By JOHN SCHWARTZ (Seen in the New York Times)
Published: December 13, 2007
Published: December 13, 2007
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 a third and the center of mass shifts forward considerably.
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 leaning puts even more pressure on the spine and muscles.
And that, report researchers from Harvard and the University of Texas in the current issue of the journal Nature, is where evolution enters the story.
Anthropologists have long known that the lower spine in humans developed a unique forward curve to help compensate for the strains that arose when the primate ancestors began walking upright. Researchers looked for a mechanism that compensated for pregnancy’s additional burden 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, Dr. 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 shifting 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 Dr. Liza Shapiro, an associate professor of anthropology, 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 pressures of 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. Of the two samples, she found the three-vertebra arrangement in one sample and not in the other. Separate evidence suggested that the extra-curvy spine belonged to a female and the other to a male. “It was very exciting” to have the fossilized puzzle fall into place, Dr. Whitcome said.
As solutions go, the extra flexibility is only partly successful, Professor Shapiro said, since women still commonly complain of back trouble during pregnancy. And that is the difference between the way that evolution works and the way that actual designers do their job, Dr. Whitcome said: nature tinkers. 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.”
Karen R. Rosenberg, an associate professor and chairwoman of the anthropology department at the University of Delaware, characterized Dr. Whitcome’s work as “way cool.” Dr. Rosenberg, who studies the evolution of childbirth, said the paper was the first published research that asked whether pregnancy caused evolutionary changes in the skeleton. “In hindsight,” she said, “Duh, of course it does.”
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 leaning puts even more pressure on the spine and muscles.
And that, report researchers from Harvard and the University of Texas in the current issue of the journal Nature, is where evolution enters the story.
Anthropologists have long known that the lower spine in humans developed a unique forward curve to help compensate for the strains that arose when the primate ancestors began walking upright. Researchers looked for a mechanism that compensated for pregnancy’s additional burden 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, Dr. 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 shifting 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 Dr. Liza Shapiro, an associate professor of anthropology, 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 pressures of 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. Of the two samples, she found the three-vertebra arrangement in one sample and not in the other. Separate evidence suggested that the extra-curvy spine belonged to a female and the other to a male. “It was very exciting” to have the fossilized puzzle fall into place, Dr. Whitcome said.
As solutions go, the extra flexibility is only partly successful, Professor Shapiro said, since women still commonly complain of back trouble during pregnancy. And that is the difference between the way that evolution works and the way that actual designers do their job, Dr. Whitcome said: nature tinkers. 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.”
Karen R. Rosenberg, an associate professor and chairwoman of the anthropology department at the University of Delaware, characterized Dr. Whitcome’s work as “way cool.” Dr. Rosenberg, who studies the evolution of childbirth, said the paper was the first published research that asked whether pregnancy caused evolutionary changes in the skeleton. “In hindsight,” she said, “Duh, of course it does.”
No comments:
Post a Comment