Anthropology of the human foot – the past, present and future

Kartik Hariharan

Kartik Hariharan is a Consultant Foot and Ankle surgeon at Aneurin Bevan University Health Board. He is Past President of BOFAS and delivered the Naughton Dunn lecture in 2015 on which this article is based.

From single cell amoeba to the human species

Locomotion more than any other single factor is, in my eyes, the most important attribute for the successful survival of a species. Locomotion allows the species to find sources of nutrition when one area is depleted, allows rapid movement from one area to another in times of adversity, allows for successful reproduction and the propagation of species DNA in areas of plenty.

Perhaps most important of all, it allows preferential mutations to accumulate by the cross transfer of genetic material thereby avoiding inbreeding and its attendant genetic degradation. From a single celled amoeba to a vast collective tribe of wildebeest, every species has to move to survive and flourish.

This incredible attribute was not bestowed upon us nor was it a single event or sequence of events. It is one that went through many millions of years of subtle, sudden, random but serendipitous changes, often going down dead ends and failed transformations and eventually to where we are today.

We are not the only species to have this attribute, but perhaps, are one of the few species to exploit it, to dramatically alter the functions of other aspects of our body: the ability to use our hands, transform our brains to think and to transform the very environment, which in itself is the most potent driver of evolution.

Therefore in essence this article is a bird’s eye view of human bipedalism: a process that took several million years of change and refinement to reach the stage we are in today.

Evolution at work

Initially vertebrate land animals were waddling tetrapods like our modern crocodilian species. Therapsids perished because like our modern alligator they couldn’t breathe when they walked or ran due to compression of the respiratory system. If we were to look at the skeletal anatomy of a primitive tetrapod limb, we would see three tarsal rows (Figure 1). This was however a simplistic arrangement that lacked sophisticated concepts such as a subtalar joint, so this foot was rudimentary but adequate in function for its day.

Figure 1: Primitive tetrapod limb showing three tarsal rows.

A very important evolutionary event that took place to change a Therapsid limb to a mammalian foot was the introduction of a construct where a talocalcaneal articulation capable of independent movement developed.

Let us take a marsupial foot for example. The foot of a bush tailed possum is clearly a prehensile one, used for clutching climbing grooming and mating. What is impressive is that the fibula is as much a weight bearing bone as the tibia with wide articulations with the talus and the Os Calcis. Closer inspection of their ankle joint reveals a fascinating structure; a meniscus that separates the articular surfaces of the tibia and fibula whilst also contouring the articular surface into a trochlear shape somewhat replicating a human talus.

As we follow the evolution from ancient marsupial to modern man, one can clearly see the decrease in the articulating area of the fibula and the compaction of the talus into a trochlear structure within a mortise between the tibia and fibula. What also happened is that there is now clear separation into ankle, subtalar, and talonavicular joints by synovial septae.

Now why did these early marsupials need such unstable and unrestrained ankles? The reason probably was climbing; scaling vertical tree trunks with the plantar surface of the foot opposed against them. This required significant rotary motion of the tibia relative to the fibula somewhat analogous to the pronation and supination seen in the upper limb between the radius and ulna.

Development of bipedalism

There are many types of bipedalism. Facultative bipedalism is often seen in the primate population sometimes assisted by knuckle walking. This form of locomotion only manifests in about 2 to 6% of all mileage covered by most apes including the bonobos and the orangutans our closest relatives. As the name suggests habitual bipedalism is the main mode of locomotion in many species including the Australian marsupials and of course all birds which are obligatory bipeds with forelimbs transformed into wings.

The ungulates are a group of herbivores including horses, sheep, rhino hippo and pigs. Also curiously they include whales and porpoises who have lost terrestrial locomotion. They are characterised by walking on one toe: in the horse this is the middle toe which is remarkably adapted for speed and agility. They can use these adaptations to generate remarkable speeds! Not unsurprisingly sesamoiditis is not uncommon in horses and arthroscopy of the MTP joint is not an uncommon veterinary procedure.

As we move on to the Avian group, birds morphed their forelimbs into wings which still bear the skeletal pattern of any other forefoot or hand. Although birds walk bipedally, the avian foot’s main function is grasping either prey or branches of trees where they land and roost. We now enter the realm of the tree climbing primates.

Many lower primates have horizontal bodies during climbing which require the use of all four limbs and their tail such as in golden tamarins. The orangutans and the bonobos exhibit a much more sophisticated type of climbing with an orthograde body and effective utilisation of all four limbs. Their feet are very prehensile with an abducted opposable hallux and long phalanges which are curved to help with grasping and climbing functions.

