One key hallmark of being human is walking on two legs. It was a seismic shift seen in no other primates. Like much of evolution, it happened in fits and starts. The 4.4 million year-old Ardipithecus of Ethiopia was a tree climber with a grasping toe that would walk upright 3.2 million year old Lucy had a pelvis brought upright walking closer, with flaring hip blades for bipedal muscles.

Some of that legacy remains in our closest relatives, the African apee, e.g. chimpanzees, bonobos, and gorillas, have upper hipbones (ilia) that are tall, narrow, and oriented flat front to back which anchor large muscles for climbing.

Unlike blades when viewed from the side, human hipbones have rotated to the sides to form a bowl shape. That is why it is called a “pelvis”, derived from the Latin "basin". Our flaring hipbones provide attachments for the muscles that allow us to maintain balance as we shift our weight from one leg to another when walking and running.


A putative gene regulatory network (GRN) underlying the dual structural novelties of the human ilium. See study for detailed notes. https://www.nature.com/articles/s41586-025-09399-9/figures/6

A new paper analyzed 128 samples of embryonic tissues from humans and nearly two dozen other primate species from museums in the United States and Europe along with human embryonic tissues collected by the Birth Defects Research Laboratory at the University of Washington. They took CT scans and analyzed histology (the microscopic structure of tissues) to reveal the anatomy of the pelvis during early stages of development and found that evolution reshaped the human pelvis in two major steps. First, a growth plate by 90 degrees to make the human ilium wide instead of tall. Later, another shift altered the timeline of embryonic bone formation.

We can still see that history now. In the early stages of development, the human iliac growth plate forms with growth aligned head-to-tail just as it did in other primates but around day 50 day our growth plates shift perpendicular from the original axis, shortening and broadening the hipbone.

Why is still a mystery, they identified over 300 genes at work, but three have outsized roles—SOX9 and PTH1R (controlling the growth plate shift), and RUNX2 (controlling the change in ossification).

The authors say that these changes began with reorientation of growth plates around the time that our ancestors branched from the African apes, somewhere between 5 million and 8 million years ago. As brains grew bigger, the pelvis came under another selective pressure known as the “obstetrical dilemma”—the trade-off between a narrow pelvis, advantageous for efficient locomotion, and a wide one, facilitating the birth of big-brained babies. They believe larger brain sizes resulted from this new way of making the pelvis.