Those of you who have been pregnant, or have been a partner to someone who has been pregnant, are familiar with one among many common consequences: lower back pain. It’s not surprising—pregnant women are carrying this low-slung 7kg (15lb) weight, and the closest we males can come to the experience would be pressing a bowling ball to our bellybutton and hauling it around with us everywhere we go. This is the kind of load that can put someone seriously out of balance, and one way we compensate for a forward-projecting load is to increase the curvature of our spines (especially the lumbar spine, or lower back), and throw our shoulders back to move our center of mass (COM) back.
Here’s the interesting part: women have changed the shape of individual vertebrae to better enable maintenance of this increased curvature, called lordosis, and fossil australopithecines show a similar variation.
The first step is to document the phenomenon. The posture and gait of pregnant women were studied kinematically through their pregnancy, and yes, they do extend their lower spines to shift the COM. The diagrams at the bottom, below, show the situation: on the left (c) is a non-pregnant woman with the center of mass marked. It’s located at the base of the spine, roughly centered over the hip joint. In (d), the woman is braced to hold her spine in the non-pregnant posture, and the weight of the fetus pushes the COM forward, to a place that would put her out of balance. In (e), you can see the preferred posture in pregnancy: the s-curve of the back is increased, moving the COM back over the hip joint.
Now let’s take a look at the anatomy. These are not measurements from the same women in the kinematic study, but from collections of skeletal remains where age and sex of the individuals were well-established from post-mortem records. The qualitative observation is that the three caudalmost lumbar vertebrae, and especially L3, show a characteristic sex difference. L4 and L5 in both sexes are typically wedge-shaped, to bend the spine forward; L3 in men tends to be more columnar, while L3 in women is also wedge-shaped, promoting a greater curvature.
The quantitative measurements back the observation up. The charts on the left above show the shape of the vertebra, and the area and angle of the zygapophyseal process. Just look at (a), though. That chart illustrates the degree of wedging of individual vertebrae; a bar that goes downward means the vertebra is trapezoidal, with the narrow end to the back, bending the spine foward; bars that extend upwards means the vertebra is wedge-shaped in the other direction; and bars that are close to the baseline mean the vertebra is columnar. Women’s vertebrae are black, men’s are white. Look in particular at L3; women’s L3 has a bar that is downward, while the men’s L3 is more columnar.
The variation is very large, though, which I’ll come back to later.
Now here are some measurements from a pair of australopithecine spines. It doesn’t quite have the same distribution as the above graph, but keep in mind that this one is from two individuals, while the modern human skeletal data is from 59 males and 54 females. Basically, though, the data show that one individual has more columnal vertebrae, while the other is more wedge-shaped. There is no independent determination of the sex of these two individuals, though, so all we can really say is that we see a morphological difference in the lumbar vertebrae that parallels a sex difference we see in modern populations.
So far, so good. I can believe that the authors have identified a statistical difference in the anatomy of lumbar vertebrae between males and females. However, I have some significant disagreements with the evolutionary interpretations of the paper. They claim to have identified evidence of an evolutionary novelty, but they haven’t tested the alternative hypothesis, that this is not an evolutionary adaptation, but a physiological one, and they haven’t adequately distinguished cause and effect.
My first thought on reading the results was that this is an example of developmental plasticity. Bones are flexible; they respond to stress with changes in shape and size that accommodate them to the pattern of activity they experience. This is an indirect evolutionary adaptation, of course — that bones have this response is a product of their genetic and developmental potential. However, the shape of an individual vertebra may not be so precisely specified, but may emerge as a product of the strains put upon it.
I’d make an alternative hypothesis. The female L3 vertebra is not wedge-shaped because women need to bear a fetal load, but instead, because women bear a fetal load, the L3 vertebra is wedge-shaped. In particular because their own data shows a significant amount of variability in vertebral shape, I’d be hesitant to assign a direct genetic cause on the pattern.
Unfortunately, the data in this paper do not touch on this possibility. All of it is from either pregnant women, or from skeletal remains of adults of child-bearing age. What I’d like to see is some developmental information, especially measurements of lumbar vertebrae in pre-pubertal children. If the difference precedes the child-bearing experience, then I’d agree that they’ve found a sexual dimorphism that could have an evolutionary cause.
Other data I’d like to see: is there a difference in vertebral morphology between women who have had children and those who have not? Another sex difference that could generate variation in vertebral morphology besides pregnancy is breast size; like carrying a fetus, women have another forward projecting weight that can shift the center of mass. Do large-breasted women have a consistent change in vertebral morphology that isn’t found in small-breasted women or men? How does obesity affect vertebral shape?
The authors have identified an interesting sexual dimorphism, but I think the paper was far too quick in assigning an evolutionary selective cause for the difference, and that it did not adequately examine the more likely (to my mind, at least) explanation of physiological adaptation.
Whitcome KK, Shapiro LJ, Lieberman DE (2007) Fetal load and the evolution of lumbar lordosis in bipedal hominins. Nature 450(7172):1075-8.