Will the availability of C-sections give humans bigger brains?

Blogging on Peer-Reviewed Research

While Steve Jones might think human evolution has stopped, I have to say that that is impossible. If human technology removes a selective constraint, that doesn't stop evolution — it just opens up a new degree of freedom and allows change to carry us in a novel direction.

One interesting potential example is the availability of relatively safe Cesarean sections. Babies have very big heads that squeeze with only great difficulty through a relatively narrow pelvis, so the relationship in size between head diameter and the diameter of the pelvic opening has been a limitation on human evolution. We know this had to be a factor in our evolution: the average newborn mammal has a cranial capacity that is roughly 50% of the adult size, chimpanzee babies have heads about 40% of the adult size, but human babies have crania that are only 23% of what they will be in adults. While our brains have gotten larger over evolutionary time, they have not gotten proportionally larger in utero, because large-headed babies increase the difficulty of labor and cause increased mortality in childbirth. If childbirth could bypass the pelvic bottleneck, that would allow for fetal heads to grow larger without increasing the risk of killing mother and/or child.

And childbirth is a risky proposition for women; 529,000 die every year from this natural process (although only about 1% of those deaths occur in places where women have access to good, modern medical facilities — hooray for modern medicine). About 8% of those deaths occur from obstructed labor, where the fetus is unable to proceed through the birth canal for various reasons, and these are the kinds of birth problems that can be circumvented by C-sections. In practice, teaching health care workers how to carry out emergency C-sections has been tested in regions in Africa, where it has actually worked well at reducing maternal mortality.

This is the subject of an article by Joseph Walsh in the American Biology Teacher, which suggests that C-sections will have an effect on human evolution.

"Nothing in biology makes sense except in the light of evolution." This was the title of an essay by geneticist Theodosius Dobzhansky writing in 1973. Many causes have been given for the increased Cesarean section rate in developed countries, but biologic evolution has not been one of them. The C-section rate will continue to rise, because the ability to perform a safe C-section has liberated human childbirth from natural selection directed against too small a maternal pelvis and too large a fetal head. Babies will get bigger and pelves will get smaller because there is nothing to prevent it.

The evidence so far is entirely circumstantial, but Walsh makes an interesting case. There are several correlations that imply an effect, but I can't help but think there are alternative explanations that may swamp out any heritable, evolutionary effect. The kinds of evidence he describes are:

  • A known trend for increasing birth weight in the US, by about 40 g over 18 years in one study. It's there, all right, but these studies don't demonstrate a genetic component to increased size — it could be a consequence of better nutrition and medical care.

  • An increasing frequency of C-sections. Again, this isn't necessarily genetically based at all, but could be a consequence of fads in medicine, or social factors, such as an increase in the likelihood of medical malpractice suits making doctors more cautious.

  • Walsh describes a couple of studies that seem to show that cephalopelvic disproportion (small pelvis or large babies or both together) does have a genetic component. So at least it is likely that there are heritable variations in these parameters that could influence the likelihood of obstructed labor.

  • There is statistical variation in neo-natal mortality that varies with birth weight in a suggestive way. Low birth weight clearly puts infants at risk, and there is an optimum weight around 3600 grams for newborns that minimizes mortality. Death rates also rise with increasing birth weight above the optimum. There is some data that suggest that availablity of modern medical care and C-sections reduces infant mortality at larger birth weights.

That increasing availability of C-sections might lead to an evolutionary shift towards increasing cranial capacity at birth is a reasonable hypothesis, but I'm not convinced that it has been convincingly demonstrated yet. There are too many variables that effect brain size at birth to make a clean analysis possible; in addition, many of the measures are indirect. Often, we use birth weight as a proxy for cranial capacity, and that means the numbers and correlations are sloppier than they should be. Many of the measurements made are of factors that are readily influenced by the environment, which makes it difficult to imply that these are the product of genetics.

So the idea is weakly supported, but tantalizing. Even as a purely theoretical exercise, though, what it does say is that it is obvious that human culture cannot end human evolution…all it can do is shape the direction in which it can occur.

Walsh J (2008) Evolution & the Cesarean Section Rate. The American Biology Teacher 70(7):401-404.