The human spine evolved differently in males and females in order to alleviate back pressure from the weight of carrying a baby, according to research spearheaded at The University of Texas at Austin.
The lumbar differences are documented for the first time in the Dec. 13 issue of Nature.
The researchers believe the adaptation first appeared at least two million years ago, in the early human ancestor Australopithecus. The male-female difference does not appear in chimpanzees, meaning the evolution to walking upright led to the adaptation.
"Natural selection favored this adaptation because it reduces extra stress on a pregnant female's spine," said University of Texas at Austin anthropologist Liza Shapiro, who conducted the research with graduate student Katherine K. Whitcome, now a postdoctoral fellow at Harvard University. "Without the adaptation, pregnancy would have placed a heavier burden on back muscles, causing considerable pain and fatigue and possibly limiting foraging capacity and the ability to escape from predators."
Harvard anthropologist Daniel Lieberman also contributed to the study, which shows the key differences between males and females appear in the lower back, or lumbar portion of the spine.
Human spines have a unique forward curve in the lumbar region, but the curve extends across more vertebrae in females. That helps offset harmful forces that might occur on the spine when pregnant women lean back or hyperextend their spines to balance the weight of the fetus, Shapiro said. The joints between the vertebrae also are larger in females and angled differently than in males to better support the extra weight.
"Any mother can attest to the awkwardness of standing and walking while balancing pregnancy weight in front of the body," Shapiro said. "Yet our research shows their spines have evolved to make pregnancy safer and less painful than it might have been if these adaptations had not occurred."