Evolving an arch across the foot’s width helped hominids walk upright

The arch running across the width of the human foot might be a big part of the reason that people can walk and run upright, a new study suggests.

People have a prominent arch along the insides of their feet from ball to heel — a structure that helps make feet stiff to withstand forces on the foot caused by walking and running. But there’s another, less obvious arch. Bones in the middle of the foot, called metatarsals, are arranged in a curve across the foot’s width. This bend, called the transverse tarsal arch, stiffens the foot lengthwise and may have evolved more than 3.4 million years ago, a step toward ancient hominids gaining the ability to walk and run on two feet unlike other primates, researchers report February 26 in Nature.

Scientists knew that the arch on the inside of the foot, called the longitudinal arch, makes the foot more rigid, thanks to the arch’s shape and elastic tissues stretching beneath it like a bow and string. How much the bend across the metatarsals helps make feet more firm was unknown.

The role that the transverse tarsal arch plays in foot stiffness is like what happens when a piece of paper is somewhat curled. “Hold [a dollar bill] with your fingers at one end of its length, and it flops down,” says Madhusudhan Venkadesan, a mechanical engineer at Yale University. “But press down with your thumb to slightly curl it along the width, and the bill will stiffen and become straighter.”

Knowing how human feet evolved to walk and run could help experts design better prosthetics or treat people with flat feet. Most prosthetic feet, for instance, are designed for walking. Those who want to run need something different — a stiff prosthetic shaped like a blade. 

The study is “exciting for those of us that live and die for foot evolution,” says Patricia Kramer, a paleobiomechanist at the University of Washington in Seattle who was not involved in the work. The study is an excellent example of combining traditional biological anthropology with engineering principles to better understand aspects of the foot, she says.

Venkadesan and his colleagues tested the stiffness of three types of curved structures, including a thin sheet, mechanical structures mimicking feet and two feet from cadavers, and found that a transverse arch makes materials more rigid. When the researchers cut tissues between the bones that comprise the transverse arch in cadavers, the foot’s stiffness went down by up to 54 percent.

While it’s clear that the transverse arch is important for foot stiffness, exactly how important will need further study, Kramer says. Experiments with the mechanical mimics of human feet, for instance, didn’t account for the parts of the foot that touch the ground — details that might be important in calculating the foot’s overall stiffness, she says. “A modified human foot does not tell you how a hominin foot worked, but it could be used to validate a model that then is modified to represent a hominin foot.”   

The researchers also examined skeletal remains and fossils from ancient human ancestors, searching for the first appearance of curved foot arches. Nonhuman primates like chimpanzees — and probably their last shared ancestor with humans — have much flatter feet than people do. But around 3.4 million years ago, a humanlike transverse tarsal arch had developed in a foot from an unknown human ancestor (SN: 3/28/12).

The appearance of that central arch “probably played a big part in us becoming bipedal,” Venkadesan says. The structure “adds an essential and missing ingredient for the stiffness of human feet.”