Early humans ditched tree-swinging for striding on two legs, and now scientists have pinpointed how tweaks in pelvic bone development made that evolutionary leap possible.
By comparing human embryos to those of apes and mice, researchers uncovered two key shifts in how our hip bones form, reshaping the pelvis for balance and baby-carrying. The study, published in Nature, reveals genetic changes that flipped bone growth sideways and delayed hardening, creating our signature bowl-shaped hips.
Gayani Senevirathne and Terence D. Capellini, along with colleagues, zeroed in on the ilium — the upper pelvic bone that went from tall and blade-like in apes to short and wide in humans. Using rare fetal samples, they scanned and sliced tissues to track development from day 45 to 72 in gestation.
In apes and mice, ilium cartilage grows tall, with cells stacking vertically like a primate’s narrow frame. But in humans, it pivots early: growth plates reorient horizontally by day 53, expanding side-to-side instead of up-down. “The human ilium cartilage growth plate underwent a heterotopic shift, residing perpendicular to the orientation present in other primate (and mouse) ilia,” the team notes.
Then comes ossification, when cartilage turns to bone. Apes and mice harden from the middle inward, quickly. Humans start at the back edge, hardening outward in a slow, radial fan — delayed by weeks compared to leg bones. This lets the front keep growing, flaring the ilium for glute muscles that stabilize our wobbly upright gait.
Genomics fingered culprits: networks involving genes like SOX9, ZNF521, and RUNX2, with human-specific tweaks in regulatory switches. Mutations here cause narrow hips in rare disorders, echoing ape shapes. Muscle attachments for walking and running hook on early, possibly tugging the bone into form.
These innovations, the team says, built a stable base for bipedalism around 6-8 million years ago, later adapting for big-brained births. “Second, we observe heterochronic and heterotopic shifts in ossification that are unlike those in non-human primate ilia or human long bones.”Unraveling this could inspire fixes for hip defects, spotlighting how tiny developmental flips sparked humanity’s stride.
Journal Reference: Senevirathne, Gayani, et al. “The Evolution of Hominin Bipedalism in Two Steps.” Nature DOI: 10.1038/s41586-025-09399-9
