User:Humanfoot/sandbox

=Evolution of the Human Foot=

The human foot is physiologically and functionally different from those of hominin ancestors and currently living nonhuman primates. The feet of modern nonhuman primates and hominin ancestors had feet that were primarily grasping organs, while modern human feet act as a lever during bipedal locomotion. Major physiological changes occured in the foot, particularly in the joints of the ankle, the medial longitudinal arch of the foot, as well as the hallux and phalanges. The evolution of bipedalism placed many selective pressures on the body, and studying fossils of hominin ancestors provides insight into the mechanism of the evolution of the human foot.

Demands of Bipedalism
The last common ancestor of currently living primates is thought to have used quadrupedal locomotion and was primarily arboreal. As body began to position itself upright, the feet and lower extremities began to bear more weight. As a result, there was a greater need for increased stability and ability to disperse forces in the foot. Furthermore, the advent of bipedalism allowed for the development of excellent endurance running capabilities, which was particularly important for early hunting strategies. Humans are the only modern primates capable of endurance running. In order to be able to maintain a running speed for a significant period of time, physiological adaptations were necessary to increase the efficiency of the exchange between kinetic and potential energy during the motions.

Support
The ability of the human foot to both bear weight and maintain enough mobility to endure the forces of locomotion is unique to Homo sapiens. The foot has developed several features to distribute force more evenly through the surface area of the sole, as well as muscular and ligamentous adaptations that act as springs to reduce metabolic costs during activity. Balance is critical for proper bipedal locomotion. Balance is maintained through a combination of rigidity in the the metatarsus as well as the flexibility in the ankle to adjust positioning of weight as needed. Finally, the first phalanx, also known as the hallux, is critical for balance during standing and during stride.

Grasping
As the supportive and lever functions of the foot began to take precedence, the grasping function was lost. Grasping in arboreal primate feet is important for maneuvering through trees and holding rounded branches. Arboreal primates' feet do not need to support as much of their weight on their hind legs for significant periods of time, and the hands of modern H. sapiens more closely resemble the feet of homonid ancestors than modern feet. As a result, the foot lost much of its dexterity, particularly in the hallux.

Ankle
When the human ankle is compared to the ankle of other primates, it is notably different. Firstly, it can be noted that the bone structure of the joints within the ankle are quite robust and suited for bearing weight. The major bones in the ankle are the talus, calcaneous, navicular, and cuboid, which form the talo-navicular and calcaneo-cuboid joints, together known as the midtarsal joint. In many homonins, these two joints became oblique, which limits the movement of both joints at once. This effectively stacks the joints on top of eachother within the ankle, which aids in stabilizing the foot during propulsive bipedal movement. Another physiological change in the ankle is the more pronounced Achilles tendon in humans. This tendon connects the heel with the plantar flexors in the foot and acts as a spring during running. The tendon is crucial for efficient endurance running and reduces the metabolic cost of this activity by almost fifty percent. The properties of the Achilles tendon in fossil specimens can be determined by comparing the point of insertion, the transverse groove of the calcaneous, with human and other modern primate specimens.

Arch
A very important feature in determining bipedal gait and efficiency is the presence of the medial longitudinal arch in the foot. This arch acts as another spring in the foot while providing plantarflexion in the toe-off portion of the stride. It also provides some shock absorption during heel strike, as well as maintaining rigidity in the mid-tarsal zone. It is thought that the medial portion of the arch arose before the longitudinal portion due to initial changes in the ankle and phalanges, which increased weight bore by the metatarsal heads during toe off. Evidence for arch development in fossilized specimens can be seen by examining the robusticity of the navicular tuberosity to determine to what extent it was weight bearing, as well as searching for grooves on the talus which provide points of insertion of plantar calcaneo-navicular and cubo-navicular ligaments. Another way to study the arches of extinct species is to examine fossilized footprints and compare the instep in the impression to the instep made in a similar material by other species.

Phalanges
One of the most distinct differences between the feet of humans and other primates is the morphology of the phalanges, or toes. In many nonhuman primates, the hallux is significantly more adducted than the hallux of modern humans, where the average angle of adduction is eight degrees. It is adaptive for the hallux to be more adducted in other species due to their arboreal nature and grasping needs. Another distinct feature of H. sapiens phalanges is their relative robusticity. The average human metatarsal robusticity sequence is I>V>IV>III>II, with metatarsal I representing the hallux. The robusticity sequence of more chimpanzee-like primates more closely resembles the robusticity sequence for the human hand. The hallux and associated metatarsal need to be robust in human bipedal walking in order to support the forces presented during the phase of the stride where weight is transferred to the metatarsals, known as the 'ball of the foot', and then the toes to complete the toe-off phase. The short length of the toes is more appropriate for bipedal locomotion in order to create a more efficient stride. Long toes result in a high stepping gait in order to gain foot clearance, which can cause increased stress on the joints of the lower body.

