Scientists Study 3.5-Million-Year-Old Origins of Unique Human Spring
Scientists have proven the arch running across the top of the human foot evolved in a 3.5-million-year old human ancestor, enabling us to walk and run upright unlike all other primates.
Sports scientists and evolutionary biologists have greatly overlooked the transverse arch when attempting to explain the way the human foot deals with weight dispersal, but now a team of researchers have determined that the foot’s transverse tarsal arch (TTA), which provides almost half the foot’s stiffness enabling us to walk and run on two feet, evolved in an ancient human ancestor around 3.5 million years ago.
The scientists findings published in the journal Nature demonstrate a “key step in human evolution” and Dr. Glen Lichtwark and Dr. Luke Kelly, from the University of Queensland , said in their paper that the human arched foot is a “unique feature” crucial for human bipedalism and that it is not present in other primates. They added that the TTA was a “quirk of human anatomy hidden in the foot,” which answers the long standing problem of ‘how’ humans evolved to walk and run on two legs.
Human footprints in the sand showing the shape of the human foot and arch. ( kozlik_mozlik
/ Adobe stock)
Tracking the Ancient Origins of Walking
Other primates have much flatter and stiffer feet than humans and cannot walk or run upright, but now the secret to how humans evolved to walk and run on two legs has been resolved. It’s all down to the TTA, where the tarsal bones meet the metatarsals over the top of the foot. The scientists inform that this one arch provides 40 per cent of the foot’s stiffness and permitted our early hominin ancestor, Australopithecus afarensis, to evolve bipedalism as early as 3.5 million years ago, explaining how we started walking on two feet.
The traverse arch provides the foot with a stiffened lever that passes the forces generated by one’s leg muscles as they push upwards against the ground and the scientists said the TTA also “retains sufficient flexibility” so that it functions similarly to a spring by storing and releasing “mechanical energy.”
According to a report in National Geographic , most studies of the human foot have focused on the medial longitudinal arch (MLA) running from the heel to the ball of the foot, but Mr. Madhusudhan Venkadesan, an assistant professor at Yale University, analyzed the feet of human cadavers and created mechanical models trying to identify the source of the TTAs stiffness.
Schematic of a human foot skeleton showing the arches and typical loading pattern
(M. Venkadesan / Nature)
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Slicing ligaments across the transverse arch caused stiffness to fall by more than 40 per cent. Putting this in context, when the scientist cut the plantar fascia ligaments, which runs along the bottom of the foot, the measured stiffness only reduced by 23%, which proved the TTA played a much greater role in stiffness than the MLA.
Computer Simulations and Robotic Feet
The researchers generated computer simulations and plastic models to study the structure and function of ancient feet in Homo erectus , Homo habilis and Homo naledi . Mr Venkadesan said the new evidence suggests an ancient human-like transverse arch must have evolved “over 3.5 million years ago”, which he says is 1.5 million years before the emergence of the genus Homo, therefore its development was a key step in the evolution of modern humans.
The plastic models with more pronounced TTAs were found to be stiffer and less susceptible to bending than flatter ones. Professor Mahesh Bandi, from the Okinawa Institute of Science & Technology ( OIST) said that in contrast, on the plastic models, an increase was noted in the curvature of the LA (longitudinal arch), which had “little effect on the stiffness,” indicating the transverse arch plays a bigger role in foot stiffness than the longitudinal arch.
Schematic showing bending test on a cadaveric human foot. (M. Venkadesan / Nature)
The Future of The Human Foot
This new research also found that when a human foot is planted and weight is loaded, the far end of the metatarsal bones fatten out on the ground and the proximal head of the bones (furthest from the toe and closest to the ankle) all rise to different levels, line up, twist and lock firmly in place producing an arch.
Then the torsion (twisting due to an applied torque) was measured in the fourth metatarsals, which provides the foot’s rigidity. Dr. Lichtwark and Dr. Kelly speculate that because some people have noticeably flat feet, whereas others have a high arch, maybe those with flat feet have less curvature of their transverse arch, reducing stiffness compared with those whose feet have high arches.
Measuring the transverse arch of the human foot using metatarsal torsion of a single bone. (M. Venkadesan / Nature)
But the scientists also consider it possible that people with flat feet have “sufficient transverse-arch curvature,” which they say compensates for their low longitudinal arches and maintains the stiffness needed for effective walking and running. The doctors also think it conceivable that a range of new treatments might grow from this greater understanding of the mechanics of the transverse-arch for various foot disorders and also in the design of prosthetic limbs and walking robots.
Top image: Image of human foot demonstrating the arch. Source: Alessandro Grandini
/ Adobe stock
By Ashley Cowie