By Jeremy DeSilva of Boston University.
Jeremy will discuss the question “Why walk on two legs?” at a SciCafe at the American Museum of Natural History on April 1, 2015. This article is an excellent introduction to the pros and cons of bipedalism.
Humans are weird. We are mammals, yet we have very little body hair. We are primates, yet unlike most primates, we are generally uncomfortable in the trees. Like other animals, our brain is energetically expensive to grow and maintain, yet unlike other animals, we have somehow been able to evolve a brain six times larger than expected for our size. And, perhaps oddest of all, humans navigate their world perched up on extended hindlimb.
Look around you today. You might see cats, dogs, squirrels, or cows. They, and most other mammals, move around on all-fours. Humans? Nope—we have released our front limbs from the duties of locomotion and left that responsibility entirely to the hindlimbs. Certainly if this behavior, and the accompanying anatomical adaptations, have evolved in the human lineage, it must have been beneficial for our ancestors. But, evolutionarily speaking, “good ideas” tend to evolve multiple times in different lineages—something called convergent evolution. For instance, the streamlined body form of sharks, marlin, ichthyosaurs and dolphins independently evolved because it is the biomechanically most efficient shape for navigating quickly through the water. The wings of bats, birds, and butterflies allow different lineages to have independently taken to the skies.
But what about bipedalism? While many mammals can certainly rise up on two legs and even take a few steps (think about a threatening bear, or a vigilant meerkat), there is no other mammal that habitually strides around on its back legs like humans do. Now, that is not to say we are the only animals who do this. Ostriches and other large terrestrial birds are also striding bipeds and so were their theropod dinosaur ancestors. While over 300 million years of evolution separate the ancestors of birds and mammals, the comparison between humans and terrestrial birds is not entirely useless. It allows us to see what evolution can do in a few hundred million years (the time it has taken for birds to refine bipedalism) versus a mere 5 million (the length of time our own lineage has been bipedal). And it is therefore instructive to compare the foot of an ostrich to your own.
The human foot is composed of 26 bones. With your two feet, you have 52 bones in your feet—this means that roughly a quarter of all of the bones in your skeleton are in your feet. When two bones come together, they form a joint. And, motion is possible at joints—in fact, the 26 bones in your foot result in 33 joints in the foot and lots of potential for motion. This seems a tad odd given that you need your feet to be a stable platform that converts into a rigid lever when you push-off the ground. What about ostriches? Well, all of the bones of the ankle and the sole of the foot have fused together into a single rigid structure called the tarsometatarus. The toes are reduced in number, and in total, there are only 8 bones in an ostrich foot. In fact, the foot of an ostrich looks a lot like the new design for human foot prosthetics—a single, flexible, but rigid “blade” that stores and releases elastic energy during gait and a stable, rigid base for propulsion. If ostriches and engineers have figured out this “design”, why does the human foot look the way it does?
The answer is that evolution does not create the best “design” out of scratch. Evolution does not create perfection. It molds previous structures to produce anatomies just good enough to survive. Humans do not have feet like ostriches because our lineage has not been feathered and bipedal for 250 million years. Instead, we evolved from apes. These apes benefitted from having mobile feet, with lots of moving joints, to assist with navigation through the trees. That is the raw material from which the human foot evolved and evolution can only jerry-rig these pre-existing structures. How? Ligaments and subtle shape differences of certain foot bones have resulted in a slightly stiffer and less mobile foot in humans compared to modern apes, or our ape ancestors. This is the evolutionary equivalent of using duct-tape and paper clips to stiffen up an otherwise quite mobile structure. But, it works- kind of.
Podiatry (foot medicine) is a billion-dollar industry. Humans twist and sprain their ankles; we suffer from collapsed arches, plantar fasciitis, shin splints, bunions, and hammer-toes. Let’s face it, everyone has foot problems, and if you don’t yet, just wait. Undoubtedly, some of these ailments are a result of our wearing restrictive shoes. But, there are fossils of early human ancestors (who undoubtedly did not wear shoes) with healed ankle fractures, sprained ankles, flat feet, compression fractures, and other foot problems humans suffer from today. These afflictions have been with us from the very beginnings our unusual form of locomotion and they will continue to be with us for millennia. Why? Because we walk on modified ape feet. It is not an anatomy one would design from scratch, because we were not designed from scratch. We evolved and we retain our ape legacy all through our bodies, including in our feet.
When your feet ache after a long day, or your shin splints flare after a short jog, you may want to curse your ancestors. But, I think you should instead thank them. Without their survival, there is no “you”, or even “us”. And, they had it much, much worse. When you break your foot, or have a severe case of plantar fasciitis, you can go to a hospital or visit your podiatrist. But, there was no such thing as a podiatrist on the predator-laden African savannah 3 million years-ago. So, how did our ancestors survive in such conditions? How did a broken ankle not guarantee death for these individuals? Certainly some of these individuals became leopard food, but we have fossil evidence that these injuries often healed. How? I propose that these fossils are evidence that as far back as 3 million years-ago, our ancestors were taking care of one another in a rather human-like way. It is just possible that the development of care and compassion in our human ancestors—qualities we hold so dear and regard as so human-like—may have developed in the context of our imperfect “design”. Bipedalism may have only worked as an evolutionary innovation in a lineage that already caring to some degree for the sick and injured. Compassion would have thus evolved as a by-product of, among other things, moving around bipedally on faulty equipment. It is a tough hypothesis to scientifically test, but worth consideration. And while you do just that, please sit—those feet could use a rest.
Jeremy DeSilva is a functional morphologist and Professor of Anthropology at Boston University.