How humans became upright

Charles Darwin published On the Origin of Species in 1859, but only in 1871 did he declare that humans, too, had evolved. In The Descent of Man, he argued that one of humanity’s most striking traits was upright walking: “Man alone has become a biped.”

Now scientists are uncovering the molecular steps behind this transformation. A new study in Nature suggests that early ancestors became bipeds when old genes took on new roles. Some genes became active in unexpected parts of the embryo, while others switched on and off at novel times.

All vertebrates have a pelvis, but only humans use it for upright, two-legged walking. Fossil evidence shows that bipedalism emerged about 5 million years ago, yet the developmental process remained unclear.

“Everything from the base of our skull to the tips of our toes has been changed in modern humans in order to facilitate bipedalism,” says Tracy Kivell, a palaeoanthropologist at the Max Planck Institute. She notes that the study advances understanding not only of humans but also of ancient relatives, such as Denisovans.

As humans evolved, the pelvis took on a wide, bowl-like form essential for walking upright. But how that change occurred puzzled scientists. “The human pelvis is dramatically different from what you see in chimpanzees and gorillas,” says study co-author Terence Capellini, a developmental geneticist at Harvard University.

To investigate, researchers examined human pelvic development across stages, comparing it with mouse embryos and other primates, including gibbons and chimpanzees. They focused on the ilium, the pelvic bone that supports internal organs and anchors muscles for stability. Samples came from museum collections, some preserved for over a century.

The analysis revealed two crucial steps in shaping the human ilium. The first occurs around seven weeks of gestation. Like in other primates, the ilium begins as a vertical cartilage rod. However, in humans, it soon rotates 90 degrees, resulting in a short, broad pelvis. The second step comes much later, around 24 weeks, when cartilage is replaced by bone. In humans, ossification is delayed compared to other primates, allowing cartilage to preserve the ilium’s distinctive shape as it grows.

These developmental quirks, driven by shifts in gene activity, ultimately sculpted the pelvis for efficient bipedalism. Together, they help explain how an ancient evolutionary puzzle gave rise to one of humanity’s most defining features — the ability to walk on two legs.