By Bruce Latimer, PhD
The foot ranks high among the most ill-constructed elements of the modern human musculoskeletal system—an observation that necessarily begs the question: Why is this so? Upright, bipedal gait is the defining characteristic of our lineage; humans and our immediate ancestors have been practicing it for at least 5 million years. Why hasn’t evolution perfected this hallmark adaptation?
Indeed, sprained ankles, stress fractures, fallen arches, bunions, plantar fasciitis, and misalignment maladies of the great toe are among a host of common injuries likely to be encountered by many of us in the course of our lives. The persistence of such maladies, and some of the etiological factors involved, can be explained by examining the evolutionary history of our species, in which the seemingly jury-rigged aspects of the human foot in fact make sense.
The Foot: Many Pieces Mean Greater Risk of Failure
The human foot comprises 26 diminutive bones, 33 joints, and more than 100 muscles, tendons, and ligaments. That, by any standard, is an extremely complex biomechanical system, comprising a lot of moving pieces. As in any mechanical system, the chance of malfunction increases with the number of moving elements.
The latter point is particularly relevant in a large-bodied species that locomotes bipedally and, in doing so, concentrates reaction forces on a single extended limb. Moreover, the forces that can exceed multiples of body weight during such activities as running and leaping are not confined to a single anatomic plane but alter dramatically in directionality from a posterolateral direction at heel-strike to an anteromedial direction at toe-off. It is clear that, during high-energy activities, the multiple, diminutive skeletal elements held tenuously together by diverse and elaborate ligamentous and muscular tethers are at serious risk of irreparable damage.
In Descending From the Tree…
That said, if one were designing a human foot “from scratch,” the complex foot we possess today would not appear to be the best choice. It is important to recognize, however, that biological evolution does not result in novel adaptations. Rather, evolution “tinkers” with preexisting materials and is therefore constrained in its ability to adapt to new environmental challenges. This means that humans inherited their seemingly overly complicated foot from their African ape ancestors.
Among living primates, humans are the only species to have relinquished tree-climbing for obligate terrestrial bipedality. The highly flexible primate foot, with its grasping hallux, is an obvious adaptation to tree climbing. Humans, in adapting to bipedality, have rearranged the elements that comprise the ape foot: notably, they have uniquely forfeited the abductable, grasping great toe, permanently aligning this element to the long axis of the foot.
It is clear, then, that the stiffened, multisegmented modern human foot is an evolutionary consequence of our tree-climbing ape ancestors. Once having left the trees, this pliable, accommodating, grasping foot adapted to the requirements of full-time, upright, terrestrial bipedality.
Role and Workings of the Pedal Arch
The diverse adaptive modifications for habitual bipedality are numerous and range throughout the human body. For this discussion, however, my focus is on the foot—specifically, the arch—and its role in 2-legged locomotion.
- Two critical factors must be accommodated by the human foot:
- The foot is a propulsive lever during impulse production.
The foot is an energy dissipater during force-generating situations, such as running and jumping.
The latter function is critical; many distinctive adaptations in the human lower limb are methods to ameliorate these dangerous loading scenarios and protect the gracile foot bones and, importantly, the nonrenewable cartilaginous articular surfaces.
The primary energy-absorbing mechanism in the foot—the longitudinal pedal arch—is an exclusively human adaptation. This idiosyncratic structure was achieved by permanently adducting the hallux and, in doing so, completely negating its grasping function. It is among the oldest of bipedal adaptations. (The 3.6 million-year-old footprint trails from the fossil locality of Laetoli, Tanzania, show clearly that this transition had already taken place. Pedal elements from the fossil site at Hadar, Ethiopia, where the famous fossil bone pieces that came to be known as “Lucy” were discovered, also indicate that, by 3.5 million years ago, the first ray was no longer capable of functional abduction. DeSilva and colleagues1 have provided a detailed description of the paleontological evidence for these observations.)
During the human gait sequence, foot-flat is that period in stance phase during which both the metatarsal heads and calcaneus are in contact with the substrate. At this time, the ground reaction will result in flattening of the longitudinal and transverse arches (Figures), which, in turn, allows eccentric (lengthening) contraction of the intrinsic muscles of the foot as well as elongation of passive ligamentous elements. The strong pennation and consequential short contractile distances of the numerous intrinsic foot muscles allow energy dissipation and mitigation of potentially dangerous reaction forces. This energy-absorbing function explains why humans maintain a large number of intrinsic pedal muscles; clearly, they serve little purpose in mobility.
In short, the pedal arches and attendant muscular and ligamentous structures are evolutionary adaptations that protect our residually complex, gracile foot bones and their intervening, cartilage-covered articulations from damage.
Pathology: One Consequence of Adaptation
Associated with these sui generis human adaptations are several similarly unique pathological conditions that beset only our species. In the absence of the arch (pes planus) and its force-dissipating action, military recruits and athletes often develop stress fracture of the metatarsals, fibula, and tibia. In addition, pes planus can also result in arthritis of the foot and ankle and likely plays a causative role in shin splints.
Other maladies that are limited to humans and associated with a nonfunctioning or poorly developed arch include hallux valgus and hallux varus, problems in which the great toe is forced out of its normal anteroposterior alignment. This creates bunions and disorders related to the misalignment of the flexor tendons and consequent maladies related to displacement of the hallucal sesamoid bones. Because only humans (among extant mammals) possess a longitudinal arch and a permanently adducted hallux, these problems are also specific to our species.
The seemingly overly complex human foot is a consequence of our having evolved from tree-climbing apes. Upon becoming bipedal, the foot adapted to the terrestrial environment, with its enhanced and altered reaction forces, by relinquishing the grasping hallux to create the force-dissipating arch. The unique pedal arch, however, resulted in novel maladies that continue to plague our species.
Biological evolution does not—cannot—lead to perfection; it works with available materials, and resulting adaptations can have novel pathological consequences. The arch of the human foot evolved as a mechanism to permit long-distance terrestrial upright walking, making bipedality the defining characteristic of our species. The maladies associated with that adaptation also define us uniquely.
Bruce Latimer, PhD, is Professor, Departments of Anatomy, Orthodontics, and Cognitive Science, Case Western Reserve University School of Medicine, Cleveland, OH.