Function and Evolution of The Arches of The Foot
The medial longitudinal arch in particular creates a space for soft tissues with elastic properties, which act as springs, particularly the thick plantar aponeurosis, passing from the heel to the toes. Because of their elastic properties, these soft tissues can spread ground contact reaction forces over a longer time period, and thus reduce the risk of musculoskeletal wear or damage, and they can also store the energy of these forces, returning it at the next step and thus reducing the cost of walking and, particularly, running, where vertical forces are higher. The non-human apes (the gibbons, mountain and lowland gorillas, orang-utan, chimpanzee and bonobo) tend to walk on the lateral side of the foot, that is with an 'inverted' foot, which may reflect a basic adaptation to walking on branches. It is often held that their feet lack longitudinal arches, but footprints made by bipedally walking apes, which must directly or indirectly reflect the pressure they exert to support and propel themselves do suggest that they exert lower foot pressure under the medial part of their midfoot. However, human feet, and the human medial longitudinal arch, differ in that the anterior part of the foot is medially twisted on the posterior part of the foot, so that all the toes may contact the ground at the same time, and the twisting is so marked that the most medial toe, the big toe or hallux, (in some individuals the second toe) tends to exert the greatest propulsive force in walking and running. This gives the human foot an 'everted' or relatively outward-facing appearance compared to that of other apes. The strong twisting of the anterior part of the human foot on the posterior part tends to increase the height of the medial longitudinal arch. However, there is now considerable evidence that shoe-wearing also accentuates the height of the medial longitudinal arch and that the height of the medial longitudinal arch also differs very considerably between individuals and at different speeds. It is not yet agreed to what extent the early human ancestor Australopithecus afarensis, (3.75 million years ago onwards) had acquired a functionally human-like foot, but the medial twist of the forefoot evident in fossil footbones of this species, and in the Laetoli footprint trail in Tanzania generally attributed to this species, certainly appears less marked than is evident in fossil footbones of Homo erectus (sometimes called Homo georgicus) from Dmanisi, Georgia (c. 1. 8 million years ago) and the roughly contemporaneous fossil footprint trail at Ileret, Kenya attributed to Homo erectus ergaster.
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