Literature

The human species exhibits the phenomenon of sexual dimorphism in a multitude of subtle ways. A fascinating study by Whitcome et al allows a closer look at one of these differences - relating to the base of the spine. The shapes of these vertebrae are critical for bipedalism, and those of us who do not look after our backs will learn the hard way that posture and appropriate exercise are important. Particular challenges face pregnant women:

"Pregnancy augments the mass of the human female abdomen by as much as 31% (6.8 kg), translating the position of the maternal COM [Centre of Mass] forward and increasing the torque exerted by the upper body around the hip joints. Although this shift in mass does not disrupt postural stability in quadrupeds, it uniquely destabilizes bipeds whose supporting joints and twofooted support base lie solely under the hips. Such gravid instability can be counteracted by muscles, but sustained recruitment risks muscle fatigue and increases the likelihood of spinal injury.
Pregnant mothers habitually compensate positionally to fetal load by extending the lower back. Our longitudinal study of 19 pregnant human females shows that adjustments to lumbar lordosis permit mothers to maintain a stable anteroposterior position of the COM as gestation progresses and fetal mass increases."

It is found that men and women have differences in the morphology of the lower vertebrae, and the research paper links these to pregnancy. Men have two wedged vertebrae where women have three. Whilst this may not sound much, with all the other minor adjustments, pregnant women have the ability to adjust their posture so that the COM is in the right position and the shearing forces on the back are not excessive.
The instinctive reaction of some of us is to recognise lumbar lordosis as a design feature. However, the authors approach their research from within the Darwinian paradigm:

"Given the demands of fetal load and the importance of pregnancy for fitness, one predicts that natural selection has operated on the unique anatomy of the hominin lumbar region to mitigate the biomechanical problems that females confront."
and:
"The evidence for lumbar sexual dimorphism in humans which improves maternal performance in posture and locomotion suggests that the distinctive hominin lumbar curve has been subject to strong selection pressures."

To illustrate how these words are paradigm-driven, they have been rewritten below from a design perspective:

"Given the demands of fetal load and the importance of pregnancy for fitness, one predicts that design modifications have been made to the unique anatomy of the hominin lumbar region to mitigate the biomechanical problems that females confront."
and:
"The evidence for lumbar sexual dimorphism in humans which improves maternal performance in posture and locomotion suggests that the distinctive hominin lumbar curve is the result of intelligent design engineering."

As a result of the authors presupposing a Darwinian adaptationist story, they feel no need for rigour in defending their interpretation of the data. They comment:

"It is reasonable to hypothesize that fatigue and pain in the lower back muscle affected early hominin mothers just as they do modern mothers, possibly limiting foraging efficiency and the ability to escape from predators, leaving the gravid female at risk of nutritional stress and injury or death."

There is no attempt here to move beyond storytelling, no assessment of what a coherent Darwinian explanation should look like, and no consideration of behavioural modifications that might be adopted as an alternative to normal practice. The reviewers of the paper should have objected to the phrase "It is reasonable . . ." because it is not. When all the other morphological and hormonal changes in pregnancy are taken into account, the inference to design remains compelling.

Fetal load and the evolution of lumbar lordosis in bipedal hominins
Katherine K. Whitcome, Liza J. Shapiro & Daniel E. Lieberman
Nature 450, 1075-1078 (13 December 2007) | doi:10.1038/nature06342

As predicted by Darwin, bipedal posture and locomotion are key distinguishing features of the earliest known hominins. Hominin axial skeletons show many derived adaptations for bipedalism, including an elongated lumbar region, both in the number of vertebrae and their lengths, as well as a marked posterior concavity of wedged lumbar vertebrae, known as a lordosis. The lordosis stabilizes the upper body over the lower limbs in bipeds by positioning the trunk's centre of mass (COM) above the hips. However, bipedalism poses a unique challenge to pregnant females because the changing body shape and the extra mass associated with pregnancy shift the trunk's COM anterior to the hips. Here we show that human females have evolved a derived curvature and reinforcement of the lumbar vertebrae to compensate for this bipedal obstetric load. Similarly dimorphic morphologies in fossil vertebrae of Australopithecus suggest that this adaptation to fetal load preceded the evolution of Homo.

See also:

Quill, E. Keeping Mom in a Full, Upright Position, ScienceNOW Daily News, 12 December 2007

Coppedge, D. Walking Upright Is Not Just for Pregnant Females, Creation-Evolution Headlines (12/13/2007)

Sandeman-Allen, C. Question of balance, The Times (Letter, 14 December 2007)