By Tara Haelle

Photo by Tara Haelle

Before pregnant women can even think about how they will balance motherhood in their own lives, their bodies are finding a certain balance in their bones. The discovery that women’s bodies have adapted throughout the evolutionary process to keep them from tipping over while carrying a baby earned a UT researcher, a UT alumna and a Harvard researcher the tongue-in-cheek Ig Nobel Prize, an annual award from Harvard’s Annals of Improbable Research magazine.

Although illuminating why pregnant women don’t “topple over” is not quite what their research aspired to show, University of Texas anthropology professor Liza Shapiro is excited that the research has garnered attention from an award that honors “achievements that first make people laugh, and then make them think,” as the Improbable Research Web site states.

“I am thrilled to be a part of this,” Shapiro said. “I have received many congratulatory notes from colleagues and other UT scientists, some of them quite accomplished with other accolades, but nevertheless envious of the coveted Ig Nobel.”

Co-authors and fellow researchers Katherine Whitcome and Daniel Lieberman accepted the award at the 19th annual Ig Nobel Prize Ceremony Oct. 1 this year. Whitcome, who went on to do her postdoctoral research at Harvard after graduating from UT, is the lead author on the paper, “Fetal Load and the Evolution of Lumbar Lordosis in Bipedal Hominins.” The research outlined in the paper comprises the bulk of Whitcome’s doctoral dissertation after she went to work with Shapiro because of Shapiro’s research on evolution of the spine in primates.

Whitcome began by studying the vertebrae of contemporary men and women, using the skeletal collections at the Cleveland Museum of Natural History and the Terry Collection at the Smithsonian’s National Museum of Natural History. She said these are two of the best-documented skeletal collections in the U.S. because they offer additional information, such as body type and disease history, about the specimens.

“We want to look at fit individuals because we’re looking at evolutionary questions,” Whitcome said. “You don’t want skeletons with osteoporosis or other conditions that would complicate the research.”

By taking measurements of the vertebrae, Whitcome determined that females have more of a curve in their lower back, formed by three of the lower vertebrae found in women. Only two of the three vertebrae in men contribute to the curvature, which enables humans to walk upright effectively.

“Those features that are unique to women are explained by the challenges they face during pregnancy in terms of maintaining their balance during pregnancy,” Whitcome said. She said the center of mass in pregnant women moves about 3.2 cm ahead of its normal place during the latter stages of pregnancy. “That’s not a huge distance, but it has a big consequence in terms of energy required to stay balanced.”

She said a pregnant woman’s options are to rely on lower back muscles to stay balanced or to lean back. Because constantly relying on those muscles could lead to strain, women lean back to realign the center of mass over their hip joints, thereby loading the vertebrae and bending the vertical column backward a bit.

“If you think of our ancestors foraging for food, a pregnant woman would need more food to provide energy for a growing baby and probably be foraging more hours of the day,” Whitcome said. “It was important to stay afoot, and anything that would compromise a pregnant woman’s ability to be active would lower her reproductive success.”

After finding the gender differences in the vertebrae of contemporary skeletons and realizing they are related to the biomechanics of pregnancy, Whitcome did a comparative study of humans’ closest primate relatives.

Liza Shapiro

They did not find the adaptation in chimpanzees, orangutans or other primates, who walk on four limbs and carry a fetus below them.

“There are certain features associated with bipedalism,” Shapiro said. “The bone responds to the activity of walking.” When a baby starts walking, the full extent of the curvature develops simultaneously  The larger curvature in females, Shapiro said, appears to be a genetically based evolutionary advantage.

“Having to carry the fetus to term puts a strain on your body,” Shapiro said. “Early in evolution, without medical care, females would have to get through that without strain or back pain. This is unique to walking on two legs.”

Bipedalism dates back about six million years, so Whitcome looked for the oldest skeletal materials available of early hominins and found two early bipeds dating back about three million years: a female Australopithecus Afarensis named STS14 and a male Australopithecus Afarensis called STW431. Despite the difficulty of finding well-preserved vertebrae—a spongy bone that crushes easily during fossilization—both skeletons fortunately had well preserved skeletal lumbar remains.

“The female had the same features as modern human women, and the Australopithecus Afarensis male showed very similar anatomical features to modern human males,” Whitcome said. “The pattern is evident from the early fossil records.”

Whitcome said their findings are preliminary, so they are cautious since they have only these two specimens and sexing early fossils is not always reliable. But the finding in hominin ancestors and its absence in non-bipedal primates is strong evidence of the importance of the evolutionary adaptation.

“Those unique female characteristics are a solution to a biomechanical problem that came about six million years ago when our ancestors started walking on two feet,” Whitcome said.

Shapiro said scientists have discovered that changes in the spine occurred before another female adaptation for birth: the enlargement of the pelvis to accommodate the larger brains of later hominins, our own genus Homo.

“Bipedal walking is the first human-like feature we see,” Shapiro said. “Before stone tools, before big brains, it’s the hallmark of the evolution of humans.”

That hallmark has not necessarily come without a price though. Shapiro has always been interested in the evolution of bipedalism and the impact that walking has on our bodies.

“We try to figure out what is bipedalism good for and why did it evolve, but it also comes with costs,” she said. “There are many aspects of our body that suffer from walking on two legs at all times. What are the compromises your body is making because of this walking and how does this affect your body negatively?”

Shapiro explained that evolution must work with what was already present in a creature. The spine curvature helps with balancing the upper and lower body, but it puts the vertebrae in a precarious situation.

“There’s new interest in medicine and in taking an evolutionary perspective in understanding the body,” Shapiro said.

If people understand there is a natural weakness that can lead to problems like fractures and slipped disks, then it may help in understanding how to strengthen those muscles to lessen the likelihood of injury.

“Everybody is susceptible to that, and it’s interesting to me that it comes from evolutionary reasons,” she said.

She described how leaning back while pregnant is necessary for countering the mass of the fetus on the front, but it puts a lot of pressure on the spine. As that curve is accentuated, the vertebrae begin pulling apart and pushing into one another, creating a stress that can damage the vertebrae. The extra curvature in women is one way evolution has corrected somewhat for that weakness.

“Natural selection would act to select for an adaptation in the spine that would enhance the survivability of females that are pregnant,” she said. “If you have to find your food and escape predators, you can’t hobble around while pregnant. Whoever had a spine could better survive, and there’s nothing more important in evolution than your babies surviving.”