Within just two generations, the cesarean section has revolutionized the birth process for millions of women and their babies. Now researchers are making headlines with the theory that the dramatic rise in the procedure is set to affect the very way we evolve as a species.
Human birth poses an obvious—and much studied—predicament: Walking on two legs requires a smaller pelvis, while giving birth to our babies (and their large heads) is most easily done with a large one. In the bargain, somewhere between 3 and 6 percent of women giving birth worldwide have babies too large to fit through their narrow birth canal. This predicament, known as obstructed labor, often results in death for one or both.
The cesarean section has all but eliminated the danger, and as a result, saved a group that might have been biologically doomed. The babies who would have previously been lost are making it into the world, leading to an evolutionary change in the way natural selection responds to the obstetric dilemma, and removing the constraints, a team of researchers led by Dr. Philipp Mitteroecker at the University of Vienna, theorize in a new study published in the Proceedings of the National Academy of Sciences. Basing their theory on a mathematical model built using existing data sets, the authors say nature is not reacting by eliminating these women and their progeny, but instead report an up to 20 percent increase of women with smaller pelvises giving birth to too-large babies.
“Women with a very narrow pelvis would not have survived birth 100 years ago,” Dr. Mitteroecker told BBC News. “They do now and pass on their genes encoding for a narrow pelvis to their daughters.”
The study, which again collected no data, is based on a number of assumptions and its findings, as the researchers fairly note (and journalists often ignore) comes with limitations.
That 20 percent reported change in obstructed birth is extrapolated from an already small number of incidences compared to the total number of women giving birth. Over fifty years, the troublesome births increased from 30 in 1,000 to just 36 in 1,000 births. In the U.S. alone, four million babies are born each year, meaning there were around 25,000 more obstructed births last year than in 1960.
But even if the data was complete and flawless, assigning triggers for human evolution in real time is wholly unrealistic, Dr. Louise Johnson, a Lecturer in Population Genetics at the University of Reading’s School of Biological Sciences, told The Daily Beast.
“Evolution can be pretty fast, but there has to be a lot of death,” Johnson said. “Our whole environment is in so much flux. It can be very hard to disentangle it all.”
Additionally, built into Mitteroecker’s model are assumptions about the supposed evolutionary benefits of smaller pelvises, and cursory attention paid to other factors in the environment like maternal weight, genetic offering of men, and the tendency especially in Western women to give birth later in life—meaning new first-time mothers are shorter, heavier, and older, all which can affect pelvis size.
The study neither takes into account the risks associated with cesarean births, which increase with each child, nor the underlying factors driving the skyrocketing rates of medically unnecessary cesarean births over the last few decades in developed countries. In the United States one-third of all babies are born by cesarean, up from 5 percent in 1970 and 20 percent in the 1990s, and they don’t always end with improved outcomes.
“Humans probably are evolving but we’re not very good at working out why. Even if you have a population that you can study very very intently— there are sets scientists who have some sheep on an island and know everything about every single individual and its ancestry and how much its weighed on every birthday for its entire life. Even in those situations it’s very hard to say, ‘This is the selection pressure. This is the change in the population that’s resulted.’ It’s hard stats to do that and I don’t think we can say that about humans here,” Johnson said.
“To say that this change in humanity has been wrought by cesarean sections over the past hundred years would need quite a bit more evidence behind it before I would believe it.”
For Karen Rosenberg, a biological anthropologist at the University of Delaware who studies human evolution and childbirth practices, the finding that cesarean births affect human evolution is obvious, even if, she says, Mitteroecker’s mathematical study failed to demonstrate how or why.
“It would never occur to me that cesarean sections would not have an effect,” Rosenberg said. “The idea that human behavior affects our evolution is a central idea in understanding of evolution. All kinds of things—when we cook our food, when we share food, when we build shelters—everything we do as cultural animals has the potential to affect our biology.”
Rosenberg notes that cesareans aren’t the first kind of childbirth advancements likely to prompt an evolutionary upshot and should be considered alongside other, non-surgical birthing practices employed by midwives and doulas that have also made birth easier and safer for women.
“Cesarean is one extreme from form of assistance during childbirth, but we have been assisting in childbirth in many ways for a very long time in our evolutionary history. Just having someone present at birth is a kind of assistance and that’s something that human women do while other animals generally do not. Having someone receive the baby as it passes, guide it out of the birth canal, or help with a cord around the neck, and very importantly, but often overlooked, provide moral support for women, all of these have been shown to have significant impact on birth outcomes.”
“The adaptation that humans have to the world is cultural whether that’s having a midwife to help you or cesarean section, I think of it more as points on a continuum than as different processes both of which have the potential to affect our evolution,” she said.
So while our dependence on cesareans—a drastic change in the way we bring our young into the world—is sure to have some effect on the future of human evolution, this study does little to illuminate what the eventual benefits or ramifications to our species may be. It seems doubtful we’ll know any time soon.