Human Skeleton Illustration

Why do people walk upright? Harvard biologists solve the mystery

Compared to chimpanzees and gorillas, the shorter and wider realignment of our pelvic blades allows humans to walk or balance more easily.

A new study shows how the pelvis evolved for upright walking.

If evolutionary biologist Terence D. Capellini ranked the body parts that define us as humans, the pelvis would be at the top.

After all, thanks to its design, humans can walk upright on two legs (unlike our primate cousins) and mothers can give birth to children with huge heads (meaning big brains). The pelvis is well understood anatomically, but when it comes to how and when this very essential structure takes shape during development, our understanding begins to fail.

That’s changing thanks to recent research by Capellini’s team. The study published in the journal scientific advances, demonstrates when the pelvis forms during pregnancy and identifies the genes and genetic sequences that drive the process. The research could one day provide insight into the genetic origin of bipedalism and pave the way for the development of treatments or predictors of hip joint diseases such as hip dysplasia and hip osteoarthritis.

“This paper really focuses on what all humans have in common, which is these pelvic changes that allowed us to walk on two legs and give birth to a large fetal head,” said Capellini, a newly hired professor from the Department of Human Evolutionary Biology at Harvard University and senior author of the study.

The study shows that many of the essential characteristics for human walking and childbirth form around the 6 to 8 week mark during pregnancy. These include key features of the pelvis that are unique to humans, such as its curved and basin-like shape. Formation occurs while the bones are still cartilage, allowing them to bend, twist, stretch, and grow easily.

The researchers also discovered that when other cartilages in the body begin to transform into bone, the developing pelvic region stays as cartilage longer so it can mature properly.

“There seems to be stalling, and that stalling keeps the cartilage growing, which was quite interesting and surprising,” Capellini said. “I call it a safe zone.”

The researchers used

RNA
Ribonucleic acid (RNA) is a DNA-like polymeric molecule that is essential in various biological roles in the coding, decoding, regulation, and expression of genes. Both are nucleic acids, but unlike DNA, RNA is single-stranded. A strand of RNA has a backbone made up of alternating sugar (ribose) and phosphate groups. Each sugar has one of four bases attached to it – adenine (A), uracil (U), cytosine (C), or guanine (G). Different types of RNA exist in the cell: messenger RNA (mRNA), ribosomal RNA (rRNA) and transfer RNA (tRNA).

” data-gt-translate-attributes=”[{” attribute=””>RNA sequencing to determine which genes in the area are actively triggering pelvic formation and slowing ossification, which usually converts softer cartilage to hard bone. They discovered hundreds of genes that are turned either on or off throughout the 6- to 8-week period to form the ilium in the pelvis, which is the largest and uppermost bone of the hip with blade-like structures that curve and rotate into a basin to support walking on two legs.

Compared to chimpanzees and gorillas, the shorter and wider reorientation of our pelvic blades makes it so humans don’t have to shift the mass of our weight forward and use our knuckles to walk or balance more comfortably. It also helps increase the size of the birth canal. Apes on the other hand have much narrower birth canals and more elongated ilium bones.

The researchers started the study by comparing these differences in hundreds of skeletal samples of humans, chimpanzees, and gorillas. The comparisons demonstrated the striking effects that natural selection has had on the human pelvis, the ilium in particular.

To see when the ilium and pelvic elements forming the birth canal began to take shape, the researchers examined 4- to 12-week-old embryos under a microscope with the consent of people who had legally terminated their pregnancies. The researchers then compared samples from the developing human pelvis’ with mouse models to identify the on and off switches triggering the formation.

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

“Walking on two legs affected our pelvic shape, which affects our disease risk later,” Capellini said. “We want to reveal that mechanism. Why does selection on the pelvis affect our later disease risk of the hip, like osteoarthritis or dysplasia? Making those connections at the molecular level will be critical.”

Reference: “The developmental impacts of natural selection on human pelvic morphology” by Mariel Young, Daniel Richard, Mark Grabowski, Benjamin M. Auerbach, Bernadette S. de Bakker, Jaco Hagoort, Pushpanathan Muthuirulan, Vismaya Kharkar, Helen K. Kurki, Lia Betti, Lyena Birkenstock, Kristi L. Lewton and Terence D. Capellini, 17 August 2022, Science Advances.
DOI: 10.1126/sciadv.abq4884

The study was funded by Harvard University, the National Science Foundation, and the Milton Fund, 


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