For decades, researchers have puzzled over how humans became the only species to walk upright on two legs. A new study published in Nature may finally provide answers, mapping the key developmental changes in the pelvis that allowed early humans to adopt bipedalism while still accommodating the birth of large-brained babies.
The research, led by a team of developmental geneticists and paleoanthropologists, compared the embryonic growth of the pelvis in humans with that of other mammals, including primates such as gibbons and chimpanzees. The scientists identified two crucial evolutionary steps during pelvic development that set humans apart from our ape relatives.
“The human pelvis is dramatically different than what you see in chimpanzees and gorillas, so we wanted to set out to try and understand what’s happening there,” said Terence Capellini, a study co-author and developmental geneticist at Harvard University.
The first change occurs during the early formation of the ilium, a large pelvic bone that supports organs and stabilizes walking. In both humans and other primates, the ilium begins as a vertical rod of cartilage at about seven weeks of gestation. But in humans, the cartilage rotates 90 degrees shortly after, giving the pelvis its distinctive short, broad shape — essential for upright walking.
The second change comes later, around 24 weeks of gestation, when bone gradually replaces cartilage. Unlike in other primates, some human bone cells form much later, allowing the pelvis to grow while retaining its distinctive bowl-like structure.
“These quirks of development help explain how our pelvis evolved into the perfect structure for bipedalism,” said Tracy Kivell, a paleoanthropologist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
Beyond anatomy, the team also pinpointed genetic factors involved in pelvic development. They identified five genes that regulate cartilage growth and bone formation in the ilium, providing new insight into how molecular signals shape human skeletal evolution.
Daniel Schmitt, a biological anthropologist at Duke University, called the work “incredible,” adding that it highlights mechanisms of bone-shape change previously unknown. “We can now consider those mechanisms all throughout the body,” he said.
Kivell noted that the findings could open the door to exploring DNA from fossilized hominins, such as Denisovans, to understand how different genetic pathways influenced skeletal growth in ancient humans.
The study not only sheds light on a defining feature of humanity but also deepens our understanding of how evolutionary biology and genetics intersect to shape the human form. As Nature reported, the pelvis tells a story stretching back five million years — one that continues to shape our species today.
