Ever Wonder Why Pregnant Women Don’t Tip Over?

The reason is ... drumroll please ... evolution! Yes, 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. [...]

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. [...]

And that is the difference between the way that evolution works and the way that actual designers do their job, Ms. Whitcome said: nature tinkers. “A designer wouldn’t build something that has a tendency to fracture your vertebrae,” she said. For natural selection to favor one feature over another, “It doesn’t have to be an ideal solution,” she said. “It just has to be better.” In any case, she noted, “Without these adaptations, there would be more problems.”

If evolution provided relief for women in pregnancy, one might ask, what about the equally awkward morphology of men with beer guts? “You’re not the first one to ask this,” Prof. Shapiro said with a laugh, and said that their research shows that “men would not be as well adapted to a beer gut than a woman.”

Dr. Whitcome noted that in terms of the time that the evolutionary shift occurred, some two million years ago, “finding extra calories wasn’t likely,” so an early hominid primate with a potbelly would have been quite a rarity.

Sorry guys ... women rock!