The Obstetric Dilemma

The difficulty of childbirth in humans is brought about by the relatively small pelvis of the mother, and the large size of the foetal head and shoulders. The "obstetric dilemma", is the result of a shrunken birth canal, due to the requirements of bipedal locomotion and the evolution of the human brain size (Wittman & Lewis 2007).

For a permanency in bipedalism, the skeleton required major changes, mainly the pelvis, with respect to childbirth. The ilium length and shape in quadrupeds is long and blade like, whilst in humans, it has shortened and broadened. Coupled with a lateral reorientation of the pelvis walls, (Jordaan 1976, Abitbol 1987, Lovejoy 2005) an increase in the attachment area for the gluteus medius and minimus is evident, increasing the stability of the torso upon standing on one support. Broad and pushed posteriorly, the human sacrum better supports body weight during an erect stance. (Abitbol 1996, Jordaan 1976) All of these pelvic changes, increases bipedalism efficiency during movement, by maintaining the balance on the central axis of the body (Abitbol 1987).

The broadened sacrum aids parturition, by increasing the traverse diameter of the birth canal (Jordaan 1976). At the same time, the ischial spine, moving medially, has become more pronounced, providing a greater area for attachment of the ligaments that support the torso whilst in an upright posture. This support for bipedalism also restricts the midplane of the pelvis, which complicates birth. (Abitbol 1996, Wittman & Lewis 2007) The evidence for these changes in the pelvis are apparent in a 3.2 million year old fossil of Australopithecus afarensis ("Lucy") (Ward 2002). Even though bipedalism predates Lucy, she is the oldest fossil which exhibits the effects of bipedalism on the pelvis structure (Tague & Lovejoy 1986).

These changes, maintain the balance, as well as the upright position of the body, thereby altering the architecture of the pelvis from a simple cylinder into a complex structure. Consequently, the planes of the pelvic inlet, midpelvis and pelvic outlets have become misaligned. The dimensions have been minimised to a point where a series of rotations of the foetal head and shoulders are required in order to pass through the birth canal. These changes have increased the complexity of childbirth (Wittman & Lewis 2007).

This all leads to the question of how bipedalism became a selection pressure. The various theories which have been hypothesised are difficult to test, leaving us with unverifiable hypotheses, leading to speculation (Wittman & Lewis 2007).

The dominant theory of the selection pressure for bipedalism was to free the hands for tool manipulation. However, in the mid-20th century, fossil discoveries invalidated this theory, with the results predating bipedalism to tool use by at least 1.5 million years. One of the most influential pieces of fossil evidence was footprints,