Walking upright on two feet is one of the crowning achievements of human evolution. Now, scientists have discovered exactly what it is about how our feet are structured that gives us this unique and crucial skill that separates us from the animals.
Turns out that what makes the human foot so unique is the transverse tarsal arch.
The transverse tarsal arch is a short area that runs across the foot, from left to right. It sits in between the ball of the foot and the heel. And according to a study published Wednesday in the journal Nature, this is the defining foot feature that differentiates your feet from those of chimps and gorillas, our closest animal relatives.
“Many different disciplines have acknowledged and identified the presence of the transverse arch in the foot, but no function was assigned to it before,” Shreyas Mandre, associate professor at Warwick University and researcher on this study, tells Inverse. “Now we have assigned a function to it.”
The results shine a light on how the transverse tarsal arch creates the stiffness that makes your foot uniquely human, allowing you to walk, jog, sprint and run a marathon in a way that animals don't do.
“The foot is one of the least understood structures of our body in terms of biomechanics."
In fact, the transverse tarsal arch is actually responsible for 40 percent of your foot stiffness, according to the paper.
These feet are made for walking
“There is a key mechanical question to ask, which is how does the foot sustain the forces that it has to apply in order to be used the way it is?” Madhusudhan Venkadesan, the lead researcher on this study and professor at Yale University, tells Inverse.
“What we found was that the transverse arch itself contributes to the stiffness of the foot quite a bit. And the contribution appears to be as much or more than the longitudinal arch," he says. The longitudinal arch runs from front to back on the foot, and is the subject of more research.
But the new study suggests the importance of the shorter, transverse arch to understanding how and why we walk on two feet has been overlooked and underestimated.
To understand the function of the transverse tarsal arch, the researchers modeled it using a combination of computer simulations and physical experiments on mechanical feet mimicking humans'. Then, they did bending tests on two human cadavers' feet.
With the human feet, they cut the skin between the toes and the tissue of the ball of the foot — snipping these transverse tissues decreased foot stiffness by 44 percent and 54 percent respectively, for the two feet.
“Observations in day-to-day life really inspired this interpretation of the function of the transverse arch,” Mandre says.
“It has to do with, for example, when you eat pizza or when you are trying to insert a currency note in a vending machine. The pizza normally droops downwards or other currency noted droops down and in order to make it straight, what we do is curl it slightly width wise and that makes it stiffer. And it was that observation that also gave us the idea that the foot is also arched in the transfers direction. Will that also make it stiffer?"
“The foot is one of the least understood structures of our body in terms of biomechanics," Kristiaan D’Aout, professor of Musculoskeletal Biology at University of Liverpool, tells Inverse. D’Aout was not involved with this research.
"Yet, it is one of the most important ones because many people suffer from foot related health problems, or issues “higher up” in the body (in the knee or back) that are caused by foot issues."
The foot's complex anatomy, which varies a lot between people, also makes it difficult to study, he says. As a result, it is hard to draw a line between anatomy and function.
"It is always surprising that the very everyday things that we use or we see remain unexplained."
“This study is very elegant because it presents a very simple model, yet it might be able to explain a major part of foot function,” D’Aout says.
“With this knowledge, many things become possible. We can focus our attention better to 'the important parts' of fossil feet and interpret them better.”
Feet: An evolutionary miracle
The findings also suggest novel ways to study the foot on an evolutionary level.
"I think this is one of the most innovative and important studies on the human foot. It’s a game-changer," Daniel E. Lieberman, a paleontologist at Harvard University. Lieberman was not involved in this research.
"It helps explain why and how people with flat feet are able to walk just fine, and how our ancestors were able to walk for millions of years before the longitudinal arch evolved."
By analyzing a database of how the arches in feet have evolved across all primates, the researchers found that it took various evolutionary steps for our arches to form — and only the genus Homo had both fully developed arches.
"This is very exciting. Clearly modern human-like feet with longitudinal arches don’t appear until about 2 million years ago, but we have evidence that hominins were walking bipedally, starting about 7 million years ago when we split from the chimpanzee lineage," Lieberman says.
"So this new model gives us a whole new way to think about the evolution of human walking and running."
This means that it might be what makes the difference between how you run, and how an ape does, and this tiny arch may have played an important part in making humans two footed creatures. The paper suggests that these inferences need to be tested with additional fossils incorporating more aspects of foot morphology, but this is a good start to finding the right direction.
“This is a case of new tricks for an old pony,” Venkadesan tells Inverse. “It is always surprising that the very everyday things that we use or we see remain unexplained. And finding explanations of those seems like a basic, not only intellectual requirement, but a requirement to improve our own view of the world.”