The assumption of natural selection for obstetric adequacy would predict the occasional discovery of skeletal evidence for inadequacy, yet such cases are rare. Here stress injuries to the pelvic ring are associated with an asymmetrical pelvis caused by partial agenesis of the right sacral costal process. The small, middle-aged woman lived about 2000 years ago and pursued a physically vigorous life as a coastal forager on the South African Cape coast. The right sacral costal process (ala) is about 5 mm narrower than the left. This sacral hemiagenesis led to pelvic asymmetry, and concomitant instability. The six articular faces of the pelvis show the normal surfaces replaced by disorganised, porous new bone and extensive eburnation. The left pubic face shifted dorsally, making contact with the ventral aspect of the right pubis. The most distinctive aspect is the presence of osteitis pubis, or pubic symphysis stress injury, which is clinically associated both with difficult childbirth and with vigorous athletic activities. Adjustments to the altered pelvis include erosions and osteophytes at sites of ligament attachment, and prominent bilateral insertion areas on the tibiae for the stabilising iliotibial band. While habitual activities may explain some pelvic features, such as the bilateral bevelling of the acetabular borders, it is postulated that childbirth exacerbated the joint instability and the subsequent deterioration. This case joins a small literature on the condition of pubic symphysis stress injury, and illustrates the mechanisms through which natural selection for pelvic size and shape characteristics may have occurred. Copyright © 2010 John Wiley & Sons, Ltd.