A comparison of two rare male pelvises from the Neanderthal lineage and 91 pelvises from modern humans suggests overturning a long-held assumption: the ancient structure was preserved in Neanderthals and modern women, while a spring-like mechanism evolved in men. In an interview with Hidan, Prof. Yoel explains why he relies on the small fossil sample and how the hypothesis can be tested.
For decades it has been perceived Pelvis The wide and sturdyNeanderthals As an unusual structure that requires explanation. New research led by Prof. Yoel only מTel Aviv University Suggests reversing the perspective: The Neanderthal pelvis may not be the exception at all. Rather, the male pelvis Homo sapiens It has undergone a significant evolutionary change.
The study, published in the journal Scientific Reports, compared two nearly complete male pelvises from the Neanderthal lineage—the pelvis of Kabera 2 m.Kabra Cave in Carmel and the basin of Sima de los Vesos in Spain — to 63 basins of contemporary men and 28 basins of women.
Despite their size and robust structure, Neanderthal pelvises resembled modern women's pelvises in many more anatomical dimensions and relationships than modern men's. The researchers suggest that Neanderthals and modern women retained a relatively primitive configuration, while the Homo sapiens male pelvis underwent specialization related to the mechanics of walking.
Only two basins — but a big difference
Relying on only two fossils immediately raises the question of sample size. In an interview with Hidan, Prof. Rak said that this is an inevitable limitation of thePaleoanthropology, but he said the differences found were too great to ignore.
"The differences are so large that we are calm even when the sample is small," he said only in an interview with the Hidan website. "In paleoanthropology, the samples are small. There are two options: to address them or not to address them, and we prefer to address them — with all the limitations."
According to him, these are the only two nearly complete male pelvises from the Neanderthal lineage that allow us to examine the relationships between the parts of the pelvis and its three-dimensional geometry.
"For many years, the Neanderthal pelvis was a complete mystery," he explained. "There were fragments of pelvises that suggested that something special or strange was happening in it relative to modern humans, but there was no complete pelvis that allowed us to figure out the relationships within the pelvis, especially the three-dimensional geometry, which is very significant to us."
The researchers found further support in fragments of Homo erectus pelvises. Although they are partial, they say their features are more consistent with the Neanderthal configuration than with the pelvis of a male Homo sapiens. Therefore, they serve as a kind of evolutionary comparison group that strengthens the possibility that the Neanderthal structure is the ancient structure.
The hip joints moved forward.
One of the main differences is in the location of the hip sockets—the sockets in the pelvis to which the heads of the femurs attach. In modern men, the hip joints are located further forward on the pelvic ring than in modern women and Neanderthal males.
The forward movement changed the relationships between the hip joints, the body's line of gravity, and the forces transmitted from the ground during stride. According to the model proposed by the researchers, muscles that attach to the front of the pelvis can stretch when the pelvis tilts backward during stride, storing energy and releasing it immediately afterward.
Thus, according to the hypothesis, the modern man's pelvis becomes a system similar to a shock absorber and a spring: it softens the drop in the body's center of mass with each step and helps lift it again for the next step.
The change did not occur alone. It was also associated with a shortening of the ramus of the pubic bone, a thickening of the bone, and a deepening of the anterior part of the pelvis, probably to withstand the mechanical forces created by the change in the position of the hip joints.
Possible advantage of walking long distances
If the model is correct, the structure gives men a certain advantage in shock absorption, energy return, and long-distance walking efficiency. This does not mean that every man walks better than every woman, or that the pelvis alone determines walking performance. This is an average difference in skeletal structure and a biomechanical model that still needs to be directly tested.
Prof. Rake noted in an interview that previous studies have already found differences between men and women in pelvic movement and the forces acting during walking. However, direct testing of the proposed mechanism will require dedicated experiments.
