With all that growing weight up front, how is it that pregnant
women don't lose their balance and topple over? Scientists think
they've found the answer: There 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 Thursday's
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 easy,"
Whitcomb said. "They are experiencing a pretty impressive
challenge. Evolution has tinkered ... to the point where they can
deal with the challenge.
"It's absolutely beautiful," she said. "A little bit of
tinkering can have a profound effect."
Walking on two feet separates humans from most other animals.
And while anthropologists still debate the evolutionary benefit of
walking on two feet, there are notable costs, such as pain for
pregnant females. Animals on all fours can better handle the extra
belly weight.
The back changes appear to have evolved to overcome the cost of
walking on two feet, said Harvard anthropology professor Daniel
Lieberman.
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.
But what about men with stomachs the size of babies or bigger?
What keeps them from toppling over?
Their back muscles are used to compensate, but that probably
means more back pain, theorized Shapiro, who added: "It would be a
fun study to do to look at men with beer bellies to see if they
shift their loads."
(Agencies via China Daily December 13, 2007)