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.
Whitcome now argues that this arching is possible because of an evolutionary adaptation in the female spine. She and her colleagues studied 19 women throughout the duration of their pregnancies, asking them to stand on force plates and walk through motion sensors. The data, presented online 12 December in Nature, shows that a woman's center of mass shifts as much as 3.2 centimeters forward during pregnancy. To compensate, the angle of curvature in the lower spine increases by as much as 28°, creating an S shape that pulls the shoulders back and redistributes the weight so the center of mass is again above the hips rather than out in front.
By comparing skeletal samples of men and women between the ages of 20 and 40, the researchers pinpointed the differences that allowed women to perform this balancing act. Women had three wedged vertebrae whereas men had only two, allowing the women to create a more extreme curve. In addition, the women's facet joints, which connect the vertebrae and prevent twisting of the spine, were larger relative to their body size and positioned differently to redirect the weight and enhance resistance.
Women still experience back pain during pregnancy because of the baby's weight, Whitcome says, but without these spinal features, the discomfort might be worse and moms would be unsteady. "Though the system isn't perfect, it gives [pregnant] women the advantage they need to stay afoot," she says.
A final experiment gave clues to when this adaptation may have arisen. When Whitcome's team compared the spines of one male and one female Australopithecus africanus, an early bipedal hominid that lived roughly 2 million years ago, it found differences in the number of wedged vertebrae. This indicates, Whitcome says, that the evolution of the spine to deal with carrying a baby's weight began quite some time ago.
However, anatomist Owen Lovejoy of Kent State University in Ohio is not sure the spine evolution story holds water. Although it's clear that pelvic adaptations evolved to open the birth canal, he says, the spinal changes that Whitcome's team observed may simply be a response to this development rather than an adaptation in its own right. "This is a secondary consequence," he says. Regardless of the mechanism, paleoanthropologist Karen Rosenberg of the University of Delaware in Newark says that the findings emphasize the unique pressures childbirth puts on a woman's frame. "Once you have bipedalism, you have to make other accommodations," she says.