Human and Chimpanzee Foot Anatomy and Function
Human and chimpanzee feet have many similarities, however their differences show how they are each highly specialised for purpose.
Humans are unique among primates in being obligate bipeds: we stand and walk bipedally as a matter of normality. Nonhuman primates may walk bipedally for short distances, however they move in a variety of other ways with more efficiency. The last common ancestor of humans and chimpanzees lived about seven million years ago; since then, the human foot has evolved to be a highly specialised and complex structure allowing us to walk with an energy-efficient bipedal gait, while the chimpanzee foot has become adept at arboreal locomotion.
Foot Anatomy
Human and chimpanzee feet consist of twenty-six bones: seven tarsals, five metatarsals and fourteen phalanges. The tarsals include the talus, which forms the ankle along with the distal tibia and fibula of the leg, and the calcaneus, which sits beneath the talus and forms the heel. In front of, and articulating with, the talus, is the navicular bone (so-called as the shape resembles a primitive boat), and next to it, in front of and articulating with the calcaneus, is the cuboid, a cube-shaped bone. These two articulations form the talonavicular and calcaneocuboid joints which run continuously together, constituting the midtarsal joint.
In front of the navicular are the three cuneiform bones (medial, intermediate and lateral), the lateral also articulating with the cuboid. The first three metatarsals sit in front of the cuneiforms, and the last two metatarsals are in front of the cuboid. The proximal phalanges of the toes articulate with the heads of the metatarsals, and the intermediate and distal phalanges of the toes follow.
So How Are They Different?
Whilst chimpanzee feet contain all the same bones as human feet, the arrangement, sizes and lengths of these bones are different in each species leading to markedly different foot functions. Humans have shorter toes than chimpanzees (other than the big toe which is longer in humans for reasons we shall see) as they are not required for grasping, and much longer tarsal bones to help the rigid lever action of the foot when walking. Humans also have a big toe that is aligned with the other toes, and a medial longitudinal arch, made up of the calcaneus, talus, navicular, cuneiforms and first three metatarsals, with the midtarsal joint sitting at its apex. Chimpanzees, and indeed all other primates, have opposable first digits much like our thumbs, and flat, compliant feet without an arch.
The Gait Cycle
Both humans and chimpanzees, when walking bipedally, use three phases of gait. Firstly contact, when the heel makes contact with the ground; then midstance, when the foot is flat on the ground; and finally propulsion, when the foot leaves the ground (and the other foot makes contact). However, the midtarsal joint works very differently in each species, contributing to our propulsive gait and the less energy-efficient chimpanzee bipedality.
The Midtarsal Joint
In humans, at contact, the midtarsal joint is relaxed which enables the arch to elongate and absorb shock. As it moves through midstance, the bones of the midtarsal joint become tightly packed together, limiting movement at the midtarsal joint, tightening the arch and making the foot a rigid lever for propulsion as the heel lifts. Weight then transfers to the big toe – larger and more robust than that of chimpanzees – to propel the body forwards. The midtarsal joint itself remains inflexible throughout: its effectiveness comes from its ability to change from a relaxed to a tightly-packed bone structure.
Chimpanzees, however, flex at the midtarsal joint. This is called the ‘midtarsal break’, and it is from here that chimpanzees gain the propulsion that humans acquire from the rigid midtarsal joint. Whereas in humans, when the heel lifts, weight is transferred to the big toe, in chimpanzees weight is initially transferred to the front of the midtarsal joint where it breaks, and then to the digits. The opposable first digit, whilst prehensile and of great advantage when moving through trees, is of little use in propelling the body forward in terrestrial bipedal gait. The foot is too mobile for effective propulsion, but is advantageous in a largely arboreal lifestyle.
Human and chimpanzee feet are wonderfully complex and specialised for purpose. Whilst similar in some ways, they are also very different and function in ways that reflect their very different primary forms of locomotion: bipedality and arboreality.
References
Crompton, R. H., Sellers, W. I. And Thorpe, S. K. S. Arboreality, terrestriality and bipedalism. Phiosophoical Transactions of the Royal Society, 365, pp.3301-3314.
Lovejoy, C. O., Latimer, B., Suwa, G., Asfaw, B. And White, T. D. Combining Prehension and Propulsion: The Foot of Ardipithecus ramidus. Science, 326 pp.72.
Kidd, R. The Past if the key to the present: thoughts on the origins of human foot structure, function and dysfunction as seen from the fossil record. The Foot, 8 pp.75-84.