March 2017

No one needs to tell a new mother that human childbirth can be challenging, to say the least. While monkeys and apes (our closest evolutionary relatives) spend about two hours laboring during birth, human mothers spend closer to nine hours. And human childbirth is dangerous. In some impoverished areas, for every 100 live births, one mother dies in childbirth and four infants die in the first few days of life. However, access to resources and modern medical interventions, such as C-sections, saves many of those lives. Doctors estimate that 10-20% of births require a C-section for mothers and babies to have the optimal health outcomes. In the United States, one in three babies is born via C-section. Worldwide, C-sections are performed more frequently than any other surgery. C-sections have become so common in some populations that biologists have begun to wonder if the procedure, as it saves lives, could be shaping the course of human evolution…

Where's the evolution?

To understand how C-sections could influence our evolutionary future, we need to dive deep into humanity’s evolutionary past. Around five million years ago, through the action of natural selection over many generations, our African ancestors began to walk on just two legs. This major evolutionary transformation involved a host of skeletal and anatomical changes, resulting in the ability to walk upright and, as a by-product, narrow hips and a narrow birth canal. Then, about 2 million years ago, natural selection began favoring individuals with larger brains, and hence, larger heads. And thus began the evolutionary tug of war. After birth, individuals with large heads, large brains, and large bodies were better able to survive and pass on their genes to the next generation. However, babies whose heads were too large would have been caught in the birth canal and died.

This evolutionary process, in which individuals with extreme values of traits (in this case small or very large heads) are selected against, is called balancing selection. As in an evenly matched tug of war, balancing selection results in an equilibrium of sorts — in this case, an equilibrium where the value of a trait remains fairly constant over generations. Balancing selection also keeps genetic variation for the trait in the population. Since genetic variation is the raw material of evolution, balancing selection leaves a trait poised to evolve readily in the future if the environment changes and selection begins acting in a different way.

Of course, in the case of human head size, one might wonder why the human lineage did not evolve wider hips and birth canals, which would have allowed larger-headed babies to thrive. After all, it certainly seems as though these trait would have been favored by natural selection. Researchers have investigated this question, but so far, it is still a mystery. For whatever reason, these traits did not evolve. The human birth canal remained narrow, and the evolutionary tug of war continued, favoring babies with genes for large heads, but not too large.

Then in the 20th century, C-sections became safer and, in some populations, common. C-sections have the potential to resolve this tug of war in favor of very large-headed babies by removing much of the disadvantage these individuals face during the birth process. After all, if more large-headed babies survive birth, these individuals are likely to grow up to have similarly proportioned babies themselves. This could change the evolutionary scenario from one of balancing selection, to one of directional selection, leading to larger- and larger-headed babies over the course of many generations.

Playing out this scenario focusing on maternal survival also leads to an interesting conclusion. In our past, one might imagine, very narrow-hipped women had a survival disadvantage since they were more likely to die during childbirth. If C-sections save many of these women’s (and their babies’) lives, more genes that lead to narrow hips should remain in the human population. If C-sections are influencing human evolution in these ways, more large-headed babies and more narrow-hipped moms could lead to even more C-sections in a self-reinforcing cycle!

So is this evolutionary change actually happening in human populations? It’s far too soon to tell. Recent research suggests that, over time, even weak selection favoring large-headed babies has the potential to cause evolution when C-sections remove much of the risk that these babies and their mothers face during childbirth; however, C-sections have been common for only a few generations and data supporting this conclusion have not yet been teased out. While the rate of C-sections has increased in many societies (as one might expect if head size were evolving), there are a wide variety of known causes for this uptick; we can’t necessarily ascribe any portion of the increase to evolutionary change in head sizes and birth canals. Nevertheless, it’s an intriguing possibility that scientists will continue to investigate. Since key elements of all evolutionary processes are birth and death, anytime that large numbers of lives are saved or lost, we should consider whether this process might be contributing to evolutionary change. By saving lives, modern medicine may shape the human lineage’s evolutionary future in a wide variety of ways that we are just beginning to understand.