Experts at Harvard University suggest that while the transition to upright walking had major consequences for human evolution, it posed a problem for pregnant females.
The answer was to evolve a larger curve in the lower section of their spines, enabling women to adjust their posture to maintain balance, and reducing discomfort during pregnancy.
The findings are published in the journal Nature and anthropologist Liza Shapiro said: "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.
"Any mother can attest to the awkwardness of standing and walking while balancing pregnancy weight in front of the body.
"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."