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27.08.2025 15:30

Two big steps toward the evolution of bipedality

Dr. Gesine Steiner Pressestelle
Museum für Naturkunde - Leibniz-Institut für Evolutions- und Biodiversitätsforschung

The pelvis plays a key role in the evolution of human upright locomotion and has changed radically over millions of years to allow humans to walk on two legs.
However, which steps were involved in this evolutionary transformation has remained mysterious. A new study of international scientists under the lead of two Harvard scientists and with contributions from scientists from the Museum für Naturkunde Berlin now sheds new light on the steps that were involved in this transformation.

"What we've done here is demonstrate that in human evolution there was a complete mechanistic shift,” said Terence Capellini, Professor and Chair of the Department of Human Evolutionary Biology at Harvard University and senior author of the new paper. “There's no parallel to that in other primates. The evolution of novelty—the transition from fins to limbs or the development of bat wings from fingers—often involve massive shifts in how developmental growth occurs. Here we see humans are doing the same thing, but for their pelves.”

The closest human relatives, chimpanzees, bonobos, and gorillas, have upper hipbones (illia) with a tall, narrow shape and a flat front to back orientation offering anchor points for musculature required for climbing.

In humans on the contrary, the hipbones form a bowl shape and have rotated sideways in turn providing attachment surfaces for the muscles that allow us to maintain balance as we shift our weight from one leg to another during upright walking and running.
In a paper published Wednesday [August 27] in Nature, the team of international scientists now identified some of the key genetic and developmental shifts that led to this evolutionary resculpturing in the evolution of bipedal locomotion.

“What we have tried to do is integrate different approaches to get a complete story about how the pelvis developed over time,” said Gayani Senevirathne, lead author of the study. “I think that is one of the strengths of this paper."

Senevirathne analyzed 128 samples of embryonic tissues from humans and nearly two dozen other primate species from museum collections in the United States and Europe. This included important material of prenatal chimpanzees from the embryological collection of the Museum für Naturkunde, which was CT scanned at the MfN as part of the dataset for this study.

“This study is a good example of the great potential integrative research approaches as taken here under Gayani’s lead have for answering large scale evolutionary questions” said Nadia Fröbisch, professor for development and evolution at the Museum für Naturkunde and Humboldt University Berlin and one of the co-authors of the study.
“It highlights the immense value natural history collections have and how much continues to be learned from specimens that have been housed and curated in museums for hundreds of years” said Dr. Vivien Bothe, researcher at the Museum für Naturkunde and co-author of the study.

The study revealed that evolution reshaped the human pelvis in two major steps. First, it shifted a growth plate by 90 degrees to make the human ilium wide instead of tall. Later, another shift altered the timeline of embryonic bone formation. Most bones of the lower body take shape through process that begins when cartilage cells form on growth plates aligned along the long axis of the growing bone. This cartilage later hardens into bone in a process called ossification.

Interestingly, the human iliac growth plate forms with growth aligned head-to-tail just as it did in other primates during early stages of development. However, by day 53, the growth plates shifted perpendicular from the original axis leading to a shortening and broadening the hipbone.

“Looking at the pelvis, that wasn't on my radar,” said Capellini. “I was expecting a step-wise progression for shortening it and then widening it. But the histology really revealed that it actually flipped 90 degrees—making it short and wide all at the same time."

Another major change involved the timeline of bone formation. Most bones form along a primary ossification center in the middle of the bone shaft. In humans, however, the ilia do something quite different. Ossification begins in the rear and spreads radially from there, but this mineralization remains restricted to the peripheral layer of the bone while ossification of the interior is delayed by 16 weeks. This allows for the bone to maintain its shape as it grows and fundamentally changes the geometry.

Senevirathne also identified the molecular control driving this shift by identifying more than 300 genes at work of which three are of particular importance as underscored in diseases of modern-day humans caused by their malfunction.

The authors suggest that these changes began with reorientation of growth plates around the time that our ancestors branched from the African apes, estimated to be between 5 million and 8 million years ago. As brains grew bigger, the so called “obstetrical dilemma” came into play, the tradeoff between a narrow pelvis (advantageous for efficient locomotion) and a wide one (facilitating the birth of big-brained babies). They suggest that the delayed ossification probably occurred in the last 2 million years of evolution.
The oldest pelvis in the fossil record is the 4.4 million year-old Ardipithecus from Ethiopia—a hybrid of an upright walker and tree climber with a grasping toe) and it shows hints of humanlike features in the pelvis. The famous 3.2 million year old Lucy skeleton, also from Ethiopia, includes a pelvis that shows further development of bipedal traits such as flaring hip blades for bipedal muscles.

Capellini believes the new study should prompt scientists to rethink some basic assumptions about human evolution. "All fossil hominids from that point on were growing the pelvis differently from any other primate that came before,” said Capellini. “Brain size increases that happen later should not be interpreted in a model of growth like chimpanzee and other primates. The model should be what happens in humans and hominins. The later growth of fetal head size occurred against the backdrop of a new way of making the pelvis."

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