Hominin Foot Physiology
There are more than ten species and subspecies that are thought to be ancestral to the current human population, and as more fossil evidence of hominin species is found, it is likely that the number of recognized species will grow. Classifying extinct species using fossil morphology can be challenging, as not only do many materials accumulate significant wear, but post-cranial skeletal elements can be difficult to find. As a result, many species are classified based on data obtained from fossilized skulls, and foot bones have not been found for every species. The species discussed below include species that have fossilized bone evidence associated with them, as determined by experts in fields such as anthropology and evolutionary biology.

Australopithecus afarensis
Australopithecus afarensis was a mostly terrestrial homonid biped that lived between 3-4 million years ago. As with all homonids, A. afarensis appears to have had a mosaic of both human traits, and those of other primate species. Several foot specimens have been found, including the famous Laetoli footprints, and there is some evidence of plantar arch development. The lateral cuneiform is elongated, as in humans, and has points of insertion for plantar ligaments which aid in plantarflexion and the arch structure. Also, the talus has grooves for insertion of the plantar calcaneonavicular and cubonavicular ligaments, which suggests that A. afarensis had a small medial arch, supporting the hypothesis that the medial arch in the foot arose before the longitudinal arch. However, the navicular bone has an enlarged medial tuberosity, similar to that of modern chimpanzees. This finding suggests the navicular bone bore weight during motion. Upon examination of the calcaneous, the transverse groove closely resembles the transverse groove present in extant chimpanzees. This type of transverse groove is common to many Australopithecus specimens, and suggests that their bipedal gait was primitive and most likely did not resemble that of H. sapiens. It also suggests that the Achilles tendon that is seen in humans likely arose no sooner than approximately three million years ago.

Australopithecus africanus
Australopithecus africanus is another homonid that is estimated to have lived between 2-3 million years ago. The most complete and informative evidence of this species is a partially complete skeleton of an individual, known as "Little Foot" due to its small, gracile stature. Although this species is thought to have been primarily arboreal, the morphology of the specimen is intermediate between human and nonhuman ape traits, and there is some evidence of physiological adaptations that would allow bipedal locomotion. The hallux is notably adducted from the rest of the foot, which would have been advantageous in its arboreal environment, and this feature is not represented in humans. The navicular bone appears to have had traits resembling those of the extant pongid group more than humans, as the navicular head is robust suggesting that it bore weight much more than the navicular head of humans. This species is another example of the how modern bipedal gait developed slowly, with skeletal changes occurring over large volumes of time.

Homo habilis
The foot morphology of Homo habilis is intermediate between that of humans and chimpanzees like A. afarensis, and A. africanus, but it is one of the oldest known species which appears to be much closer to human morphology than any other extant primate. The evidence of foot morphology in this species is very well represented in the Olduvai Hominid 8 specimen that was recovered in the Olduvai Gorge of Africa, which is estimated to be approximately two million years old. The OH8 foot is composed of an almost complete tarsus and metatarsus of the left foot. H. habilis was likely a habitual biped that still spent some of its time climbing and living in trees. The metatarsals are robust in order to support weight during the toe-off of a bipedal stride. The cuboid bone is significantly more narrow than what is observed in chimpanzees or in older hominin species. The medial cuneiform in this specimen is flat, which is interpreted as a transition from the convex surface seen in other apes to the concave surface seen in humans. The lateral cuneiform also shows modifications as it is rectangular like the cuneiform in humans, unlike the squared bone seen in chimpanzees. These changes, along with the navicular bone's sites for the cubonavicular and plantar calcaneonavicular ligaments, indicate that H. habilis had both the bone morphology and primitive medial longitudinal arch necessary for a bipedal gait quite similar to what is seen in humans today.

Homo erectus
Much of the data about the feet of Homo erectus comes from observations of fossils obtained from the Koobi Fora formation footprints, which are estimated to be between 1.51 and 1.53 million years old. The depth of different areas of the footprints as well as their shape provide ample information regarding the morphology and gait of H. erectus, although there is still some disagreement whether these prints represent H. erectus or Homo ergaster, and whether they are two distinct species. The hallux in these footprints is quite adducted at an angle of 14 degrees, compared to an angle of 8 degrees present in humans and 27 degrees present in the older Laetoli prints. The decrease in angle with time is evidence for the gradual shift towards a less adducted hallux which is appropriate for bipedal locomotion. The impression of the hallux is quite deep, as is found in a human footprint, due to the pressure placed on the toes and metatarsal heads during toe-off. The metatarsal heads themselves are represented in the footprint by a very deep impression that represents the "ball" of the foot seen in modern humans. The lateral toes also make an impression, and they observed to be shorter and less robust than those of older extinct homonids. An instep, or medial longitudinal arch, is also represented by the relatively shallow impression on the medial side of the footprint, further supporting the idea that H. erectus had a modern gait with an efficient stride.

