The upright gait is a key feature of humans, it distinguishes us from our closest relatives in the animal kingdom. Our pelvis is a decisive prerequisite for stable upright walk. In contrast to all great apes, it is shell -shaped, the intestinal stone forms two flat, wide shovels. Now a study reveals how this unique human pelvic form came about. Accordingly, this was done in two steps: During the embryonic development, the growth plate of the intestinal plate of the intestinal plate, which is made of cartilage and bone predecessor cells, is 90 degrees. The second step is a modification of the normal ossification of the intestinal plate for mammals: instead of the center, it ossed it only on the edge, the middle only becomes a bone later. This favors the attachment of the important muscles for our upright gear. Only both together gave our ancestors the ability to balanced upright gear.
The upright walk was a crucial breakthrough in human evolution. Because only through the stable two -legged, our ancestors got our hands free to produce and use more complex tools. Although great apes – our closest relatives – can also straighten up for a short time. However, they lack the anatomical conditions to bring their upper body and head vertically over their legs and feet. As a result, gorilla, chimpanzees and Co cannot stand and stay in balance.
Beck shovel instead of elongated hip
Fossils and footprints suggest that the upright gait in our ancestors has only gradually developed. The Vormensch Ardipithecus ran temporarily around 4.4 million years ago and showed first adjustments to the skull base and hips, but he probably still climbed most of his time. That changed about 3.2 million years ago with the Australopithecus: “Lucy” and her relatives already had the necessary anatomy and were able to go straight and typical of human being. One of the decisive innovations that distinguishes us and our ancestors from all other primates is our pelvic anatomy. While monkeys and all other mammals have narrow intestinal bones pointing backwards in the direction of the body axis, the intestinal (Ilium) is turned and widened with us humans. As a result, it forms the typical pelvic peel, which supports our inner organs and on which the muscles that are important for our balance can set well.
Gayani Senevirathne from Harvard University and her colleagues have now informed how our unique pool has developed. “We have combined different approaches in such a way that we can fully follow the development of our hips over time,” explains Senevirathn. To do this, she and her team carried out histological, genetic and anatomical analyzes of tissue samples, which came from human embryos in various stages of development and from primate embryos conserved in museums. This enabled them to reconstruct how the intestinal bone is created in human embryo – and where its development deviates from that of the other primates. It showed that the typical human pelvic shape arises in two crucial steps.
Garbing intersection and changed ossification
The first change happens in the early embryonic stage, around 53 days after fertilization: Before this time, the growth plate of the intestinal plate consisting of cartilage and bone precursor cells is directed to the rear parallel to the body axis. But then the growth direction of the cells suddenly changes and instead of backwards, the intestinal growth plate grows forward and back. “I had expected a gradual change, but histology shows that the intestinal plate rotates 90 degrees directly,” explains senior author Terence Capellini from Harvard University. “This makes the intestinal stone short and at the same time wide.” This change is complete on the 72nd day after fertilization – the human fetus now has typical human pelvic shovels.
The second crucial change concerns the ossification of the intestinal. In other primates and most other bones of humans, this conversion of the cartilage bone in the bone into bones follows a solid pattern. The bone cells first ripen in the middle of each bone, from this ossification center the ossification then spreads further and further towards the bone ends. “All non -human primates and the mouse have intestinal bones that follow this typical icing pattern,” explain Senevirathn and her colleagues. But when they examined this process in the human embryo, a completely different picture was shown: “The formation of the intestinal bone bone begins unilaterally on the back near the crossbone instead of in the middle,” reports the team. This icing front then moves along the edge and leaves the middle of the shovel -shaped intestinal. Their ossification is delayed by 16 weeks, so that this part of the pelvis remains cartilaginous until the 24th week. “This pattern is unique and only occurs in the human intestinal,” explain the researchers. There is also no other primate.
“A complete mechanical change”
“These findings demonstrate that human evolution made a complete mechanical change here,” explains Capellini. “There is no parallel to one of the other primates for this.” Analyzes of genetic activity showed that more than 300 genes are involved in this comprehensive changeover of human pelvic development. However, three genes play the most important role: SOX9 and PTH1R control the 90-degree rotation of the intestinal cartilage science. If these genes are defective in a human being, they are born with anomally narrow, misprinted pools. The third gene, Runx2 controls the ossification and is responsible for the unusual icing pattern of the human intestinal, as the researchers determined.
The team assumes that these mechanical and genetic steps to the human pelvis have developed after the other one after the other. It started with the rotation of the intestinal growth plate eight to five million years ago. “This occurred when the locomotion of the hominins changed from an affectable passage to the optional two -leggedity and the muscle function adapted to ensure greater side stability,” write Senevirathn and her colleagues. “When the hominins then passed from the optional bipedalism five to two million years ago, additional molecular changes ensured that this orientation of the intestinal plate was fixed.” The Australopithecines already had these characteristics. Around two million years ago, the unique icing pattern and its delay came about – around the time when the first representatives of our genus were created.
Source: Gayani Senevirathne (Harvard University, Cambridge) et al., Nature, DOI: 10.1038/S41586-025-09399-9
