October 3, 2018

What Makes the Human Foot Unique?

Adding a chapter in the story of what makes us human

This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American

Our feet stand us up. The bones that make up the feet represent a quarter of the human skeleton, and yet, despite comprising such a large percentage of the body, they have largely eluded us in the fossil record until recently. This is frustrating because it's clear that this story — the evolution of the human foot — has captivated us for hundreds of years. It is after all linked to the pivotal development of bipedalism in our history. Understanding differences between our feet and those of other apes (both ancestral and contemporary), can give us clues into the changes that were necessary for bipedalism — and perhaps for bipedalism itself. What has emerged is a story of diversity in locomotion that supports a case for mosaic evolution making the story of the foot overall (not just ours) a remarkable one.

A recent review article from researchers Ellison McNutt and colleagues tracks the literature on the evolution of the human foot. The quest to know ourselves begins in 1699 with an anatomical assessment of modern chimpanzees by Edward Tyson who labeled them quadrumanous, meaning all of their appendages were adapted to function as hands. In 1863 Thomas H. Huxley would make comparisons to gorilla feet and call out that while they were also inverted and possessed grasping tendencies, they also shared muscular similarities with the human foot. In 1935 anatomy professor Dudley Morton proposed the modern human foot is the result of two distinct transitions. In the first instance, the foot would have possessed more “ape-like” qualities with greater grasping abilities and flexibility, and notably an elongated midfoot region. In later stages, the foot would have moved away from these traits, although it may have retained the “grasping” ability with the big toe.

These ideas were fine, but what we needed were fossils. The discovery of OH 8 in 1960 by the Leakey team in Olduvai Gorge propelled us forward. OH 8 refers to Olduvai Hominid number 8 and dates to about 1.8 million years old. It belonged to a member of Homo habilis family and includes the the left tarsal and metatarsal bones — the tarsals are a series of bones that comprise the plane of the foot leading to the toes — but no actual toes. Taken together with the discovery of Lucy, as well as the Laetoli footprints, OH 8 served the story that the human foot was adapted from the arboreal chimpanzee foot. However, in 1995, scientists proposed that the anatomy of the ankle joint and heel (the hindfoot) existed before the anatomy of the human forefoot as it pertains to bipedalism. This was based on the discovery of “Little Foot,” an almost complete Australopithecus fossil skeleton recovered from Sterkfontein, South Africa dating to 3.3 million years ago, which exhibited similar hindfoot traits. Scientists believed that the foot of Australopithecus was adapted for bipedalism but it also allowed this early human ancestor to take refuge in the trees if needed. In piecing together these discoveries, it became clear that the evolutionary story of the human foot wouldn’t be explained linearly. The human foot evolved independently of other developments within human evolution and at different rates between species.

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One of the things we can say with certainty is that the modern human foot did not evolve from the chimpanzee foot. As reasonable as it may seem to draw comparisons between the two, the clear divergence between the genera from a last common ancestor (LCA) means that hominins and panins evolved feet to suit their needs. A major difference between the two stems from flexibility. The former’s foot is adapted for a stiff push-off which is necessary for bipedal locomotion. The latter’s feet maintains greater flexibility overall and grasping abilities that enable climbing trees as well quadrupedalism on the ground. Close anatomical analysis reveals that many of the differences between the two orient around the stiffness of one versus the flexibility of the other with robust versus gracile features that support different musculature and movement. For example, the big toe of humans is thick in comparison to that of a chimpanzee, and is aligned with the other toes, which allows the foot to push off the ground. This "big toe" is not only more gracile in chimpanzees, but it curves toward the other toes enabling a greater flexing motion. These characteristics are true of the toes in general also. In humans these bones are more robust and may help absorb some of the pressure of the push-off, while in chimpanzees these bones are extended and curved with higher degrees of flexibility.

The chimpanzee foot most likely contains derived elements that were adapted to support their arboreal lifestyle. If we want to unlock the "ancestral foot," the modern human foot may hold clues. We haven’t yet identified the last common ancestor we shared with chimpanzee, but we do have some very old hominid fossils, such as Pierolapithecus catalaunicus (11.9 million years old) and hominin fossils like Ardipithecus ramidus (4.4 million years old). The latter is the oldest hominin fossil we have with a relatively complete foot. Between these bookends and other Miocene apes, we can do the same thing that Morton did and generate a proposal for the LCA. We can predict that it may have possessed gracile features to facilitate grasping and flexibility. It would have been inverted, but it may also have passed a stiffer, more robust midfoot to allow for terrestrial activities.

Following the transition from Australopithecus to Homo, toes decreased in length and curvature, the ankle and corresponding musculature reduced in size, and full foot arches emerged. The big toe shifted to align with the other toes rather than curving inward enabling a more efficient push-off for bipedalism. There are some exceptions to these developments. In Homo naledi, for example, the toes are more curved than they are in the Homo genus overall. And Homo floriensis has an elongated forefoot, which most closely resembles that of bonobos. These kinds of variations aren’t unusual; the heel bones of modern great apes vary between species. These cases illustrate a diversity in foot evolution and locomotion, which with time may offer greater contextual clues about the lives of these groups.

The story of the human foot is still unfolding. It is unique because it is best suited to our style of bipedal locomotion. The variations that scientists have found in foot bones for australopiths suggest there was variation in how they walked even among themselves, which is true of humans today: we have different stride lengths and different ways of coming down on our feet. Some have a more forceful step than others, to say nothing of how the feet of dancers are changed with years of training. Our story has the added complexity of the impact of habitual shoe wearing. It has changed the way we walk and undoubted changed the morphology of our feet. It will need to account for prosthetics and accessibility options. The story, while incomplete, remains no less fascinating that it was hundreds of years ago when all we had was a comparative assessment.

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Referenced:

McNutt EJ, Zipfel B, DeSilva JM. The evolution of the human foot. Evol Anthropol. 2018;1–21. https://doi.org/10.1002/evan.21713