"You can test the amount of softening that accompanies a man's walking," he said. Another way, he said, is to measure muscle activity and see if, when the pelvis tilts back, they actually stretch and store energy. "These are things we suggest. Now the physiologists will go and test it."
The prediction could be tested using a combination of force surfaces, 3D motion imaging, and electromyography, which measures muscle activity. Such research would need to examine whether the geometry of the male pelvis does indeed reduce the shock of stepping and return energy, and how significant the effect is in normal walking and long journeys.
Who maintained the ancient structure?
When comparing two different structures, it is not enough to identify the difference; one must find out which one is closer to the ancient state and which is a later specialization. Researchers also use a functional criterion for this purpose: if in one structure there is a mechanism with a defined function that does not exist in the others, there is a basis for considering it a specialization.
"We propose a mechanism that exists in one and does not exist in the other two," said Rake. According to him, the proposed shock-absorbing mechanism appears fully in the male Homo sapiens, but not in the Neanderthal male or the female Homo sapiens.
The known pelvis fragments of Homo erectus also fit, according to the researchers, with the general configuration seen in Neanderthals. The combination of the comparison with ancient fossils and the proposed functional advantage supports the interpretation that the modern male pelvis is the specialized structure.
Why didn't the change fully occur in women?
Researchers suggest that the demands of childbirth have limited the ability of the female pelvis to undergo the same set of changes. Delivering a baby with a large head and body requires a wide birth canal and a pelvis that is not too deep. Therefore, women retain a structure that is closer to their original configuration.
This explanation does not mean that the female pelvis is "less developed" or evolutionarily inferior. On the contrary: it is adapted to a different set of evolutionary pressures, chief among them safe childbirth. Evolution does not progress towards a single, perfect structure, but shapes different solutions to different functional requirements.
A separate study published almost simultaneously in PNAS examined the pelvis of a Neanderthal female from Sima de las Plumas and the pelvis of a Neanderthal infant from Dedria Cave. The authors found that the pelvic parts associated with childbirth are functionally similar in Neanderthals and modern humans, although the overall shape and components related to movement are different.
They suggested that the main evolutionary trade-off was not necessarily between childbirth and walking, but between childbirth and maintaining pelvic floor stability.
Reck, who read the article in preparation for the interview, does not see it as a contradiction to his research. He says the similarity between the pelvis of a Neanderthal female and that of a modern woman in areas related to childbirth is consistent with the claim that both groups retained ancient characteristics that are not fully present in modern men.
However, the two papers examine different fossils and different questions. The Scientific Reports study focuses on male differences and the perspective of the walking mechanism; the PNAS study focuses on pelvic development, birth, stability, and movement. Only further biomechanical studies will be able to determine how much the new mechanism actually explains the differences in movement.
The fossils change the question.
The importance of the study is not just in proposing a new mechanism. It also lies in changing the perspective. For years, researchers have asked why the Neanderthal pelvis looked different from that of modern men. The new study suggests asking the opposite question: Why did the modern male pelvis deviate from the structure shared by Neanderthals, modern women, and at least some of our earliest ancestors?
The answer suggested by Rak and his colleagues is that the change created a more efficient system for absorbing shock and returning energy during walking. This is still a biomechanical hypothesis and not fully experimentally proven, but it makes clear predictions that can be tested in living humans.
"The questions about human evolution are not just in the distant past," said Prof. These are among, Research Associate To the Phys.org websiteUnderstanding the evolution of the walking mechanism may also contribute to biomechanics, musculoskeletal medicine, rehabilitation, and injury prevention. In this case, two rare pelvises from the past may help to better understand the living human body.
For the original publication: Opening the original publication
More on the subject on the science website
- The human body is not a perfect machine: this is how evolution improvised solutions that work well enough
- Earliest evidence of interbreeding between Homo sapiens and Neanderthals discovered in Israel
- The last Neanderthals in northwestern Europe were more connected than we thought
- Lucy is evidence that upright walking preceded brain growth
- Running—the key to human evolution