Finding Lucy

About 3 - 4 million years ago there appears to be a breakthrough in evolutionary terms. Nature seems to have stumbled upon a winning formula to start the trend of modern obligatory bipedal walking.

On a hot and sweltering mid-day in Hadar, Ethiopia on the 24th November 1972, Donald Johansson and his colleague Tom Gray were returning from a fossil hunt when they chanced upon a fossilised ulna. Further digging revealed an almost complete skeleton. She was named Lucy during a post expedition party when ‘Lucy in the Sky with Diamonds’ was playing repeatedly. She was 3.2 million years old and was the oldest hominid or erect walking ancestor found to that date (Figure 2). Lucy did walk upright but her hallux was divergent, her upper limbs were long and gangly, her phalanges were curved and her knees were not straight; all of these were attributes of climbing creatures but her pelvis was orientated like that of homo sapiens as was her femur skull and spine and ankles. Was she a hominid a creature that walked upright? Did she or could she still climb trees? Were her feet prehensile? These are questions that still wait to be answered by anthropologists worldwide.

Figure 2: Cast of Lucy.

In 1978 the celebrated anthropologist Mary Leakey and her associates discovered footprints in solidified volcanic ash in Laetoli Tanzania which were thought to be made by two individuals possibly a male and female of the Australopithecus Afarensis species (Figure 3). Since then, several footprints of varying species have been discovered including that of homo erectus, Cro Magnon and neanderthal man. What these footprints showed us was that significant changes in the morphology of the foot were occurring as bipedalism continued to be reformatted and refined.

Figure 3: Ancient hominid footprints in Tanzania.

Sapiens

Possibly as far back as 40,000 years ago, the features we associate with modern homosapiens started to appear. By now hominin had been endowed with remarkable feet, which took on the quintessential functions of propulsive ambulation and shock absorption by several mechanisms.

Several anatomical changes took place: verticalisation of the pelvic skeleton, walking with an extended knee to create our stiff kneed gait, altered biometrics of the foot, longer legs, longer metatarsals, unopposable hallux, straighter and shorter phalanges, stable mortised ankle joints, rigid mid-feet, and a complex subtalar joint. Remarkable changes, each of which were naturally selected to promote efficient bipedalism.

So we now have this biomechanical marvel of locomotion technology; a walking organ with an array of shock absorption mechanisms from the plantar fat pad of the heel, a longitudinal arch with a sophisticated windlass mechanism which also conserves and stores energy, proprioceptors and motion sensors, proximal muscles such as the Triceps Surae and the tibialis anterior contracting eccentrically to reduce the shock of ground reaction force: not to forget a torque converter that is the subtalar joint with its pitched calcaneus and complex midfoot articulations. The ability to use our hands and our brains to make tools, plan hunting and resource management strategy, carrying food and flint for fire, carrying offspring etc is now much easier because of the luxury of our modern feet and lower limbs.

The price of evolution

There has been a price to pay for these remarkable changes though. Two feet can’t cope with the shock absorbing functions of four, so we traded speed for efficiency and economy of energy. As we have retained the double curve of our spines, we suffer backache and other musculoskeletal problems. We are cursed with enthesopathy, stress fractures and the fragile soles of our feet that can’t stand the damage of rough or sharp objects. As we lost speed and were better able to absorb shock, we lost bone mass. We have the thinnest lower limb bones amongst all modern apes. Indeed, Osteopenia is a finding almost only in humans amongst all primates.

Technology can overcome almost any deficiency. Our walking needs are decreasing with rapid transit systems cars with endless solar and renewable power taking over. But where are we heading in the future? With climate change almost certainly accelerating and the surety of massive increases in sea levels, are we heading for a watery world where the need for swimming and webbed feet starts to ascend? Or are we going to inhabit worlds with lesser gravity so the need for shock absorption becomes less important? Or is the function of the subtalar joint going to become redundant with shoes, orthoses, and ultra-predictable walking surfaces? The consequences of unpredictable and random evolution with natural selection are something that we are now familiar with and can understand; but our terrifying ability to manoeuvre and engineer this remarkable phenomenon is in my opinion, leading us to unchartered territory (Figure 4).

Figure 4: Future human evolution?

The best part of the story of the human foot is that it is never ending.

Conclusion

Spontaneous and random mutations will continue to shape the features and functions of the human foot for as long as humanity exists and endless forays into selection of favourable attributes will continue to occur through our time on this earth and shape its destiny. I hope I have, in some small way, kindled your interest in the anthropology of the human foot and its connotations and echoes in the modern world. There is so much to learn and understand, for I do believe that very many abnormalities and dysfunctions of the foot have their origins in human pedal evolution. I do hope you will look at the foot in your clinics somewhat differently and perhaps treat its disorders with an evolutionary ear to the ground.