The pelvis is often described as the keystone for upright locomotion. Over millions of years, it has undergone significant transformations, enabling us to walk on two legs more effectively than any other part of our lower body. However, the specifics of this remarkable adaptation have been largely unknown. Recent research has uncovered two crucial genetic changes that reshaped the pelvis, permitting it to evolve into the upright structure that our ancestors utilized while traversing the Earth.

Anatomists have long recognized that the human pelvis is distinct among primates.

In our closest relatives, African apes (chimpanzees, bonobos, and gorillas) possess hipbones that are tall, narrow, and flat from front to back. When viewed from the side, they resemble thin blades.

The pelvic structure of an ape supports large muscles essential for climbing.

In contrast, human hip bones rotate sideways, forming a bowl shape. This flaring of the hip bones allows for muscle attachment critical for maintaining balance while shifting weight from one foot to the other during upright locomotion.

Nonetheless, the mechanisms behind this transformation have been elusive until now.

In a recent study, Professor Terrence Capelini from Harvard University and his team pinpointed vital genetic and developmental shifts that facilitated the evolution from the pelvis of tetraleaf monkeys to bipedalism.

“Our findings illustrate a complete mechanistic shift in human evolution,” stated Professor Capelini.

“There is no parallel to this among other primates.”

The researchers analyzed 128 samples of embryonic tissue from humans housed in museums in the US and Europe, along with nearly 20 other primate species.

These collections included specimens over 100 years old, preserved on glass slides or in bottles.

Using CT scans and histological analysis, they investigated pelvic anatomy during the early stages of development.

Their research revealed that the evolution of the human pelvis unfolded in two major phases.

Initially, the growth plate shifted 90 degrees, widening the human ilium instead of extending its height.

Following this adjustment, the timeline for embryonic bone formation was altered.

Typically, bones in the lower body develop when chondrocytes align along the long axis of the growing bone.

This cartilage becomes rigid through a process known as ossification.

At the early stages of development, similar to other primates, human growth plates formed from the head and continued to develop.

However, by day 53, the growth plate had notably shifted vertically from its initial orientation, resulting in a shorter and broader hip joint.

“When I examined my pelvis, it wasn’t initially on my radar,” Professor Capelini remarked.

“I anticipated a gradual modification to shorten and widen it, but histology indicated a complete 90-degree reversal.”

Group of Australopithecus afarensis. Image credit: Matheus Vieira.

A further significant alteration was the timeline of bone formation.

In most cases, bones develop along the primary ossification center located in the center of the bone shaft.

However, in humans, the ilium diverges from this norm, with ossification beginning at the posterior region in the sacrum and expanding radially.

This mineralization remains restricted to the peripheral layer, while internal ossification is postponed by 16 weeks, allowing bones to grow and maintain their shape during their geometric transitions.

To uncover the molecular mechanisms driving these changes, the team employed techniques like single-cell multiomics and spatial transcriptomics.

The researchers identified over 300 active genes, including three with notable roles: Sox9 and PTH1R (which control growth plate shifts) and runx2 (which governs ossification changes).

The significance of these genes is underscored by diseases arising from their dysfunction.

For example, mutations in Sox9 can lead to Campomelic dysplasia, a disorder characterized by an abnormally narrow hip joint lacking lateral flaring. Similarly, mutations in PTH1R result in narrow hip joints and various skeletal disorders.

The scientists propose that these adaptations began with the reorientation of the growth plate around the time our ancestors separated from African apes, estimated to have occurred between 5 and 8 million years ago.

They believe the pelvis has served as a focal point for evolutionary transformations over millions of years.

As brain size increased, the pelvis encountered selective pressures known as the obstetric dilemma—the trade-off between a narrow pelvis for efficient movement and a broader one for accommodating the birth of larger babies.

Researchers suspect that the delay in ossification likely occurred within the last two million years.

The oldest pelvic fossil, dated at 4.4 million years, belongs to Ardipithecus from Ethiopia—a species exhibiting a blend of upright walking and tree-climbing features, with pelvic characteristics akin to those of humans.

The renowned 3.2 million-year-old skeleton of Lucy (Australopithecus afarensis) showcases further adaptations for bipedalism, including the distinctively flaring hip blades.

“From that point onwards, all hominin fossils displayed pelvises that diverged significantly from those of earlier primates,” stated Professor Capelini.

“The implications of brain size and its subsequent changes should not be interpreted through growth models applicable to chimpanzees and unassociated primates.”

“Models should focus on the developments between humans and their own lineage.”

“Post-fetal growth occurred against the backdrop of novel methods for constructing the pelvis.”

This study is set to be published in the journal Nature.

____

G. Senevilas et al. The evolution of hominin bipedal walking in two steps. Nature Published online on August 27th, 2025. doi:10.1038/s41586-025-09399-9