clock menu more-arrow no yes mobile

Filed under:

Has the rise in C-sections affected human evolution? This scientist predicts yes.

Evolutionary theory and mathematical modeling suggest babies' heads may be growing slightly larger.

Getty Creative Images
Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

Human ingenuity increasingly allows us to fight back against “natural selection” and, in effect, influence the path of our own evolution.

Take Cesarean sections, the procedure in which babies are born via surgical incision rather than through the mother’s birth canal. Some form of the procedure has been around for hundreds of years, but only in the past few decades has it become commonplace.

In the US, C-sections now account for 30 percent of all births, according to the Centers for Disease Control and Prevention. But back in 1970, that figure was around 5 percent. So while C-sections have only been widely available to mothers for just a couple of generations, already scientists are speculating that the procedure is affecting human evolution.

Here’s why.

Human evolution has favored big heads and small hips. This is great for running and thinking. It’s less ideal for childbirth.

There’s a reason human childbirth is so long, painful, and historically dangerous: A human baby’s head is designed to just barely squeeze through a mother’s pelvis. This makes humans more prone than other primates to a condition called fetopelvic disproportion, a life-threatening condition for the mother and child that’s implicated in around 3 percent of all births. With this condition, either the baby’s head is too big or the mother’s hips are too narrow for a natural birth.

Most other primates (and there are exceptions) have a pelvis-infant ratio more favorable for easy childbirth. We’re in the bottom right, the genus Homo.

Why would we have evolved such a misproportioned birthing process that can lead to the death of the child and mother? It’s the result of an evolutionary trade-off scientists call the “obstetric dilemma.”

The “dilemma” theorizes that over the course of our evolution, there have been dueling selection pressures that have resulted in tight-fitting childbirth — it’s beneficial for us to have both big heads and small pelvises.

Big heads mean more room for big brains. Narrower pelvises make it easier to be bipedal, to stand, walk, and run on two feet. Nature has found an optimal middle ground that works, most of the time. But that middle ground has limited the degree to which our hips can grow narrow or our heads can expand.

Thanks to better availability of the C-section, the “obstetric dilemma” is now actually becoming less and less of a dilemma.

Where C-sections are available, the risk of an obstructed pregnancy drops considerably. “Most cases of fetopelvic disproportion … were lethal without C-sections,” Philipp Mitteröcker, an evolutionary biologist at the University of Vienna, writes me. “Hence these mothers were not able to pass on their genes encoding for a narrow pelvis and or a large fetal size to the next generation.”

C-sections changed birth survival rates, “which is, per definition, a change of selection pressure,” Mitteröcker says. And when selection pressures change, we evolve.

Mitteröcker theorizes that might be happening now in a new paper published this week in the Proceedings of the Academy of Science. With the C-section, he posits, human can keep getting narrower hips and bigger brains without suffering the consequences of more babies and mothers dying in childbirth.

Mitteröcker doesn’t have direct evidence for this; rather, he and his colleagues have worked out a mathematical model that predicts how many more cases of fetopelvic disproportion ought to be occurring now that C-sections are commonplace. And according to this model, the rate of fetopelvic disproportion has risen from 3 percent to around 3.66 percent in the past few decades.

Put more simply: He predicts more babies whose heads are too big for their mothers’ hips, because, presumably, the genes that code for narrow hips and big heads have been allowed to propagate.

This is just a prediction, he says. “To my knowledge, this has not been shown empirically yet.”

That’s partly because while there’s good data on the number of C-sections performed in the US, and in many countries around the world, it’s hard to know how many of those C-sections were done for fetopelvic disproportion and not something else — like other medical complications or giving birth to multiples. (Many health care experts now argue that C-sections, which are costly and can be risky, have become an overused procedure.)

”C-sections that matter for evolution are only the ones that actual saved lives and hence were really necessary,” Mitteröcker says. “Our model says nothing about the many C-section carried out for other reasons.”

There are some other clues that this evolution might actually be happening. There are studies that suggest our skulls are larger than those of people who lived 150 years ago. And average birth weight increased in recent decades as well, suggesting there are more big-headed babies than ever before. (Though these changes may also be signs of improving nutrition and health care.)

In any case, Mitteröcker says it’s unlikely that our heads will increase in size forever. “The selection toward larger babies is limited by the mother's metabolic capacity,” he writes. Carrying huge babies for nine months is hard work.

Again, this is just a hypothesis. Mitteröcker says it would take a study of many generations of births, complete with hereditary data and skeleton size data, to confirm the prediction. “We are about to carry out these studies,” he says.

But it’s not too far-fetched. There are signs of evolution in our own bodies if you know where to look.