Why Humans Walk Upright

Author: Harvard University
Published: 2022/09/12 - Updated: 2023/01/04 - Peer-Reviewed: Yes
Contents: Summary - Main - Related Publications

Synopsis: Study shows that many of the features essential for human walking and birth form around the 6 to 8-week mark during pregnancy. This includes key pelvic features unique to humans. This paper focuses on what all humans share: these changes to the pelvis that allowed us to walk on two legs and give birth to a large fetal head. Compared to chimpanzees and gorillas, the shorter and broader 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.

Human Pelvis

The human pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis or pelvic skeleton). In mammals, the bony pelvis has a gap in the middle, significantly larger in females than in males. Their young pass through this gap when they are born. Modern humans are, to a large extent, characterized by bipedal locomotion and large brains. Because the pelvis is vital to both locomotion and childbirth, natural selection has been confronted by two conflicting demands: a wide birth canal and locomotion efficiency, a conflict referred to as the "obstetrical dilemma." The female pelvis, or gynecoid pelvis, has evolved to its maximum width for childbirth - a wider pelvis would make women unable to walk. In contrast, human male pelvises are not constrained by the need to give birth and therefore are more optimized for bipedal locomotion.

Main Digest

The developmental impacts of natural selection on human pelvic morphology.

If evolutionary biologist Terence D. Capellini were to rank the body parts that make us quintessentially human, the pelvis would place close to the top.

After all, its design makes it possible for humans to walk upright on two legs (unlike our primate cousins), and it makes it possible for mothers to give birth to babies with large heads (therefore big brains). On an anatomical level, the pelvis is well understood, but that knowledge starts to break down regarding how and when this uber-important structure takes its shape during development.

A new study from Capellini's lab is changing that. Published in Science Advances, the work shows that the pelvis takes shape during pregnancy and identifies the genes and genetic sequences that orchestrate the process. The work can one day shed light on the genetic origin of bipedalism and open the door for treatments or predictors of hip joint disorders, like hip dysplasia and hip osteoarthritis.

"This paper is really focused on what all humans share, which are these changes to the pelvis that allowed us to walk on two legs and allowed us to give birth to a large fetal head," said Capellini, a newly tenured Professor in the Department of Human Evolutionary Biology and senior author on the study.

The study shows many features essential for human walking and birth form around the 6 to 8-week mark during pregnancy. This includes key pelvic features unique to humans, like its curved and basin-like shape. The formation happens while bones are still cartilage, so they can easily curve, rotate, expand, and grow.

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

"There appears to be a stalling that happens, and this stalling allows the cartilage to grow still, which was pretty interesting to find and surprising," Capellini said. "I call it a zone of protection."

The researchers performed RNA sequencing to show which genes in the region are actively triggering the pelvis formation and stalling ossification, which normally turns softer cartilage into hard bone. They identified hundreds of genes turned either on or off during the 6 to 8-week mark to form the ilium in the pelvis, the largest and uppermost bones 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 make 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. On the other hand, apes 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 worldwide. 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."

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This peer reviewed publication pertaining to our Anthropology and Disability section was selected for circulation by the editors of Disabled World due to its likely interest to our disability community readers. Though the content may have been edited for style, clarity, or length, the article "Why Humans Walk Upright" was originally written by Harvard University, and submitted for publishing on 2022/09/12 (Edit Update: 2023/01/04). Should you require further information or clarification, Harvard University can be contacted at the harvard.edu website. Disabled World makes no warranties or representations in connection therewith.

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