Evolution throws a helpful curve toward pregnant women

Evolution throws a helpful curve toward pregnant women

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

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 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, Whitcome found that the lumbar, or lower back, curve in women extends across three vertebrae, as opposed to 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 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, Whitcome said, women 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.

Working at the University of Texas with Liza Shapiro, an associate professor of anthropology who studies the primate spine, Whitcome found that the differences between male and female spines do not show up in chimpanzees. That suggested that the changes occurred in response to the problems caused by walking upright.

When she moved on to Harvard and started working with Daniel Lieberman, an anthropologist with expertise in primate fossils, she was able to examine two sets of fossilized vertebrae for the telltale signs of evolved flexibility. Of the two samples, she found the three-vertebra arrangement in one and not in the other. As it happened, separate evidence from those skeletons suggested strongly that the extra-curvy spine belonged to a female and the other belonged to a male.

"It was very exciting" to have the fossilized pieces of the puzzle fall into place, Whitcome said.

As solutions go, the forward spine is only partly successful, Shapiro said, since women still commonly complain of back trouble and pain during pregnancy. Even the basic forward curve that promotes balance in upright walking is "not a structurally ideal solution," she said, since it can lead to instability and even fractured vertebrae.

And that is the difference between the way that evolution works and the way that actual designers do their job, 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."

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," 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."

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.

Anthropology has extensively explored the evolution of the female hip bones, which expanded over time to accommodate the evolutionary growth of the heads of human babies. But Shapiro said that pregnancy and the lower spine constituted new ground for evolutionary biology.