A new study from the University of Vienna, Austria, suggests that Cesarean sections are changing the trajectory of human evolution, altering physical characteristics in both mothers and babies. Trouble is, the researchers presented virtually no empirical evidence to support their extraordinary claim, and the credulous media simply took it at face-value.
Theoretical biologist Philipp Mitteroecker and his colleagues say the burgeoning use of Cesarean sections since the 1950s has led to evolutionary changes in the size of newborns and possibly the mother’s pelvic dimensions, making it increasingly difficult for babies to fit through the mother’s narrow birth canal.
The research, published in Proceedings of the National Academy of Sciences, points to a 10 to 20 percent increase in the rate of “obstructed birth,” a dangerous medical complication in which newborns cannot fit through the pelvis. Without the benefit of C-sections, many of these babies would die. But, because C-sections are now prevalent, selective pressure for smaller babies, or mothers with wider pelvises, could have eased up, which in turn could account for the higher observed rate of obstructed birth today.
Mitteroecker’s conclusion led the BBC (and other publications) to breathlessly declare: “Cesarean births [are] ‘affecting human evolution.’” Yet, outside of their computer models, the authors of the new study presented no real-world evidence, experimental or observational, to support such an incredible claim. What’s more, a closer look at the study reveals an exceptionally weak “selectional effect” at play, raising doubts about whether evolution could have acted so quickly over the course of such a short timescale, and whether or not C-sections can even influence the trajectory of human evolution.
Before we get into all this, it’s important to understand why Cesareans might be influencing human evolution. Childbirth, as many mothers can attest, is difficult. Unlike other primates, newborn humans are relatively large compared to the mother’s pelvic canal. This unfortunate contradiction has puzzled biologists, who hypothesize that an evolutionary tension exists between the baby, who wants to be as big and healthy as possible, and the mother, who needs a narrow pelvis to assist in upright walking and to avoid premature births. This “obstetrical dilemma,” as it’s called, has been medically addressed in recent times by virtue of the Cesarean section, which bypasses a vaginal birth, saving the life of the newborn—and often the mother.
Cesareans have been on the rise since the 1950s, leading Mitteroecker to wonder if the reduction in deaths caused by obstructed labor might be influencing human evolution, specifically the size of babies and/or the physical characteristics of the mother’s pelvis. And indeed, the prevalence of obstructed labor, or “fetopelvic disproportion,” in the parlance of the researchers, has also increased over time, according to recent data from the World Health Organization and other agencies. This observation seems to support the hypothesis that C-sections, by preventing premature deaths, have resulted in women with narrower pelvises and/or larger babies.
To find out, Mitteroecker’s team plugged data on modern rates of obstructed labor from the World Health Organization and large birth studies into mathematical models in an effort to explain the sudden (and apparent) increase in large babies. Overall, the models estimate that the rate of obstructed labor has seen an “evolutionary increase” of 10 to 20 percent since the 1950s.
This may sound dramatic, but in reality, if the models are correct, the increase is actually quite small. The global incidence of obstructed labor in humans is around three percent, so an increase of 20 percent means that instead of obstructed labor happening in 30 out of 1,000 births, it’s now happening at a rate of 36 in every 1,000 births. It’s important to keep in mind that these projections were churned out by a computer; no analysis was done to see if these predictions match actual rates of obstructed labor in the 1950s, or to see if babies are getting bigger and/or their mother’s pelvis’s are getting narrower. Also, recorded rates of obstructed births are all over the map, ranging from three percent through to 8.5 percent depending on the region.
“They also don’t control for any other factors that could be affecting fetopelvic disproportion—and assume that C-sections are the only factor in the evolution of fetopelvic disproportion—which is an oversight that is quite shocking for a published, peer-reviewed scientific paper,” Randy Olsen, a Senior Data Scientist at the University of Pennsylvania Institute for Biomedical Informatics, told Gizmodo in an email. “I don’t think it’s appropriate for them to make such a claim in their paper, especially based on a simple mathematical model that has only a small basis in reality.”
As Olsen notes, this study leaves more questions than it answers. How could evolution produce such measurable effects across just two to three generations? To see even a 10 to 20 percent “evolutionary increase” in obstructed labor, the selective pressure to produce big babies and/or narrower pelvises must be absolutely intense. It’s not clear from Mitteroecker’s study that this is the case. If it were, then the uptick in C-sections would be self-reinforcing, with bigger babies or narrower pelvises necessitating more and more C-sections.
We don’t know if that’s true. We do know that the rise in C-sections is being driven by plenty of factors that have nothing to do with obstructed labor, including women having babies later in life, and medical conditions like diabetes.
While it’s important to consider the effects of C-sections on the traits of mothers and babies moving forward, it seems highly unlikely that C-sections could ever influence human evolution on a global scale. The rates of this procedure vary across nations and regions, owing to medical, social, cultural, and economic differences. And for a new trait to trickle through a population, it would have to be particularly strong and advantageous. Most traits, even under strong selective pressure, require dozens, if not hundreds, of generations to proliferate thoroughly enough to become a fixture of human DNA.
The reality that human evolution is a slow, gradual process was largely ignored by the media, even though the study’s lead author acknowledged it. “I expect that this evolutionary trend will continue but perhaps only slightly and slowly,” noted Mitteroecker in the BCC. “There are limits to that. So I don’t expect that one day the majority of children will have to be born by [Caesarean] sections.”
So are C-sections affecting human evolution? Given what we’ve learned from this paper, the answer is simple: We still don’t know.
[PNAS]