By Colin Barras | NewScientist
You may be walking on chimp-like feet without knowing it. At least 1 in 13 of us have feet that are specially adapted for climbing trees.
Textbooks will tell you that the human foot is rigid, which allows more efficient walking. Other apes, in contrast, have flexible feet better suited to grasping branches as they move through the trees. But the textbooks are wrong, say Jeremy DeSilva and Simone Gill at Boston University.
The pair asked 400 adults to walk barefoot around the Boston Museum of Science while they filmed their feet. This revealed that 8 per cent of people have some mid-foot flexibility, rather like that seen in tree-dwelling apes (American Journal of Physical Anthropology, doi.org/mmh). In another, soon-to-be-published analysis, Robin Huw Crompton at the University of Liverpool, UK, found that a flexible mid-foot may be even more common than DeSilva and Gill suggest.
So what does a flexible foot look like? It bends at the ball of the foot, and also halfway between the heel and the ball. All human feet have a joint at this point – but in most of us, stiff ligaments span the joint and keep it rigid. In some people, however, the ligaments are softer, allowing the mid-foot to bend. Close-ups of flexible feet as they unroll during walking make the bend obvious, but DeSilva says their owners were not aware of anything unusual, nor was their gait any different. "I was, and continue to be, surprised by this," he says.
Crompton thinks flexible feet may have been with us since the dawn of our species, as a relic of our tree-dwelling days. Other features were lost along the way. Chimpanzees, for instance, have opposable toes for gripping.
On the strength of his study, Crompton believes most feet can produce this flexibility in specific circumstances, adding vital stability. "For instance, our work shows that it is important during a sudden change of speed."
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