for National Geographic News
Wedge-shaped vertebrae in the lower back might be the key evolutionary adaptation that helps human females maintain a stable posture over the course of pregnancy.
According to anthropologists, the human adaptation is unique among primates and may have arisen shortly after early humans started walking upright.
(Related news: "Human Evolution Speeding Up, Study Says" [December 11, 2007].)
"Bipedalism challenges stable postures, because the abdomen expands in front of the body as the baby grows," said Katherine Whitcome, an anthropologist at Harvard University.
"This changes the mother's center of mass, which is a critical point in any three-dimensional body on which gravity acts."
As this center of mass shifts forward, pregnant women have to lean back and change their gait to stay steady.
This realigns the center of mass over the hips, knees, and ankles to correct the imbalance—but it creates another problem.
"It generates loading on parts of the vertebral column that are not normally under such stress," Whitcome said.
To find out how pregnant women keep their balance without damaging their spines, Whitcome and her colleagues studied 19 pregnant females between the ages of 20 and 40.
The team found that the key appears to be joints in the bony vertebrae that wrap protectively around the spinal cord. (Explore an interactive of the human body.)
These joints become heavily loaded whenever people lean back.
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