned big toe. Although varying degrees of bipedalism are seen in other anthropoids, all have straight or bowed spines, bent knees, and grasping (prehensile) feet, and all use the hands to bear part of the body weight when moving about.

Complete bipedalism in the human freed the hand to become a supremely sensitive instrument for precise manipulation and grasping. The most important structural detail in this refinement is the elongated human thumb, which can rotate freely and is fully opposable to the other fingers. The physiological requirements for speech were secondarily established by erect posture, which positions the vocal cords for controlled breathing, and by the skilled use of the hands. The latter development occurs in association with the enlargement and specialization of a brain area (Broca's convolution) that is a prerequisite for refined control of the lips and tongue.

The large (averaging 1400 cc/85.4 cu in) brain of Homo sapiens is approximately double that of early human toolmakers. This great increase in size in only 2 million years was achieved by a process called neoteny, which is the prolongation of retention of immature characteristics. The juvenile stage of brain and skull development is prolonged so that they grow for a longer period of time in relation to the time required to reach sexual maturity. Unlike the early human adult skull, with its sloping forehead and prominent jaw, the modern human skull—with biologically insignificant variations—retains into maturity a proportionately large size, in relation to the rest of the body, a high-rounded dome, straight-planed face, and reduced jaw size, all closely resembling the characteristics of the skull in the juvenile chimpanzee. Its enlarged dimensions required adaptations for passage through the birth canal; consequently, the human female pelvis widens at maturity (with some sacrifice in swiftness of locomotion), and the human infant is born prematurely. Chimpanzees are born with 65 percent of their adult brain capacity; Australopithecine, an erect, tool-using near-human of 3 million years ago, was born with about 50 percent; modern human newborns have only 25 percent of adult brain capacity, resulting in an extended period of helplessness. The many neurological pathways to the rapidly growing brain must be organized and coordinated during a prolonged period of dependency on and stimulation by adults; lacking this close external bond in the early years of life, development of the modern brain remains incomplete.