Why Is Childbirth So Hard? Neanderthal Fossils Point to a New Answer

Why is human childbirth so hard?

Most of us learned the same answer at school.

Our big-brained babies have to squeeze through a pelvis built for walking on two legs. A wider pelvis would make birth easier, but walking harder.

Scientists call this the "obstetrical dilemma". It's been the go-to explanation for decades.

Now a new study of rare Neanderthal fossils suggests the story may be wrong, or at least incomplete.

The real trade-off may not be with walking at all. It may be with something far less talked about: the pelvic floor.

The old idea: birth versus walking

Walking upright changed the human pelvis. It became shorter and bowl-shaped, to support our organs and balance our weight over two legs.

At the same time, our babies' heads got bigger. Human newborns have a tight fit through the birth canal, much tighter than in most other primates.

The obstetrical dilemma says evolution had to compromise. A wider pelvis helps birth, but supposedly makes walking and running less efficient.

That idea has been used to explain why women's pelvises differ from men's, and even why human babies are born so helpless.

But it has been challenged for years. And testing it has been hard, because we've had very few well-preserved pelvises from our extinct relatives.

The study: rare Neanderthal pelvises

Two remarkable fossils

Pelvises rarely survive in the fossil record. They're fragile and often crushed.

This team, led by researchers at the University of Zurich, worked with two unusually complete Neanderthal pelvises:

They rebuilt both pelvises digitally in 3D. Then they compared them with two other reconstructed Neanderthal adults, a woman and a man.

Comparing with modern humans

For comparison, they used anonymised medical CT scans of modern people.

That included 34 children aged 1 month to 8.4 years, and 112 adults aged 14 to 49, split equally between women and men.

They mapped each pelvis with more than 100 anatomical landmarks. That let them compare shape precisely, including the three key planes of the birth canal.

They also estimated how the hip joints and muscles would have worked during walking.

What they found

Neanderthal pelvises were different from the start

The Neanderthal child's pelvis already looked distinctly Neanderthal at 18 months old.

Its hip bones flared out more, and it was wide from side to side but short from front to back.

So these differences weren't something Neanderthals grew into. They were built in from very early on.

The birth parts matched ours

Here's the surprise. Despite their different overall shape, Neanderthal and human pelvises showed a very similar pattern of differences between females and males.

In both species, females had larger openings at all three levels of the birth canal: the inlet, the middle and the outlet.

That suggests Neanderthal mothers faced the same basic challenge as human mothers: getting a big-brained baby out. The researchers conclude that their birth mechanics were probably broadly similar.

The walking parts didn't

The parts of the pelvis linked to walking told a different story.

Neanderthals had differently angled hip joints and shorter levers for key hip muscles. That points to a somewhat different way of standing and walking.

Crucially, the differences between species and the differences between sexes were statistically independent. In other words, the female "birth" adaptations weren't tangled up with the "walking" design.

A wider birth canal didn't mean wider hips

There's another blow to the old idea. In women, a wider birth canal didn't come with a wider distance between the hip joints.

Instead, the pelvis made room internally, partly through smaller thigh-bone heads and slimmer bone in one part of the hip. So a roomier birth canal doesn't automatically mean a less efficient walking frame.

So what is the real trade-off?

The authors point to the pelvic floor. That's the sling of muscles and tissue at the bottom of the pelvis.

It holds up your bladder, bowel and, in pregnancy, the growing baby. It also keeps you continent.

A larger birth canal means a larger opening for that sling to span. Stretch a hammock wider and it sags more.

So evolution may have had to balance a canal big enough for birth against a floor strong enough to hold everything up. The authors argue that canal size, more than shape, was the main target.

Shape matters less than we thought

The team also found that the shape of the birth canal's entrance didn't differ between women and men. Only its size did.

They re-examined two classic studies that linked pelvis shape to birth complications. Their reanalysis found no statistically robust link.

That matters beyond the fossils. Pelvic floor problems, such as leaking urine or prolapse, are common after childbirth.

What earlier research found

This study joins a long-running debate. Several earlier findings had already chipped away at the walking explanation.

A wider pelvis doesn't cost more energy

A 2015 study in PLOS ONE measured walking and running in men and women.

Pelvis width didn't predict how hard the hip muscles worked or how much energy people used. Women and men were equally efficient at both walking and running.

That undermined the core assumption of the obstetrical dilemma.

The mother's energy may set the timing of birth

A 2012 study in PNAS proposed the "metabolic hypothesis" for why human babies are born so helpless.

It argued that birth timing is limited mainly by how much energy a mother's body can supply. The pelvis played a smaller role than the classic story claims.

The pelvic floor hypothesis

A 2021 study in PNAS built a computer model of the female pelvic floor and varied its size.

As the opening got wider, the floor sagged disproportionately more, and tissue stresses rose. Making the tissue thicker helped, but couldn't fully make up for it.

That's the mechanism the new fossil study now supports.

Neanderthal babies had big brains too

A 2008 study in PNAS, by some of the same Zurich researchers, reconstructed a Neanderthal newborn and two infants.

Neanderthal brain size at birth was similar to ours, and most likely faced similar birth constraints. That fits the new finding of human-like birth canals.

How it fits together

Put together, the evidence increasingly suggests that walking isn't the main villain in the story of difficult childbirth.

A mother's energy budget and her pelvic floor may matter more. And Neanderthals seem to have faced the same trade-offs, over a very long stretch of evolution.

How much should you trust this?

Early. It's careful, detailed work, but it's based on very few fossils.

What makes it convincing

• The Dederiyeh and Palomas pelvises are among the most complete ever found.
• The team used precise 3D landmarks and a solid modern human comparison sample.
• The pattern fits several independent lines of earlier evidence.
• It's published in PNAS. The authors declare no competing interests.

What makes me cautious

• Only four Neanderthal pelvises were analysed, and they had to be reconstructed.
• Bones can't show soft tissue, so the pelvic floor conclusion is an inference.
• The walking models are static and use human settings. The authors call their locomotion conclusions preliminary.
• It doesn't show how Neanderthals actually gave birth.

What this means for you

This is mainly a story about evolution, but it has a practical echo.

If the pelvic floor has been under pressure for hundreds of thousands of years, it's no surprise that it often needs support today.

• Pelvic floor problems are common and treatable. Leaking when you cough, laugh or exercise is not something you have to put up with.
• Pelvic floor exercises help. They're often recommended during pregnancy and after birth.
• Ask for help. A GP, midwife or pelvic health physiotherapist can check your technique and suggest treatment.

The NHS explains treatments for urinary incontinence, including pelvic floor training. The US National Institutes of Health also has an overview of pelvic floor disorders.

In this talk, evolutionary biologist Barbara Fischer explains why human birth is so difficult and why evolution hasn't solved it:

What we still don't know

1. How did Neanderthals actually give birth? Soft tissue and fetal head position don't fossilise.
2. How did they walk? Dynamic models with body proportions and leg length are needed.
3. Were the species differences adaptive? They could reflect natural selection, or just chance.
4. How far back does this trade-off go? Older fossils could show when it began.
5. Will more fossils agree? Four pelvises is a small sample.
6. Could this help modern medicine? Understanding pelvic floor limits may improve care after childbirth.

My take: walking may be the wrong suspect

What I like about this study is how it uses our closest relatives as a natural experiment. Same birth problem, different body plan.

And the result challenges a textbook story many of us grew up with.

I'm cautious, because four reconstructed pelvises is a thin base for a big evolutionary claim.

Still, it adds to a growing pile of evidence. The hard part of human birth may be less about how we walk, and more about what holds us together.