Ancient humans evolved to walk on two legs in two steps

Walking on two legs is the single most distinctive trait that separates humans from other primates. Unlike apes, our pelvis is short, broad, and bowl-shaped, and thus stable for walking upright while supporting internal organs and holding space to deliver large-headed infants. For more than a century, scientists have known that bipedalism set our ancestors on a unique evolutionary path. Yet the developmental origins of this transformation have been a mystery.

Traditional explanations that focused on fossil anatomy and biomechanics couldn’t reveal the molecular and developmental processes at work. Moreover, unlike other skeletal features, the ilium, which is the broad upper bone of the pelvis, shows no obvious counterparts in primates that could have revealed signs of gradual, evolutionary gradual change. Researchers from Germany, Ireland, the U.K. and the U.S. thus sought to uncover the hidden steps in embryonic development that shaped the pelvis into its human form. Their findings were published in Nature on August 28.

The team examined human embryonic pelvises across critical weeks of development, using histology to map cartilage zones and micro-CT scanning to track bone formation. They compared these with embryos of mice and of chimpanzees and gibbons held in museum collections. At the molecular level, they used single-cell multi-omics and spatial transcriptomics to catalogue which genes and pathways were active in different cell types.

The study uncovered two major innovations. First, instead of elongating vertically as in apes and mice, the growth plate of the iliac cartilage expanded horizontally, rendering the pelvis wider and shorter. Second, the onset of bone formation in humans began later, at the posterior edge of the ilium, and spread outward along the surface rather than inward through the cartilage. This unusual timing and placement allowed the pelvis to keep expanding in width before hardening into bone. These shifts were tied to a network of developmental regulators, shaped in turn by changes in human DNA.

The findings suggest bipedalism arose via a two-step reprogramming of pelvic development: by redirecting cartilage growth and delaying bone formation, human embryos gained a pelvis capable of supporting upright walking and accommodating childbirth. Understanding these pathways may illuminate the origins of pelvic malformation seen in skeletal disorders. In evolutionary terms, the authors have said this work may also clarify why fossil hominins like Australopithecus already had short, wide pelves millions of years ago.