Why is childbirth so difficult for humans? The Neanderthal pelvis provided an unexpected answer
Scientists have found a new argument against the conventional explanation for why human childbirth is so complex. A virtual reconstruction of two exceptionally well-preserved Neanderthal pelvises has shown that a wide pelvis could well have been compatible with efficient bipedalism.
Researchers suggest that the key evolutionary trade-off may not have arisen from the need to give birth to large-headed babies and walk on two legs, as had been believed for decades. The constraint may have been another challenge — maintaining a sufficiently strong pelvic floor to support the internal organs. However, this is still a hypothesis that has gained new support, rather than a definitively proven answer.
Why was childbirth linked to bipedalism in the first place?
The human pelvis fulfils several functions at once. It helps to support the body whilst walking upright, serves as an attachment point for muscles and, at the same time, forms the birth canal.
This gave rise to the famous ‘obstetric dilemma’. In its classic form, the evolution of the human pelvis was seen as a compromise: a wide birth canal is advantageous for the birth of a baby with a large head, but efficient bipedal walking supposedly requires a narrower pelvis.
As a result, human childbirth was thought to be a kind of price to pay for the combination of a large brain and bipedalism.
The problem is that this explanation has long been the subject of debate. Biomechanical studies provide no convincing evidence to suggest that a wider pelvis necessarily makes walking or running significantly less efficient.
Neanderthals provided an opportunity to test this idea using a different type of human anatomy.
The pelvic bones of ancient humans are poorly preserved, so complete fossil specimens are extremely rare.
A team led by Christoph Zollikofer and colleagues has virtually reconstructed two almost completely preserved Neanderthal pelvises.
The first belonged to a child aged around 1.5 years, found in the Dederyeh Cave in Syria. The second belonged to a young woman from the Sima de las Palomas del Cabezo Gordo site in south-eastern Spain.
The researchers used computed tomography and digital reconstruction, which allows the bone to be separated from the surrounding deposits without physically disturbing the fossil material.
The resulting models were compared with the pelvises of modern humans and previously reconstructed Neanderthal remains.
Neanderthals had a wider pelvis than we do
The overall shape of the Neanderthal pelvis differed markedly from that of modern humans.
Their iliac bones flared out more widely to the sides, and the pelvis itself was relatively wide in the transverse direction and short from front to back.
Moreover, these characteristic features were already present in a one-and-a-half-year-old child. This is an important detail: it shows that the broad shape developed during the early stages of development, rather than arising exclusively later on as a result of walking patterns, physical strain or lifestyle.
The authors also believe that this shape largely preserved the ancient pelvic structure known from earlier hominins, whereas the characteristic pelvis of Homo sapiens represents a later evolutionary variant.
But the wide pelvis did not prevent them from walking on two legs
It is precisely here that the main argument against the simple version of the obstetric dilemma arises.
Neanderthals had a wide pelvis, yet were fully-fledged bipeds. Moreover, the anatomical differences between them and Homo sapiens do not fit the simple formula ‘narrow pelvis – efficient walking; wide pelvis – less efficient walking’.
Researchers have identified differences in the position of the hip joints, the tilt of the pelvis and the mechanical levers of the muscles. According to their interpretation, the Neanderthal structure may have provided good trunk stability and powerful movements.
In modern humans, by contrast, certain pelvic features may have been more conducive to economical, sustained walking and running.
In other words, the two human lineages evolved different pelvic geometries whilst remaining successful bipeds. This challenges the idea that the need for efficient walking alone strictly limited the width of the female birth canal.
That said, childbirth was no easy matter for Neanderthals either
Despite noticeable differences in the external shape of the pelvis, the areas directly forming the birth canal in Neanderthals and modern humans turned out to be functionally similar.
Both groups exhibit marked sexual dimorphism: the female pelvis provides more space for the baby to pass through than the male pelvis.
This is particularly important because Neanderthals, like modern humans, gave birth to babies with relatively large heads.
Therefore, the evolutionary problem remains: enlarging the birth canal would indeed have made it easier to give birth to a large baby. The question is why the pelvis simply did not become even wider.
A new study suggests another possible constraint.
The problem may not have been walking, but the pelvic floor
The pelvic floor is a complex of muscles, ligaments and connective tissue that closes off the pelvis from below and supports the organs of the pelvis and abdominal cavity.
When the body is in an upright position, these tissues are constantly under strain. This strain increases during walking, running, lifting heavy objects, coughing and other activities that raise intra-abdominal pressure.
The wider the pelvic opening, the greater the area that the soft tissues have to support. According to biomechanical models, an excessively wide canal potentially increases the strain and deformation of the pelvic floor.
This gives rise to an alternative evolutionary dilemma:
a wide pelvis facilitates childbirth, but an excessive widening of the birth canal may compromise the support of internal organs.
The authors of the new study believe that it is precisely this trade-off that best accounts for the differences and similarities between Neanderthals and modern humans.
This does not mean that the mystery of the complexity of human childbirth has been solved
Despite its striking conclusion, the study does not prove that difficult human childbirth arose solely because of the demands placed on the pelvic floor.
The classic obstetric dilemma has long since ceased to be regarded as the only possible explanation. Researchers discuss a whole range of factors: the size of the baby, the mother’s energy constraints, the rate of foetal growth, the shape of the pelvis, thermoregulation and the characteristics of soft tissues.
This new study adds rare data from the fossil record to this debate and shows that efficient bipedalism is possible with a wide variety of pelvic shapes.
However, the soft tissues of the Neanderthal pelvic floor have not been preserved. Scientists cannot directly measure their strength or function – the corresponding conclusion is based on bone geometry and modern biomechanical models.
It is therefore more accurate to say that the findings support the hypothesis regarding the role of the pelvic floor, rather than proving it.
And the sample size of ancient humans remains very small
Another major limitation relates to the rarity of fossilised pelvises.
The main new reconstructions are based on just two individuals – a child and a young woman. For comparison, the authors also used other known Neanderthal remains, including those from Tabun and Kebara, but the overall sample is still small.
This means that the specific proportions of the two new pelvises cannot be considered universal for all Neanderthals who lived in different regions over tens of thousands of years.
Nevertheless, the find from Sima de las Palomas is particularly valuable: this site has yielded unusually well-preserved Neanderthal remains, including a female pelvis. The site has yielded the remains of at least 13 individuals from layers dating back approximately 130,000–50,000 years.
What this ultimately changes
The research does not make human childbirth any less mysterious — rather, it reframes the question.
Previously, the classic narrative was straightforward: humans began walking upright, the pelvis was expected to become narrower, and the growing infant’s brain required a wide birth canal. The conflict between these two requirements is what led to complicated childbirth.
Neanderthals show that the first part of this model may be too simplistic. Their pelvis was wider, and their locomotor mechanics differed from ours, yet they successfully walked on two legs.
Therefore, the key constraint on the width of the birth canal may not have been the cost of walking in itself, but the need to simultaneously ensure the birth of a large baby, the stability of the body and the normal functioning of the pelvic floor.
This is precisely the unexpected answer suggested by Neanderthal anatomy: the problem of human childbirth may have been linked not so much to the way we walk, but to the fact that the pelvis must simultaneously allow a baby to pass through whilst retaining the internal organs.
Source
Authors: Christoph P. E. Zollikofer et al.
Journal: Proceedings of the National Academy of Sciences (PNAS), 2026.