Why does a person walk upright? The secret lies in our basin

Newswise – If evolutionary biologist Terence D. Capellini were to categorize the parts of the body that make us intrinsically human, the pelvis would be near the top.

After all, its design allows humans to walk upright on two legs (unlike our primate cousins) and makes it possible for mothers to give birth to babies with big heads (and thus big brains). On an anatomical level, the pelvis is well understood, but this knowledge begins to crumble when it comes to how and when this very important structure takes shape during development.

a New study From the Capellini Lab changes that. Posted in science progressThe work shows when the pelvis forms during pregnancy and identifies the genes and genetic sequences that regulate the process. The work could one day shed light on the genetic origin of walking on two legs and open the door for treatments or predictors of disorders of the hip joint, such as hip dysplasia and hip arthritis.

“This paper really focuses on what all humans share, which are the changes in the pelvis that allowed us to walk on two legs and allowed us to have a large fetal head,” said Cappellini, a recent professor in the department. of human evolutionary biology and senior author of the study.

The study showed that many essential features of human walking and childbirth are formed by 6 to 8 weeks of pregnancy. This includes the main pelvic features unique to humans, such as its curved and pelvic-like shape. Formation occurs when the bones are still cartilaginous so that they can easily bend, rotate, expand and grow.

The researchers also saw that while other cartilage in the body begins to turn into bone, this developing pelvic part stays as cartilage longer, so it has time to form properly.

“There seems to be a pause happening and that pause is allowing the cartilage to continue growing, which is very interesting and surprising,” Cappellini said. “I call it a protection zone.”

The researchers performed RNA sequencing to show genes in the region that actively lead to pelvic formation and the arrest of ossification, which normally turns softer cartilage into hard bone. They identified hundreds of genes that turn on or off over a period of 6 to 8 weeks to form the iliac in the pelvis, which is the largest and highest hip bone with blade-like structures that bend and rotate in the bipedal support pelvis.

Compared to chimpanzees and gorillas, the shorter and wider reorientation of our pelvic blades makes them not have to shift our weight forward and use our finger joints to walk or balance more comfortably. It also helps to increase the size of the birth canal. On the other hand, apes have much narrower birth canals and more elongated ilium bones.

The researchers began the study by comparing these differences in hundreds of skeletal specimens of humans, chimpanzees and gorillas. Comparisons demonstrated the surprising effects of natural selection on the human pelvis, particularly the iliac.

To find out when the iliac and pelvic elements that make up the birth canal began to form, the researchers examined 4- to 12-week-old fetuses under a microscope with the consent of people who had legally terminated their pregnancies. The researchers then compared samples from the developing human pelvis to mouse models to determine the on and off switches that trigger the formation.

The work was led by Marielle Young, a former graduate researcher in Cappellini’s lab who graduated in 2021 with a Ph. The study was a collaboration between Capellini’s lab and 11 other labs in the United States and around the world. Ultimately, the group wants to know what these changes mean for common hip diseases.

“Walking on two legs has affected the shape of our pelvis, which influences disease risk later on,” Cappellini said. We want to reveal that mechanism. Why pelvic picking affects the risk of later hip disease, such as osteoarthritis or dysplasia. Making these connections at the molecular level will be critical.”