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.

Working at the University of Texas with Liza Shapiro, an associate professor of anthropology who studies the primate spine, Dr. 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, Ms. Whitcome said.