Human pelvis from the anthropological collections of the Department of Biology and Environmental Studies of Charles University, Czech Republic. Credit: MAKY.OREL (CC0 1.0)
A new study in Nature has revealed the key genetic and developmental shifts which reshaped the pelvises of our early hominin ancestors to make bipedalism possible.
Unlike other primates, humans permanently walk around on 2 legs. It was the evolution of this type of locomotion which set early hominins on a separate evolutionary path.
Walking is made possible by our pelvis’ bowl-like shape. Its short, wide iliac blades (upper hipbones) support internal organs and provide attachments for the muscles which maintain balance as we shift weight from one leg to another.
Chimpanzees, bonobos and gorillas – humans’ closest living relatives – have distinctly different pelvises. Their upper hipbones, which are tall, narrow and flat, anchor large muscles used for climbing.
Human hipbones have shortened, rotated to the sides and flared outwards in comparison.
“What we’ve done here is demonstrate that in human evolution there was a complete mechanistic shift,” says senior author Terence Capellini, Chair of the Department of Human Evolutionary Biology at Harvard University in the US.
“There’s no parallel to that in other primates. The evolution of novelty – the transition from fins to limbs or the development of bat wings from fingers –
often involve massive shifts in how developmental growth occurs. Here we see humans are doing the same thing, but for their pelves.”
The researchers analysed embryonic tissues from humans and nearly 2 dozen other primate species using computed tomography (CT) and histology. This revealed how the anatomy of the pelvis changes during early stages of development.
Most human bones form via a process called ‘endochondrial ossification’ in which cartilage cells form on growth plates which are usually aligned along the long axis of a growing bone. The cartilage later hardens into bone (ossifies).
The study shows that this also happens for the iliac growth plate in the early embryonic development of humans and other primates. But by day 53, something important happens.
In humans, the iliac growth plates suddenly undergo a perpendicular shift, shortening and broadening the hipbone.
“I was expecting a stepwise progression for shortening it and then widening it. But the histology really revealed that it actually flipped 90° –making it short and wide all at the same time,” says Capellini.
Comparison of human, primate and mouse iliac growth plates. Credit: Senevirathne et al 2025, Nature, https://doi.org/10.1038/s41586-025-09399-9 (BY-NC-ND 4.0)
The timing of human iliac ossification was also different.
The process started at the rear sacrum before spreading around the hips, rather than beginning in the centre of the bone as would be expected.
Ossification also began on the outer layer of the pelvis, while the interior was delayed by 16 weeks, allowing the bone to retain its shape as it grew in 3 dimensions.
“This pattern is unique to the human ilium and is not observed for human long bones at these same stages,” the authors write.
The researchers identified more than 300 genes which drove these shifts in human ilia development.
They suggest the reorientation of iliac growth plates first began when early hominins first branched from African apes between 5 to 8 million years ago (mya).
“Although fossil pelves are not available for the earliest period of hominin evolution (8 to 5 mya), it is noteworthy that some of the earliest bipedal traits present in Ardipithecus (4.4 mya) and Australopithecus (3.85 mya) include a short and wide ilium,” the authors write.
Then, as hominins shifted to obligatory bipedalism about 5–2 mya, the researchers believe new molecular changes fixed in the ilium’s new orientation.
Lastly, as hominin brains grew larger in the last 2 million years, the pelvis likely came under a selective pressure known as the ‘obstetrical dilemma’.
This is the trade-off between a narrow pelvis, which is more advantageous for running and walking, and a wider one which more easily allows the birth of babies with bigger heads and wider shoulders.
“The timing of ossification became delayed to enable enhanced growth and the retainment of a complex iliac shape,” the authors explain.