Bipedalism and Parturition
Fig. 2. Lower limbs of human and chimpanzee compared. There are immediately recognizable differences in lower limb anatomy between humans and our closest living relatives in virtually every bony element. In the pelvis, most importantly are the modifications made for weight bearing (eg, reduction in the overall vertical height of the pelvis) and for muscle position (most especially the reorientation of the small gluteal muscles). The valgus knee is also associated with bipedal locomotion. Notice that there is an accommodation at the knee to allow both an angulation of the femur from the hip to the knee but also to keep the knees parallel to the ground as well as maintaining their position directly under body weight. This angulation must by necessity be limited. Black lines how the position of the long axis of the major lower limb bones. Red lines show the effect of increasing diameter within the confines of the bony pelvis on the angulation of the femur from hip to knee. The lateral dimensions of the bony pelvis are limited by the degree of angulation possible of the femur from hip to knee while still maintaining the structural integrity of the knee joint. Given the incidence of knee injuries, especially in females with their wider hip joints, it is quiet possible we are at the limit of lateral hip expansion.
INCREASES IN BRAIN SIZE AND THE OBSTETRIC DILEMMA
Modern female humans and their infants experience a host of unique features associated with the birthing process. All of these together create a set of birthing features that ultimately result in what has been called the obstetric dilemma (but see also).22 Rosenberg and Trevathan16 summarize a unique set of features characterizing human birth: 1. Because of the tight fit of the fetus through the birth canal, the infant undergoes a series of rotations to maximize the success of the passage of the large infant head and shoulders into the changing dimensions of the maternal inlet, midplane, and outlet. 2. In general, the infant exits the birth canal in an occipital anterior position. In contrast to other primates, where the neonate is born in a face-up position23 allowing the mother to assist and lift her child to her nipples, the human infant exits
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