An unappreciated quirk of human anatomy hidden in the foot is thought to be the secret behind how humans evolved to walk and run on two legs.
The feature, dubbed the transverse tarsal arch (TTA), runs over the top of the foot from left to right and provides 40 per cent of the foot’s stiffness.
It is this stiffness that is thought to have been essential in allowing early hominins to evolve bipedalism around 3.5million years ago.
Other primates, both extinct and alive, have far flatter feet which are less stiff and poorly adapted for walking upright.
Only species of the genus Homo – of which humans are the only living member – possess the key arch.
Researchers believe their findings may explain how Australopithecus afarensis, a human relative which lived around 3.66 million years ago, could generate footprints.
A quirk of human anatomy hidden in the foot is thought to be the secret behind how humans evolved to walk on two legs. The feature, dubbed the transverse tarsal arch (TTA), runs over the top of the foot from left to right and provides 40 per cent of the foot’s stiffness
The scientists believe their findings, published in the journal Nature, show a ‘key step’ in human evolution and may also help to improve the design of robotic feet.
In a comment article, Glen Lichtwark and Luke Kelly from the University of Queensland, write: ‘Humans evolved to walk and run effectively on the ground using two feet.
‘Our arched foot, which is not a characteristic of other primates, is a unique feature crucial for human bipedalism.
‘The arch provides the foot with the stiffness necessary to act as a lever that transmits the forces generated by leg muscles as they push against the ground.
‘The arch also retains sufficient flexibility to function like a spring to store and then release mechanical energy.’
Madhusudhan Venkadesan, an assistant professor at Yale, studied the feet of human cadavers and mechanical models to discover the extremity’s source of stiffness.
Most previous studies and pre-existing research has focused on another arch, the medial longitudinal arch (MLA), which runs from the heel to the ball of the foot.
As a result, the transverse arch went overlooked and its role in human locomotion was unknown.
Dr Venkadesan took frozen cadaver feet, thawed them out and cut the ligaments that spanned across the transverse arch.
When the foot’s stiffness was assessed, the loss of these ligaments keeping the transverse arch together saw stiffness fall by more than 40 per cent.
By comparison, cutting the plantar fascia ligament along the bottom of the foot, which negates the longitudinal arch, reduces stiffness by just 23 per cent.
This finding from the new study indicates the transverse arch plays a bigger role in foot stiffness than the longitudinal arch.
It is therefore likely that it was integral in the evolution of bipedalism in our species.
Mr Venkadesan said: ‘Our evidence suggests that a human-like transverse arch may have evolved over 3.5 million years ago, a whole 1.5 million years before the emergence of the genus Homo, and was a key step in the evolution of modern humans.’
The researchers also created computer simulations and plastic models to analyse the structure and function of ancient human ancestor feet, including those of the species Homo erectus, Homo habilis and Homo naledi.
Professor Mahesh Bandi from the Okinawa Institute of Science & Technology (OIST) said: ‘We found that the plastic models and simulations with more pronounced TAs (transverse arches) were stiffer and less susceptible to bending than flatter ones.
‘In contrast, on these models, an increase in the curvature of the LA (longitudinal arch) had little effect on the stiffness.’
This finding from the new study indicates the transverse arch plays a bigger role in foot stiffness than the longitudinal arch. It is therefore likely that it was integral in the evolution of bipedalism in our species
Other primates, both extinct and alive, have far flatter feet than humans which are less stiff and poorly adapted for walking upright. Only species of the genus Homo – of which humans are the only living member – possess the key arch
The lateral side of the metatarsals all line up whewn the foot is planted. They twist and lock in place firmly (pictured). The torsion was measured in the fourth metatarsal and is what allows the transverse arch to provide most of the foot’s rigidity
Madhusudhan Venkadesan, an assistant professor at Yale, studied the feet of human cadavers and mechanical models to discover the foot’s source of stiffness and how much was due to the transverse arch. Pictured, how the feet were assessed
The role of the transverse arch can be simplified and imagined by picturing a piece of paper.
Pinch the flat sheet between two fingers and the paper flop downwards, with no rigidity.
However, bent the paper and pinch and the sheet will become stiff and support its own weight. It is the lateral curvature that firms up the entire structure.
The role of the traverse arch in the foot is more complicated but the theory remains the same.
The research found that when a human foot is planted and loaded, the far end of the metatarsal bones fatten out on the ground.
The proximal head of the bones – the end closer to the ankle that the toe – all rise to different levels, producing an arch.
It is this side of the bone which all line up, twist and lock in place firmly. The torsion was measured in the fourth metatarsal and is what provides the foot’s rigidity.
In the comment article, Dr Lichtwark and Dr Kelly speculate about what this might mean for modern humans with flat feet or overly-pronounced arches.
They write: ‘Some people have noticeably flat feet whereas others have a high arch.
‘Perhaps those with flat feet have less curvature of their transverse arch and thus potentially reduced stiffness in their feet compared with those whose feet are less flat.
‘But it is also possible that people with flat feet have sufficient transverse-arch curvature to compensate for their low longitudinal arch, thereby maintaining sufficient stiffness for effective walking and running.’