Last week the NZ Herald carried a story, based on a new scientific paper, about how evolution had affected the shape of women's spines, resulting in an adaptation for weight-bearing during pregnancy. The paper (Whitcome et al. 2007) describes how men & women differ in the shape of their lumbar vertebrae, and relates this to the weight gain of pregnancy and how this affects women's centre of mass and consequently their posture. The clincher appears to be data from some australopithecine remains that also showed this feature ie this is an ancient adaptation, and one that's missing in chimpanzees.
Spinal curvature in women is obviously different from that in chimps & this difference is related to posture. And when we're pregnant, all that weight in the front alters our centre of mass & can throw us off balance. Pregnant women tend to compensate for this by throwing their shoulders back, so that the S-shape of their spines becomes even more pronounced. The lumbar vertebrae are key to this, & the original paper describes how in women these vertebrae are more wedge-shaped than in men, permitting greater curvature (lordosis) of the lower spine.
But is this the whole story? Perhaps not… Another science blogger (PZ Myers, of Pharyngula fame) has already done a great job of reviewing the paper, so I'll summarise his key points here.
Data on posture & gait were collected from 19 pregnant women, throughout their pregnancies. And they do show greater lordosis during pregnancy. The male/female comparisons came from skeletons where the age and sex were known (59 males & 54 females). These data showed a sex difference in the lower 3 lumbar vertebrae (L3-L5), and particularly in L3, which is wedge-shaped in women & more barrel-shaped in men. However, there is quite a lot of variation between individuals.
The australopith data come from only two individuals. One had lumbar vertebrate that were more wedge-shaped than the other, & so was assumed to have been female – but there was no independent confirmation of sex (eg from something like pelvic structure).
On the basis of this information, the paper's authors concluded that these spinal differences are an evolutionary adaptation. But it could just as well be physiological – after all, we know that bones are constantly reshaped during an individual's lifetime, in response to stresses placed on them. That is, that wedge-shaped L3 vertebra could be a response to weight-bearing during pregnancy. What other information could help to distinguish the two possibilities?
- All the information's from women who are pregnant, or from adults (ie old enough to have children). What about measurements from children & adolescents?
- Do the vertebrae differ in women who've had children, & those who haven't?
- Does obesity (in either sex) have an effect on the lumbar vertebrae?
In other words, there's a lot more to be done on this one.
But do go & read PZ's article – this is only a summary, after all.
Reference: K.K. Whitcome, L.J. Shapiro, & D.E. Lieberman (2007) Fetal load and the evolution of lumbar lordosis in bipedal hominins. Nature 450:1075-1078.