The mystery of human bipedalism solved: this is how we began to walk upright

BarcelonaBeing able to walk upright on two legs is considered one of the most important milestones in human evolution. However, how the body adapted to this new habit was, until now, a mystery. Harvard researchers conclude that the origin lies in two changes in the pelvis, the cornerstone of vertical locomotion. More than any other part of our body, this bone structure has been radically altered over millions of years to allow our ancestors to be bipedal and roam the planet. a study published in the magazine Nature, scientists reveal the key genetic changes that reshaped the pelvis between 5 and 8 million years ago, when human ancestors diverged from African great apes.

According to Terence Capellini, chair of Harvard’s Department of Human Evolutionary Biology and senior author on the study, the research shows that in human evolution, there was a complete change in body mechanics that transformed the way our ancestors got around. “There are no parallels in other primates. The evolution of novelty—the transition from flippers into limbs or the development of bat wings from digits, for example—often involves massive changes in how species development occurs. Here we see humans doing the same thing, but with the pelvis,” .

Anatomists have long known that the human pelvis is unique among primates. In our closest relatives, the African apes (chimpanzees, bonobos, and gorillas), the upper hip bones, or ilia, are tall, narrow, flat, and oriented front to back. This anchors the large muscles used for climbing. In humans, however, the hip bones have rotated sideways to form a bowl shape, providing attachments for muscles that maintain balance while standing and while shifting weight from one leg to the other while walking.

Until now, it was unknown how the pelvis developed this way. To unravel the problem, researcher Gayani Senevirathne analyzed 128 samples of human embryonic tissue and tissue from various primate species held in museums in Europe and the United States. She also studied human embryonic tissue obtained from the University of Washington to observe in great detail how the pelvis forms in the early stages of development.

Embryonic development

The results showed that evolution reshaped the human pelvis in two key steps. First, a bony growth plate shifted 90 degrees, causing the ilium (the largest part of the hip bone) to be wider, not taller, as it is in other primates. Normally, the body's bones develop from cartilage that hardens in a process called ossification, but in humans the pattern turned out to be very different.

In the first weeks of embryonic development, the human iliac growth plate begins as it does in other primates, aligned top to bottom. However, around day 53, it radically reorients itself, becoming perpendicular to the original axis. This causes the pelvis to widen and shorten, a key feature for bipedalism.

Another change appears later: the ossification of the ileum does not begin in the center, but in the posterior part of the sacrum and progresses radially, while the interior of the bone delays its mineralization until 16 weeks, which allows the pelvis to maintain flexibility and acquire a different geometry.