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Harvard scientists have discovered new evolutionary changes in pelvic structure that allowed the first humans to walk upright on two legs.
The August study published in the journal Nature reveals that two distinct morphogenetic shifts—one in the growth plate and the other in the timing of bone formation—were key in altering the shape of the pelvis, enabling humans to be bipedal.
Gayani Senevirathne, the lead author of the study and a postdoctoral fellow at the Capellini Lab at Harvard, said that while researchers have long known that the human pelvis looks structurally different from other primates, these new findings highlight why the pelvic girdle looks different when it develops.
Nicholas B. Holowka, an assistant professor of Anthropology at Penn State, said that the evolution of the pelvis is the most “important change that distinguishes humans from other primates as far as locomotion and movement are concerned.”
Senevirathne and her colleagues worked with museums around the world to study rare embryonic primate samples from the 1800’s and early 1900’s.
“Every single part of this project is a collaboration,” said Terence D. Capellini, senior author and Harvard professor of Human Evolutionary Biology. “Science is beautiful that way.”
To visualize both skeletal and soft tissues, the team employed iodine staining in combination with CT scanning.
“You can immerge your specimen inside this solution, and then the soft tissues will start to absorb it. As a consequence, the absorption of iodine basically allows it to increase the contrast of the soft tissues and cartilage,” said Matteo Fabbri, a co-author of the study and assistant professor of Functional Anatomy and Evolution at Johns Hopkins School of Medicine.
“All of the sudden you can put your specimen inside the machine, and eventually you can capture morphology of the muscles, tendons, nerves, brain, cartilage, and obviously also the skeleton,” he added.
Using CT scanning, the team processed data from early-stage human embryonic tissue to create 3D models of bones, muscles, and blood vessels, helping to identify the muscle attachments important for maintaining upright posture.
“The growth of the cartilage is very weird in humans: you have two main steps, or rotation and expansion, while this is completely absent in other primates or in mice and other mammals,” Fabbri said.
According to Capellini, scientists assumed that the development of the human pelvis might mirror that of primates: shortening the blades before widening them — a “stepwise process.”
“What Gayani found wonderfully is that actually that’s not the case,” he said. “You can really change the orientation, and that satisfies actually making them shorter and wider at the same time.”
Holowka said he was interested to know more about the interaction between the developing bone that guides the reorientation and the muscles, as well as when the changes might have taken place.
“I think that’s what’s next to understand — were these changes sudden, or did they happen gradually?” he asked.
—Staff writer Mana Tsuruta can be reached at mana.tsuruta@thecrimson.com. Follow her on X @ManaTsuruta.
—Staff writer Neena Tarafdar can be reached at neena.tarafdar@thecrimson.com.
