BEIJING, Jan. 21 (Xinhuanet) -- A biological difference between men and women apparently is what keeps a pregnant woman from losing her balance and falling.

    Scientists say there's are slight differences between women and men in one lower back vertebrae and a joint in the hip, which allow women to adjust their center of gravity.

    This elegant evolutionary engineering is seen only in female humans and our immediate ancestors who walked on two feet, but not in chimps and apes, according to a study published in the journal Nature.

    "That's a big load that's pulling you forward," said Liza Shapiro, an anthropology professor at the University of Texas and the only one of the study's three authors who has actually been pregnant. "You experience discomfort. Maybe it would be a lot worse if (the design changes) were not there."

    Harvard anthropology researcher Katherine Whitcomb found two physical differences in male and female backs that until now had gone unnoticed: One lower lumbar vertebrae is wedged-shaped in women and more square in men; and a key hip joint is 14 percent larger in women than men when body size is taken into account.

    The researchers did engineering tests that show how those slight changes allow women to carry the additional and growing load without toppling over — and typically without disabling back pain.

    "When you think about it, women make it look so very damn easy," Whitcomb said. "They are experiencing a pretty impressive challenge. Evolution has tinkered ... to the point where they can deal with the challenge.

    When the researchers looked back at fossil records of human ancestors, including the oldest spines that go back 2 million years to our predecessor, Australopithecus, they found a male without the lower-back changes and a female with them.

    (Agencies)