Homo neanderthalensis
Homo neanderthalensis is known as one of H. sapiens' closest relatives, with evidence of existence ranging from approximately 300 000 to 30 000 years ago. Although the Neanderthal was much more robust than H. sapiens, it appears that it had a remarkably similar bipedal gait to modern humans. The phalanges and hallux appear to be similar in relative size to the body and function. The metatarsals were essentially the same as H. sapiens metatarsals, and there is ample evidence of a well developed medial longitudinal arch. H. neanderthalensis specimens are largely distinguished from H. sapiens specimens on the basis of their robusticity and skull morphology; the foot morphology is extremely similar and shares many derived traits.

Homo floresiensis
Homo floresiensis are known to have one of the most unique foot morphologies compared to body morphologies of all homonins. They are also known to be the most recently surviving homonin aside from H. sapiens, living between 95 000 and 17 000 years ago. The individuals were very short, leading them to be nicknamed "the hobbits", but more significantly, their feet were very long relative to their femur and tibia lengths. Their metatarsals had similar robusticity to modern humans, however there is little evidence of a medial longitudinal arch and the navicular bone was likely weight bearing. In contrast to many other homonins, phalanges II-V were quite long, with the hallux being quite short and gracile in comparison. In order to accommodate the toes, their gait must have been rather high stepping and likely did not resemble the modern human bipedal gait, with very little load sharing placed on the digits. Unfortunately, the unusual gait appears to have placed significantly more stress on the joints of the feet and legs, and several specimens present osteophytes on their phalanges. There is still some controversy among researchers as to whether H. floresiensis is a true species, or a unique population of H. sapiens or another homonin ancestor.

Comparison to Modern Primates
The extant primates, specifically discussing the apes, have a significantly different foot morphology than humans. In a human stride, the heel strikes first, then the lateral area of the foot makes contact with the surface. Next, the weight of the body is shifted onto the metatarsal heads, and finally the toe-off occurs, particularly involving the hallux. In contrast, the gorilla footstep begins with the heel and lateral midfoot making contact with the surface together, then the lateral phalanges, and then the push off occurs using both the hallux and lateral phalanges. In chimpanzees, a footstep occurs in two stages. First, the calcaneous leaves the surface and transfers weight primarily to the cuboid bone. Secondly, the weight is transferred to the digits, which complete the propulsive movement as the foot leaves the ground. In humans the cuboid bones never bear weight, and the lateral phalanges contribute very little force to the toe-off portion of the step. This difference is a result of decreased dorsiflexion and increased rigidity in the midfoot. Most notably, the midfoot region is lengthened in humans and the toes are much more short and gracile, with the hallux having a very small angle of adduction. The lengthened foot is important to distribute the forces of bearing weight, and the exaggerated medial longitudinal arch serves to maximize the efficiency of energy expenditure during bipedal walking. Furthermore, the toes are shortened in order to minimize vertical movement necessary for toe clearance.

Evolutionary Trade-offs
The human body is constantly evolving and adapting in order to be best suited to the pressures that exist in its environment, however many changes that are made have costs associated with them. In the human foot, adaptations for bipedalism have resulted in walking in a more efficient way that meets the needs of the species, but some of these modifications have also left the body more prone to developing certain injuries and diseases. Nevertheless, the advantages these changes provide are greater than the disadvantage of increased chance of some diseases, therefore the modifications persist in the human population. Plantar fasciitis is an overuse injury that results in inflammation and chronic pain in the band of connective tissue that helps form the longitudinal arch, particularly in the medial process of the calcaneal tubercle. The arch in the foot, while important for shock absorption, makes the foot more sensitive to biomechanical stress. Another common condition affecting the arch of the foot is pes planus, commonly known as flat feet. Flat feet result from prolonged stress on the foot, or can be acquired with advanced age, and the condition is simply a collapsed or undeveloped arch in the foot. Many flat-footed individuals experience no pain or negative symptoms, however those who participate in high amounts of physical activity, particularly running or jogging, may develop symptoms over time. Morton's neuroma describes the painful condition associated with the development of a fibrosis in the third digital nerve branch of the foot. It is generally caused by improper development of the arch of the foot, resulting in crowding of the second and third metatarsal spaces. The above conditions do not affect survival or reproductive ability, and they are mainly caused by overuse, misuse, or part of the aging process. They can cause significant discomfort in some affected individuals, however there is not a strong enough selective pressure for them to be eliminated from the population.