Wed. Mar 22nd, 2023
Look at the size of that head.
Enlarge / Look at the size of that head.

People have a relatively difficult time giving birth. Compared to other species, human newborns are large for their birth canal. This means humans are at a surprisingly high risk of our babies getting stuck during delivery – which can be fatal to the mother, the newborn, or both. Some estimates suggest it occurs in as many as six percent of births worldwide.

The fact that this is an ongoing problem for our species has puzzled evolutionary scientists. Why hasn’t evolutionary pressure pushed the species away from such a common and fatal situation? According to an international research team made up of biologists, a philosopher and a pediatrician, the answer lies in a complicated push and pull of several evolutionary factors between the baby and the mother. But the researchers are discovering that there’s also an opportunity for us humans to tip our own evolutionary scales — with C-sections. Using a mathematical model, published in the Proceedings of the National Academy of Sciences, the researchers discover that over time, modern medicine could push human evolution off course and bring bigger babies to the world.

Fall off a cliff

For starters, the sticking point is that there are competing evolutionary pressures for both the mother and the newborn, according to the study authors, led by biologist Philipp Mitterroecker at the University of Vienna, Austria. Humans have big brains, which is great, and you’re more likely to survive in life if you have a big brain (and a high birth weight). However, you are more likely to survive birth if you are small enough to fit through the birth canal. This means that the evolutionary pressure acting on an entire population pushes in the direction of bigger babies – until big babies suddenly become too big and the survival rate plummets.

At the same time, there seems to be some sort of evolutionary pressure that keeps the human pelvis small. Clearly, such pressure must exist, otherwise mothers with larger pelvises would simply be more likely to survive childbirth and larger pelvises would be more common in humans.

An explanation for small pelvic size is bipedalism. Human ancestors walked upright before our brains got that big, which is why babies with larger brains were born using pelvises already adapted to bipedalism. Some researchers have suggested that a wider pelvis would not be suitable for bipedalism, but the evidence for this is limited. Another explanation is that a narrow pelvis is more likely to prevent organ prolapse – that is, to prevent the intestines from sliding out – and to support a pregnancy.

So while selection pressure pushes for bigger babies, they push for narrower pelvises. This looks much like the pattern for newborn size: selection pressure acting across the population pushes toward smaller pelvises, until small suddenly becomes too small and childbirth becomes dangerous. The researchers describe this pattern as a “cliff edge,” as the survival rate steadily increases until it suddenly plummets.

If you take 100 births and compare the size of the newborn and the birth canal, there will be many different levels of difficulty and danger. In a small minority of cases, the newborn will be on the small side and the pelvis large, and birth will be relatively easy. In most cases, there is a larger newborn and a smaller pelvis and the birth will be more difficult. And at the other end of the scale, another small minority will have an oversized newborn, undersized pelvis and dangerously stunted birth.

In their mathematical model, Mitterroecker and his colleagues found that the cliff edge pattern explains the puzzle. The small minority of dangerous births persists because it is not possible to fully adapt to these competitive pressures – evolution pushes in too many different directions at once.

C-sections offer a way out

Fortunately for humans, caesarean sections offer a way around this evolutionary problem. Since the middle of the last century, the procedure has grown in safety and availability. The researchers also examined whether the growing availability of cesarean deliveries could reduce the pressure to have smaller babies, which could lead to larger babies and an increasing need for cesarean deliveries over time. Their model suggested they may already have.

This conclusion has generated headlines reporting that cesarean section changes human evolution, but it’s less clear-cut than that. Because the evidence comes from a mathematical model, the conclusions seem more like a prediction than an explanation; the findings suggest that if we look into the real-world data, there’s a chance we’ll find it. However, a model also has to make many assumptions in order to arrive at such a conclusion.

The researchers proceeded from a conservative estimate: they assumed that only three percent of deliveries are obstructed (and therefore require a caesarean section). If things like head size are indeed passed on genetically from one generation to the next (rather than being caused purely by factors such as diet), their model suggested that over two generations of cesarean deliveries we would see a 20 percent increase in the number of births that would be hindered. That brings us from three percent of births to 3.6 percent.

It’s hard to verify this in real-world data, because cesarean rates are already much higher than 3 percent, for a variety of reasons. And of course, there may be things other than stunted labor that keep babies small, such as a mother’s body’s ability to grow and support the baby. It does mean, however, that the fuss over the best caesarean rate may have a new factor to consider: Perhaps people will need them more and more over time. It’s a good example of how evolutionary pressures are still acting on our species.

PNAS2016. DOI: 10.1073/pnas.1612410113 (About DOIs).

By akfire1

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