Gravity is not kind to the pregnant woman. With 7-plus kilograms added to her tummy, a soon-to-be mother must stretch her lower back to balance the bulge. Now, a study suggests that women’s spines evolved to help them carry the extra weight. The findings show how the need to reproduce can drive evolution, say the authors, but some scientists argue that the changes in the spine stem from an already well-explained phenomenon.
Anatomists have long known that, because of the demands of childbirth, women’s bodies differ from men’s. Most notably, the female pelvis is more open, an adaptation that makes way for our big-brained species to emerge from the birth canal. Biological anthropologist Katherine Whitcome of Harvard University wondered whether women’s spines also had to adapt. When primates began to walk on two legs, they freed up their hands for other activities. But this new upright posture posed a problem for pregnant women. With a baby on board, a woman’s center of mass, the point on which gravity acts, shifts forward, away from the spine. This shift threatens to topple pregnant bipeds. (Expectant quadrupeds can resort to their hands for balance.) To realign this shifting mass, women arch their backs.