CAMBRIDGE - A hugely pregnant woman teetering along is actually a sublime feat of evolutionary engineering - ages of adaptations have gone into her backbone to make sure she doesn't fall on her face, scientists say.
Women's spines have evolved differently from men's to better carry the weight and awkward proportions of pregnancy, according to research being published today by the journal Nature. The study sheds new light on how humanity's ancestors adapted biologically to meet the rigors of walking upright.
"Several million years of adaptation went into making [human] females able to move more easily, comfortably, and safely during pregnancy," said Katherine K. Whitcome, a Harvard anthropologist and one of the authors of the study, which also found that the female spines of our hominid forebears were markedly different from those of apes. That suggests changes in the spine represent a crucial part of human evolution.
Among other things, the research implies, those changes allowed pregnant hominids to walk without tipping over.
That idea might seem ludicrous, but bipedalism - lumbering around on two feet - itself is an evolutionary feature so strange that anthropologists still can't fully explain why humans assumed the vertical position. But most scientists believe it was the ability to walk and run upright, far more than brain power, that allowed predecessors of modern humans to break from the apes and gradually emerge as the planet's dominant species.
The shape of women's spines and vertebrae represents one of the evolutionary distinctions between humans and apes, according to the new research. Nature favored the adaptation, the scientists say, because it reduced stress on the female spine and it lent extra spryness when foraging for food or dodging the fangs and claws of rival carnivores.
"Being bipedal is a big part of being human. But it's also a pretty bizarre evolution," said Daniel E. Lieberman, professor of biological anthropology at Harvard. "It created a lot of problems that had to be resolved by natural selection. Among them, how were pregnant women going to carry that extra weight without falling over?"
Human spines have an unusual forward curve in the lumbar region of the lower back. Starting more than 2 million years ago, in the early hominid ancestor called Australopithecus, spinal curvature in females spread across more vertebrae than in males. Moreover, the joints between the vertebrae became larger in females and assumed a different shape to offset "shear forces" - the sideways grinding generated by the weight of pregnancy.
This shifted the load of baby-to-be in a way that reduced stress on the spine while also giving pregnant women greater stability.
"Without the adaptation, pregnancy would have placed a heavier burden on back muscles, causing considerable pain and fatigue," said Liza J. Shapiro, a University of Texas anthropologist who conducted the study with Whitcome and Lieberman.
The female-male difference in spinal curvature does not appear in chimpanzees, meaning it was an adaptation that occurred after human ancestors started walking upright - a fact of significance for understanding human evolution.
"This broadens our understanding of how our success as a species came to include the vertebral column," Whitcome said. "These changes not only made females more comfortable, they provided an evolutionary edge."
The changes in spinal structure started long before humans were even human. Australopithecines were the first vaguely humanlike beings to walk upright, according to the fossil record. This locomotion was probably an adaptive response to dwindling jungles in Africa - the new creatures were able to traverse wider savannas and seek a broader variety of food.
These were tough times: Pregnant women had to be agile enough to escape the jaws of predators and also forage for wild tubers up until nearly the moment of giving birth. So a spine adaptation that allowed easier movement - in addition to not tipping over - was critical in the progression toward modern humanity.
"These adaptations, over time, meant life or death for hunter-gatherer societies," Lieberman said.
Such fleetness is perhaps not absolutely necessary for modern women, but stability and relative comfort remain a happy legacy of hominid evolution. An age-old saying has it that, for women, being born and succumbing to death are just brackets for a life of backache. But it could have been a lot worse, scientists say.
"Any mother can attest to the awkwardness of standing and walking while balancing pregnancy weight in front of the body," Shapiro said. "Yet our research shows their spines have evolved to make pregnancy safer and less painful than it might have been if these adaptations had not occurred."
Colin Nickerson can be reached at nickerson@globe.com.
