A new study traces how our hips changed so our ancestors could walk on two legs. It identifies two precise changes during early development that reshaped the pelvis and set the stage for stable, efficient bipedal movement.
The work follows what happens inside a forming hip, from early cartilage to bone. It clarifies when a key growth zone turns and when bone hardening is delayed, producing a short, wide, bowl-like pelvis suited for upright walking.
Rotation of hip growth plate
The research was led by Professor Terence D. Capellini, Chair of Human Evolutionary Biology at Harvard University.
The first discovery centers on the growth plate, a thin zone where cartilage cells multiply and stack as a bone lengthens. In humans, the iliac growth plate starts out like other primates, then by day 53 of development it swings 90 degrees, so growth runs across the pelvis instead of head to tail.
That single rotation turns the ilium from tall and narrow into short and wide. “What we’ve done here is demonstrate that in human evolution there was a complete mechanistic shift,” said Capellini.
Delayed bone hardening
The second change is all about timing. In a process called ossification, most bones start hardening from a center point in the shaft, then spread inward and outward.
Human ilia break that pattern. Bone begins near the back, next to the sacrum, and spreads radially along the outer rim while the inside stays cartilaginous for roughly 16 weeks. This allows the pelvis to keep its shape as it enlarges.
That delay keeps the “basin” geometry intact while muscles and ligaments find their anchors. “But the histology really revealed that it actually flipped 90 degrees,” said Capellini.
Pelvis rotation and timing
Behind those shifts are cells called chondrocytes that build cartilage and a ring of tissue called the perichondrium that helps initiate bone. The team mapped more than 300 active genes and highlighted three with outsized roles in these steps.
Mutations in the SOX9 gene can cause campomelic dysplasia, which includes unusually narrow hip bones with reduced lateral flare. That clinical pattern matches what you would expect if iliac growth plate widening fails.
Activating variants in the PTH1R gene cause Jansen metaphyseal chondrodysplasia, a disorder tied to abnormal growth plate signaling. Changes in that same pathway appear to help reorient the human iliac growth plate.
Homozygous PTH1R (a genetic state where an individual inherits two non-functional copies of the PTH1R gene) mutations are linked to Eiken syndrome with delayed bone ossification. That mirrors the second developmental shift documented in the iliac growth plate’s development.
Human hip changes began early
The research team scanned and sectioned 128 embryonic and fetal samples, covering humans and nearly two dozen primates. The human pattern was different, and it was different early.
According to the researchers, the growth plate reorientation began after our branch split from African apes. Later, as selection balanced walking efficiency with childbirth, the ossification delay likely emerged to preserve shape while the pelvis grew.
“All fossil hominids from that point on were growing the pelvis differently from any other primate that came before,” said Capellini.
Pelvis shaped walking and birth
A short, wide ilium changes how key muscles work. The flaring blades reposition gluteus medius and gluteus minimus muscles so they stabilize the pelvis when one leg is on the ground during a step.
Comparative work shows that flaring ilia reorient hip muscles and influence birth canal shape, as outlined in a review. That broader context helps explain why the pelvis must serve walking, load transfer, and childbirth together.
The study also notes early attachments for the rectus femoris and the iliofemoral ligament near the front of the ilium. Those connections fit the demands of upright gait and help resist pelvic drop during single-leg support.
Tracking changes in hips
The investigators combined micro CT with histology to see shape changes over time. They then layered single-cell multiomics and spatial transcriptomics to match gene activity with exact locations in the tissue.
That blend linked a rotated growth plate to SOX9, PTH1R, and related signals. It also tied the unusual rim-first, inside-late ossification to the RUNX2 gene and factors that govern when and where bone cells appear.
The combined picture shows a coordinated pathway that first widens the pelvic blades, then slows internal mineralization to hold geometry while size increases. That two-step sequence explains the basin-like hips required for stable bipedalism.
Many questions remain
Open questions remain about how mechanical forces from early muscle contractions might reinforce the new growth directions. It is also unclear how much variation these developmental programs show across modern human populations.
Future work can probe how regulatory DNA near these genes changed through time. That could reveal whether selection acted in bursts or nudged many switches at once.
The study is published in the journal Nature.
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