Humans are one of only a few creatures on the planet that can walk and run upright. Now, scientists have identified a key piece of anatomy that makes this possible: the long-overlooked transversal arch, which runs across the middle of the foot, just behind—and perpendicular to—our toes.
To make the finding, the researchers created a silicone model of the transversal arch, which is formed by metatarsal bones and ligaments in human feet, and measured how stiff it was with different curvatures. Just as if they were curving a long, flat piece of paper, the more they curved the arch in the model, the stiffer it became lengthwise. This property is important because feet have to be strong enough to bear more than two times the body weight when running (video above).
To figure out whether the bones in the midfoot themselves or the tissue around them were creating such stiffness, the scientists cut the ligaments and skin in feet from human cadavers and measured how rigid they remained. The tissue, acting together with the bones, accounted for more than 40% of the stiffness of the whole foot, the team reports today in Nature.
The transversal arch is absent in modern-day gorillas and chimpanzees, which have flat and flexible feet. Only the genus Homo developed the transversal arch, suggesting it appeared 3.4 million years ago, the scientists report.
The findings could help researchers develop better prosthetic feet and even build robots that walk much like we do.