One of the characteristics that distinctly separates humans from other animals is the ability to walk on two legs. The key organ that enables this is the pelvis, but how the human pelvic structure, which is entirely different from that of monkeys and chimpanzees, came to be has remained a mystery for a long time.
Researchers from Harvard University in the United States discovered how the human pelvis evolved into a unique structure through genetic and developmental processes. The research results were published in the international journal Nature on the 28th.
The pelvis supports the weight of our body and helps maintain balance when walking. The pelvis of African apes like chimpanzees or gorillas is long and narrow, which is advantageous for climbing trees or moving using their limbs. In contrast, the human pelvis is shaped like a bowl, which allows for a natural transfer of weight from one leg to the other when walking, enabling longer walking or running.
The research team analyzed 128 samples of human embryonic tissue collected from museums in the United States and Europe, as well as samples from over 20 other primate species, and discovered that two major genetic changes led to the differences in the human pelvis compared to other primates.
The first change is the rotation of the growth plate by 90 degrees during the early embryonic stage. The pelvis, which originally grew long in the direction from head to tail, became shorter and wider. As a result, the familiar bowl-shaped pelvis was formed.
The second change is the delay in the point at which the bones harden. In most bones, ossification begins in the center of the skeletal structure, but in the human pelvis, the edges harden first, and the center solidifies later. This allowed the pelvis to maintain its flared shape while growing, creating a structure advantageous for upright walking.
These two processes were associated with specific genes such as SOX9, PTH1R, and RUNX2. SOX9 and PTH1R control the movement of the growth plate, while RUNX2 regulates changes in ossification. In fact, abnormalities in these genes have resulted in rare diseases that cause the pelvis to narrow or deform.
The researchers estimated that these genetic changes began about 5 to 8 million years ago when humans diverged from African apes. As humans' brains grew larger, the difference between the narrow pelvis of mothers and the large skulls of babies put pressure on the evolution of the pelvis.
Paleontological evidence supports this. The oldest pelvis in the fossil record, from about 4.4 million years ago, belongs to Ardipithecus and shows similar characteristics to the human pelvis. The 3.2 million-year-old fossil 'Lucy' also had a wide pelvic structure suitable for upright walking.
Professor Terence Capellini, who led the research, noted, "The human pelvis has grown in a completely different way than that of other primates. This is a revolutionary transition, much like the transition from fins to limbs or the evolution of fingers into wings in bats."
References
Nature (2025), DOI: https://doi.org/10.1038/s41586-025-09399-9