A study published in Nature identified two structural innovations in the upper human pelvis that enabled bipedalism and reported associated genetic programs active during development. Researchers from Harvard University concluded that the origin of human bipedalism centered on two pelvic structural and genetic changes that were connected at the molecular and tissue levels, reported SINC.
The pelvis served as the cornerstone of upright locomotion in humans, having changed over millions of years to support bipedalism, reported El Mundo. In humans, the hip bones rotated outward to form a bowl shape that provided muscle insertions for upright balance and allowed weight to shift from one leg to the other during walking.
The first innovation reoriented a growth plate by 90 degrees, making the ilium wide instead of tall; unlike other species in which the ilium grew vertically along the body’s axis and remained tall, flat, and narrow, the human ilium became short, wide, and curved, helping stabilize the body for upright walking and running. The second innovation delayed pelvic ossification during embryonic development, preserving the advantageous new ilium shape by maintaining flexibility and enabling a different geometry.
To investigate how the human pelvis developed, lead author Gayani Senevirathne analyzed 128 embryonic tissue samples from humans and primates preserved in museums in Europe and the United States, and examined human embryonic tissues obtained at the University of Washington. The team conducted computed tomography scans and tissue microscopy, including century-old specimens on glass slides and in jars, to reconstruct early pelvic anatomy; the work identified differences in how bone cells were deposited on cartilage in the human ilium compared with other primates and with human long bones. Researchers identified hundreds of regulatory DNA sequences active during ilium development, with signs of evolutionary change, and found more than 300 genes involved in the two innovations. No single bipedalism gene was found; many small DNA switches worked together, said Senevirathne, a postdoctoral researcher in evolutionary biology at Harvard.
“Without these changes, human walking likely wouldn't have been possible, and subsequent brain size increases would have been difficult to envision,” said Terence Capellini, a biologist at Harvard University and an author of the study. “What we’ve done here is demonstrate that in human evolution there was a complete mechanistic shift,” explained Capellini. “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 involves massive shifts in how developmental growth occurs. Here we see humans are doing the same thing, but for their pelves,” he ruther said.
The authors suggested that these changes began after human ancestors separated from African apes between 5 and 8 million years ago and that the pelvis continued to evolve for millions of years. The pelvis of Ardipithecus ramidus at about 4.4 million years showed human-like traits, while the 3.2-million-year-old skeleton of Lucy showed greater development of bipedal features, including expansion of the hip lobes for bipedal muscles; these fossils indicated that the ilium-shape change had already occurred by the time these species moved across Africa, said Capellini.
The study linked the second innovation to the evolutionary obstetric dilemma, the trade-off between a narrow pelvis for efficient locomotion and a wide pelvis for giving birth to babies with large brains. In this way, the pelvis could grow in size and maintain a shape important for walking, but also preserved the shape of a birth canal that would ultimately be used to allow the passage of a baby with a large brain, said Capellini.
Bipedalism replaced movement on four limbs and freed human hands for tools, food preparation, carrying infants, and transporting materials; upright posture improved environmental visibility, reduced the body’s surface area exposed to direct sunlight, and helped cool the body in hot climates. Humans used a form of bipedalism not found in other living primates; chimpanzees sometimes walked on two legs but mainly used quadrupedal locomotion, and human bipedalism differed anatomically from that of birds, kangaroos, and dinosaurs such as Tyrannosaurus rex. “Our form of bipedalism was exceptionally efficient in its gait, allowing us to walk or run long distances with limited energy expenditure. Other primates that tried to walk bipedally used much more energy, and that was exhausting for them,” said Capellini.
“It is very interesting that one of the first changes that occurred in our evolutionary lineage was precisely the more lateral reorientation and the shorter length of the iliac bone,” said José Miguel Carretero Díaz, an associate professor of paleontology at the University of Burgos. Although bipedalism, due to its uniqueness among primates, had been considered a characteristic closely linked to the human evolutionary history, many of the mechanisms underlying this complex change remained hidden, said Juan Manuel Jiménez Arenas, an associate professor of prehistory and archaeology at the University of Granada. “It is a paper of enormous quality and relevance,” he said, adding that “studies like this contribute solidly to expanding knowledge about our particular locomotion,” and that “the conclusions are supported by solid data that came from different disciplines”.
Written with the help of a news-analysis system.