Pregnant women standing on their own two feet

The title of the Nature paper is "Fetal load and the evolution of lumbar lordosis in bipedal hominins" (Nature 450, 1075-1078 (13 December 2007) | doi:10.1038/nature06342) and the Editor's comment is given the slightly snappier title, "The mother load." (Nature (DOI:10.1038/nature06342). But I like the New Scientist version best: "Why pregnant women don't fall over."

Here's the issue. Animals that walk upright on two legs (bipedalism) have their center of mass (COM) over their hips. This means that when walking the muscles spanning the hip from trunk to leg have to contract on the weight bearing side because the COM is still in the center and the body would tip to that side otherwise. People who have had polio or have their hip muscles weakened for other reasons can't do that so they throw their COM over the weight bearing leg giving their gait a characteristic lurch (called a Trendlenberg lurch). For a pregnant woman there is a similar problem. The fetus and placenta are forward and shift the COM. To avoid falling over forward pregnant women have to shift the COM back again so that it winds up between her hips. The paper in Nature by Anthropologist Whitcome and her colleagues (Harvard and U. of Texas) show that a woman's spine is built differently from a man's in ways that enables them to do this more easily.

Until recently, hominin females spent most of their adult lives either pregnant or lactating. Pregnancy augments the mass of the human female abdomen by as much as 31% (6.8 kg), translating the position of the maternal COM 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 (Fig. 1a, b), it uniquely destabilizes bipeds whose supporting joints and two-footed support base lie solely under the hips (Fig. 1c, d). Such gravid instability can be counteracted by muscles, but sustained recruitment risks muscle fatigue and increases the likelihood of spinal injury.

Here's the figure (Fig. 1a, b, c, d) that goes with that text:

Figure 1: COM and lumbar lordosis during pregnancy.
a, Quadrupedal chimpanzee, non-pregnant. b, Quadrupedal chimpanzee, pregnant with no change in sagittal position of the COM with respect to the postural support base. c, Bipedal human female with typical lumbar lordosis and COM in approximate sagittal alignment with the hip. At a given 0.005-m COM distance from the hip, a 409-N upper body generates 2 N m torque at the hip. d, Pregnant human female with anteriorly translated COM, lacking positional adjustment of lumbar lordosis. The force of gravity, when more distant from the hip, generates a larger hip moment and an unstable upper body. With pregnancy, a 511-N upper body and a COM at 0.032 m from the hip increases the torque to 16 N m. e, Typical pregnant human female with naturally extended back and recovered COM by means of increased lumbar lordosis, a stable positional alignment with reduced hip torque (1.5 N m) but with exacerbated spinal shearing load. Open circle with cross hairs, COM in sagittal plane; filled circle, hip position in sagittal plane; arrow, direction of gravitational force.

The caption gives the torques, with and without postural adjustments afforded by lumbar lordosis (the concavity of the lower spine). Women have more wedge shaped vertebrae, permitting an increase in this swayback posture. But that adjustment comes at a cost, however: greater shearing forces bearing on specific small joints between the vertebrae, so they also have broader joint surfaces than men to prevent the shear from making the vertebrae slip. This difference is called a sexual dimorphism.

This dimorphism is also quite ancient:

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. If so, one expects these adaptations to be present in the genus Australopithecus, which is known to have been habitually bipedal at least two million years after the earliest bipedal hominins. It is intriguing that, of the two nearly complete known australopith lumbar segments, Sts 14 and Stw 431, the former has the typical human female pattern with three dorsally wedged vertebrae, whereas the latter has a more male-like pattern with fewer lordotic vertebrae. One possible explanation for this difference is that one female and one male A. africanus are sampled.

In other words, of the two skeletons of Australopitehcus we have, one appears to be a male and one a female (determined on grounds other than the vertebrae) and they seem to show the same sexual dimorphism as contemporary humans.

"I would imagine that Australopithecus women were uncomfortable during pregnancy in the same way that modern women often are," says Karen Rosenberg, a palaeoanthropologist at the University of Delaware, US.

The same spinal adaptations that served Australopithecus females - and still serve modern women - during pregnancy, most likely proved just as useful after birth, Rosenberg adds. After all, a woman carrying a baby in her arms may be even more front-heavy than one whose baby is still within. (New Scientist)

The adaptations aren't perfect, since backache is one of the most frequent complaints of pregnant women. Natural selection doesn't care about a little backache, however. All it cares about is seeing the baby gets born. The conservation of this sexual dimorphism over a few million years suggests that without these adaptations human reproduction would be much less efficient or perhaps not even possible, at least for bipedal posture.

Pregnancy is difficult enough for women without having to go six of the nine months on all fours.