Jeremy DeSilva is a paleoanthropologist at Dartmouth College and the author of First Steps: How Upright Walking Made Us Human. He is a part of the research team that discovered and described two ancient members of the human family tree– Australopithecus sediba and Homo naledi. He has studied wild chimpanzees in Uganda and early human fossils in museums throughout Eastern and South Africa. From 1998-2003, he worked as an educator at the Boston Museum of Science. He continues to be passionate about science education and travels all over the world, giving lectures on human evolution. He and his wife, Erin, live in Norwich, Vermont, with their twins, Ben and Josie.
In this interview, we discuss his book First Steps and how upright walking helps humans think critically and build communities. Enter the giveaway below to win a copy of First Steps.
Greg: In brief, how did humans come to walk on two legs?
Jeremy: We actually don’t know how humans came to walk on two legs! It is one of the great mysteries in our science and a question that my colleagues and I are seeking to better understand. There is a lot we don’t know about the origin of walking, but we can start with what we do know. We know by comparing our DNA to that of our closest ape relatives—chimpanzees and bonobos—that the common ancestor we share with them lived around six million years ago. We don’t have too many fossils from this time period, but the bones we do have from our most ancient hominin ancestors (with names like Sahelanthropus, Orrorin, and Ardipithecus) already possess anatomies consistent with at least occasional upright walking.
Of all of the ways humans differ from our ape cousins, what evolved first—what literally got us going on our own lineage—was upright walking. It is the most ancient hominin adaptation. But precisely why this form of locomotion was favored in our ancestors is not entirely clear.
Furthermore, we still aren’t entirely sure from what bipedalism evolved. The all-too-familiar image of a chimpanzee slowly standing upright and turning into a bipedal human (called the “March of Progress”) may not be correct. There are some intriguing new fossils that suggest the common ancestor of the African apes did not knuckle-walk and instead moved upright on two legs in the trees like orangutans and gibbons sometimes do. If this turns out to be true, it would mean that knuckle-walking is the more recently evolved locomotion! We are still trying to figure this out and will need to discover many more fossils in the coming years to piece together how upright walking evolved.
Greg: What are the advantages and disadvantages of walking on two legs?
Jeremy: There are many advantages to moving on two legs. Bipedalism frees the hands from the duties of locomotion and allows them to be used to carry food and tools and babies. Moving on two legs (especially fully extended legs) is energetically efficient, allowing humans to cover large distances and have big home ranges without using up too much energy. Standing upright kept our ancestors cool on the African savanna by limiting how much of our body was exposed to direct sunlight while also making evaporative cooling more efficient.
But there are costs as well. For our size, we are pitifully slow. The fastest human on the planet—Usain Bolt—topped out at 28 mph. That is half the speed of a sprinting zebra, antelope, lion, or leopard. We are also unstable and can easily fall. And on top of that, humans are particularly vulnerable to injury. A sprained ankle, torn knee ligament, or slipped vertebral disk can render an already slow biped completely immobile. Finally, the anatomical changes to the pelvis that make this part of our body adapted for upright walking changes the dimensions of the birth canal. Compared with our ape cousins, humans have longer labors and birthing a child can be difficult and sometimes dangerous.
Greg: How does walking help us think? Does running have a similar influence?
Jeremy: Anecdotally, it is well known that walking helps you think. Some of the great thinkers—from Darwin and Dickens to Thoreau and Virginia Woolf—were obsessive walkers. But only recently have scientists begun to better understand the physiological mechanism underlying this connection. A daily walk has been shown to improve connectivity in regions of the brain understood to play an important role in our ability to think creativity. In older individuals, a daily walk can maintain the size and health of the hippocampus—a region of the brain critical for memory.
But, how? In the last two decades, physiologists have discovered that muscles act as endocrine organs and when they contract, whether via a brisk walk or a run, they release molecules called myokines into the bloodstream. Hundreds of myokines have been identified, including some that target regions of the brain. Irisin and BDNF (brain-derived neurotrophic factor) are two such myokines. This is a hot area of research and primed for new discoveries in the coming years.
Greg: Do you know if there are any relationships between specific types of movement and their impact on the brain? I suspect walking is great for creativity because walking is such an easy activity that your mind can wander. But that’s not the case with a very technical trail run. Perhaps that would be better for improving memory or quick decision-making. And what about running sprints on a track or walking indoors on a treadmill versus outside in nature?
Jeremy: That’s an awesome question, but I don’t know enough about the psychology involved to answer it. In researching the book, I read some of the work by University of Chicago psychology professor Marc Berman. He has written about the environment in which we exercise and how in environments where stimuli are “softer”—clouds, trees, bird chirps, sunsets, streams (as opposed to traffic and stoplights)—there is the opportunity for reflection and restoration of our minds. The alternative is what he has called “direction attention fatigue,” in which we respond to every little noise around us and the experience is not as rejuvenating. But I ended up not incorporating this very much in the book.
To your points, though, a Stanford study by Marily Oppezzo found that the benefits of walking on creativity were even greater when it was done outside (versus on a treadmill). That, to me, is fascinating and goes beyond the simple, mechanistic impacts of myokines.
It could be that our capacity for selflessness arose out of our vulnerabilities as bipeds in a dangerous world.
Greg: What is the relationship between upright walking and accepted social virtues like compassion and empathy?
Jeremy: It is rumored that an anthropology student once asked Margaret Mead for the earliest evidence of human civilization. After a brief reflection, Mead responded, “a healed femur.” While this exchange is likely apocryphal, the point being made is an important one. I wonder, then, what Professor Mead would have thought of a two-million-year-old femur from Kenya with evidence for a healed fracture. An even older fossil (3.4 million years old) from the famous Lucy’s species—Australopithecus afarensis—has a healed ankle fracture. These ancient bipedal ancestors of ours broke bones that would have rendered them completely immobile. Yet, they survived.
To me, this is evidence that they had assistance from group members. What it took for the seeds of cooperation and altruism to burst forth in the human lineage were the colossal challenges created by upright walking. It could be, then, that one of the most mysterious aspects of the human condition—our capacity for selflessness—arose out of our vulnerabilities as bipeds in a dangerous world.
Greg: What most surprised you as you were doing research for this book?
Jeremy: Two fossils absolutely blew my mind:
1) Carnufex carolinensis: aka “the Carolina Butcher.” I always thought that crocodiles were “living fossils” and that their lineage had not undergone many noticeable evolutionary changes, but I was totally wrong! Carnufex lived 230 million years ago in what is today North Carolina. It stood nine feet tall, had a mouthful of razor-sharp teeth, and—at least occasionally—was bipedal. Can you imagine a nine-foot tall, bipedally running crocodile??!!
2) Sthenurus stirlingi was a ten-foot, 300-pound kangaroo from the Pleistocene of Australia. If an animal of that size tried to hop, it would snap its tendons—a recipe for quick extinction. So, how did this kangaroo get around? It walked. When humans first got to Australia 65,000 years ago, they would have seen hopping kangaroos and giant, striding ones. So, what surprised me in researching this book were the many bipedal experiments, in different animal lineages, that had evolved—and gone extinct—over the eons. It was a fascinating and humbling experience.