And I thought we were clumsier...

The New York Times had this piece...

Why Pregnant Women Don’t Tip Over

By JOHN SCHWARTZ

Pregnant women do not tip over, and researchers say an evolutionary curve has a lot to do with the reason why.

Anthropologists studying the human spine have found that women’s lower vertebrae evolved in ways that reduce back pressure during pregnancy, when the mass of the abdomen grows by nearly one-third and the center of mass shifts forward considerably. That increases pressure on the spinal column, strains the muscles and generally reduces stability.

Even without the benefit of advanced study in biomechanics, women tend to deal with the shift — and avoid tumbling over like a bowling pin — by leaning back. But the solution to one problem creates another, since it puts even more pressure on the spine and muscles.

And that, report researchers from Harvard University and the University of Texas in the current issue of the journal Nature, is where evolution enters the story.

The lower spine in humans had already developed a unique forward curve that helps compensate for the extra pressures that arose when the primate ancestors went from moving around on four limbs to walking upright. Researchers looked for an additional mechanism that might have compensated for the increased strain of pregnancy as well.

What they found, said Katherine K. Whitcome, a post-doctoral fellow at Harvard and the lead author of the paper, was evidence that evolution had produced a stronger and more flexible lower spine for women. After studying 19 pregnant subjects, Ms. Whitcome found that the lumbar, or lower back, curve in women extends across three vertebrae, as opposed to just two in men. And the connecting points between vertebrae are relatively larger in women, and shaped differently in ways that make the stack more stable and less prone to the bones shifting out of alignment or breaking.

Since the engine of evolution runs on the passage of genes from one generation to the next, pregnancy is a critical moment. Without that adaptation, Dr. Whitcome said, females would have been in considerably greater pain during pregnancy and might not have been able to forage effectively or escape predators, ending the pregnancy and the genetic line as well...

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