Women evolved curvier spines than men to stop them wobbling under the weight of pregnancy, researchers say.
Thank goodness she has a curvy spine
Harvard University experts say without the extra bend, humans' upright-walking ancestors would not have been able to escape predators while pregnant.
They would also have been crippled by back pain, says the Nature study.
The theory was strengthened when the difference was spotted in the female remains of Australopithecus, a prehistoric "relative" of humans.
The transition to upright walking was one of the key shifts in the evolution of man, but it had one unwanted consequence for pregnant females.
In primates, the weight of a pregnancy falls relatively comfortably under the belly, but in humans, it sits in front, pushing the body's centre of gravity forward, and throwing it out of balance.
Both men and women have a curve in the lower section of their spines, but in women, the Harvard researchers found, the curve extends over a longer section of spinal vertebrae.
The difference lets women adjust their posture to keep themselves in balance and in less discomfort from lower back pain, even in the last few months of pregnancy, when the abdomen can weigh almost 7kg more than normal.
The researchers said that if the change had happened through direct evolution following the shift to upright walking, then some hominids - our prehistoric ancestors - would also show it.
Examination of remains of Australopithicus, which lived approximately two million years after the first bipeds, revealed the same difference between males and females.
Harvard anthropologist Liza Shapiro said: "Natural selection favoured this adaptation because it reduces extra stress on a pregnant female's spine.
"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."
Spinal expert Dr Peter Dangerfield, from the University of Liverpool, said that an alternative explanation for the difference between the sexes also lay in childbirth, as a side-effect of the evolution of a female pelvis capable of giving birth to the relatively large-skulled human baby.
He said that the evolutionary shift to upright-walking had produced several consequences for humans - not all of them positive.
"Now, of course, we have the high morbidity in older adults from back pain, which is a direct result of standing upright."
STAYING UPRIGHT: SPINAL SECRETS
In primates, the weight of pregnancy does not affect the centre of gravity or place extra stresses on the spine.
In humans, the upright posture cannot cope as well, as the centre of gravity is pushed forward later in pregnancy.
Scientists believe that human females evolved a more pronounced bend in the lower spine compared with men to allow them to lean further backwards and stay in balance.